--- /dev/null
-elevator_may_queue_fn returns true if the scheduler wants to allow the
- current context to queue a new request even if
- it is over the queue limit. This must be used
- very carefully!!
-
+ =====================================================
+ Notes on the Generic Block Layer Rewrite in Linux 2.5
+ =====================================================
+
+ .. note::
+
+ It seems that there are lot of outdated stuff here. This seems
+ to be written somewhat as a task list. Yet, eventually, something
+ here might still be useful.
+
+ Notes Written on Jan 15, 2002:
+
+ - Jens Axboe <jens.axboe@oracle.com>
+ - Suparna Bhattacharya <suparna@in.ibm.com>
+
+ Last Updated May 2, 2002
+
+ September 2003: Updated I/O Scheduler portions
+ - Nick Piggin <npiggin@kernel.dk>
+
+ Introduction
+ ============
+
+ These are some notes describing some aspects of the 2.5 block layer in the
+ context of the bio rewrite. The idea is to bring out some of the key
+ changes and a glimpse of the rationale behind those changes.
+
+ Please mail corrections & suggestions to suparna@in.ibm.com.
+
+ Credits
+ =======
+
+ 2.5 bio rewrite:
+ - Jens Axboe <jens.axboe@oracle.com>
+
+ Many aspects of the generic block layer redesign were driven by and evolved
+ over discussions, prior patches and the collective experience of several
+ people. See sections 8 and 9 for a list of some related references.
+
+ The following people helped with review comments and inputs for this
+ document:
+
+ - Christoph Hellwig <hch@infradead.org>
+ - Arjan van de Ven <arjanv@redhat.com>
+ - Randy Dunlap <rdunlap@xenotime.net>
+ - Andre Hedrick <andre@linux-ide.org>
+
+ The following people helped with fixes/contributions to the bio patches
+ while it was still work-in-progress:
+
+ - David S. Miller <davem@redhat.com>
+
+
+ .. Description of Contents:
+
+ 1. Scope for tuning of logic to various needs
+ 1.1 Tuning based on device or low level driver capabilities
+ - Per-queue parameters
+ - Highmem I/O support
+ - I/O scheduler modularization
+ 1.2 Tuning based on high level requirements/capabilities
+ 1.2.1 Request Priority/Latency
+ 1.3 Direct access/bypass to lower layers for diagnostics and special
+ device operations
+ 1.3.1 Pre-built commands
+ 2. New flexible and generic but minimalist i/o structure or descriptor
+ (instead of using buffer heads at the i/o layer)
+ 2.1 Requirements/Goals addressed
+ 2.2 The bio struct in detail (multi-page io unit)
+ 2.3 Changes in the request structure
+ 3. Using bios
+ 3.1 Setup/teardown (allocation, splitting)
+ 3.2 Generic bio helper routines
+ 3.2.1 Traversing segments and completion units in a request
+ 3.2.2 Setting up DMA scatterlists
+ 3.2.3 I/O completion
+ 3.2.4 Implications for drivers that do not interpret bios (don't handle
+ multiple segments)
+ 3.3 I/O submission
+ 4. The I/O scheduler
+ 5. Scalability related changes
+ 5.1 Granular locking: Removal of io_request_lock
+ 5.2 Prepare for transition to 64 bit sector_t
+ 6. Other Changes/Implications
+ 6.1 Partition re-mapping handled by the generic block layer
+ 7. A few tips on migration of older drivers
+ 8. A list of prior/related/impacted patches/ideas
+ 9. Other References/Discussion Threads
+
+
+ Bio Notes
+ =========
+
+ Let us discuss the changes in the context of how some overall goals for the
+ block layer are addressed.
+
+ 1. Scope for tuning the generic logic to satisfy various requirements
+ =====================================================================
+
+ The block layer design supports adaptable abstractions to handle common
+ processing with the ability to tune the logic to an appropriate extent
+ depending on the nature of the device and the requirements of the caller.
+ One of the objectives of the rewrite was to increase the degree of tunability
+ and to enable higher level code to utilize underlying device/driver
+ capabilities to the maximum extent for better i/o performance. This is
+ important especially in the light of ever improving hardware capabilities
+ and application/middleware software designed to take advantage of these
+ capabilities.
+
+ 1.1 Tuning based on low level device / driver capabilities
+ ----------------------------------------------------------
+
+ Sophisticated devices with large built-in caches, intelligent i/o scheduling
+ optimizations, high memory DMA support, etc may find some of the
+ generic processing an overhead, while for less capable devices the
+ generic functionality is essential for performance or correctness reasons.
+ Knowledge of some of the capabilities or parameters of the device should be
+ used at the generic block layer to take the right decisions on
+ behalf of the driver.
+
+ How is this achieved ?
+
+ Tuning at a per-queue level:
+
+ i. Per-queue limits/values exported to the generic layer by the driver
+
+ Various parameters that the generic i/o scheduler logic uses are set at
+ a per-queue level (e.g maximum request size, maximum number of segments in
+ a scatter-gather list, logical block size)
+
+ Some parameters that were earlier available as global arrays indexed by
+ major/minor are now directly associated with the queue. Some of these may
+ move into the block device structure in the future. Some characteristics
+ have been incorporated into a queue flags field rather than separate fields
+ in themselves. There are blk_queue_xxx functions to set the parameters,
+ rather than update the fields directly
+
+ Some new queue property settings:
+
+ blk_queue_bounce_limit(q, u64 dma_address)
+ Enable I/O to highmem pages, dma_address being the
+ limit. No highmem default.
+
+ blk_queue_max_sectors(q, max_sectors)
+ Sets two variables that limit the size of the request.
+
+ - The request queue's max_sectors, which is a soft size in
+ units of 512 byte sectors, and could be dynamically varied
+ by the core kernel.
+
+ - The request queue's max_hw_sectors, which is a hard limit
+ and reflects the maximum size request a driver can handle
+ in units of 512 byte sectors.
+
+ The default for both max_sectors and max_hw_sectors is
+ 255. The upper limit of max_sectors is 1024.
+
+ blk_queue_max_phys_segments(q, max_segments)
+ Maximum physical segments you can handle in a request. 128
+ default (driver limit). (See 3.2.2)
+
+ blk_queue_max_hw_segments(q, max_segments)
+ Maximum dma segments the hardware can handle in a request. 128
+ default (host adapter limit, after dma remapping).
+ (See 3.2.2)
+
+ blk_queue_max_segment_size(q, max_seg_size)
+ Maximum size of a clustered segment, 64kB default.
+
+ blk_queue_logical_block_size(q, logical_block_size)
+ Lowest possible sector size that the hardware can operate
+ on, 512 bytes default.
+
+ New queue flags:
+
+ - QUEUE_FLAG_CLUSTER (see 3.2.2)
+ - QUEUE_FLAG_QUEUED (see 3.2.4)
+
+
+ ii. High-mem i/o capabilities are now considered the default
+
+ The generic bounce buffer logic, present in 2.4, where the block layer would
+ by default copyin/out i/o requests on high-memory buffers to low-memory buffers
+ assuming that the driver wouldn't be able to handle it directly, has been
+ changed in 2.5. The bounce logic is now applied only for memory ranges
+ for which the device cannot handle i/o. A driver can specify this by
+ setting the queue bounce limit for the request queue for the device
+ (blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
+ where a device is capable of handling high memory i/o.
+
+ In order to enable high-memory i/o where the device is capable of supporting
+ it, the pci dma mapping routines and associated data structures have now been
+ modified to accomplish a direct page -> bus translation, without requiring
+ a virtual address mapping (unlike the earlier scheme of virtual address
+ -> bus translation). So this works uniformly for high-memory pages (which
+ do not have a corresponding kernel virtual address space mapping) and
+ low-memory pages.
+
+ Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion
+ on PCI high mem DMA aspects and mapping of scatter gather lists, and support
+ for 64 bit PCI.
+
+ Special handling is required only for cases where i/o needs to happen on
+ pages at physical memory addresses beyond what the device can support. In these
+ cases, a bounce bio representing a buffer from the supported memory range
+ is used for performing the i/o with copyin/copyout as needed depending on
+ the type of the operation. For example, in case of a read operation, the
+ data read has to be copied to the original buffer on i/o completion, so a
+ callback routine is set up to do this, while for write, the data is copied
+ from the original buffer to the bounce buffer prior to issuing the
+ operation. Since an original buffer may be in a high memory area that's not
+ mapped in kernel virtual addr, a kmap operation may be required for
+ performing the copy, and special care may be needed in the completion path
+ as it may not be in irq context. Special care is also required (by way of
+ GFP flags) when allocating bounce buffers, to avoid certain highmem
+ deadlock possibilities.
+
+ It is also possible that a bounce buffer may be allocated from high-memory
+ area that's not mapped in kernel virtual addr, but within the range that the
+ device can use directly; so the bounce page may need to be kmapped during
+ copy operations. [Note: This does not hold in the current implementation,
+ though]
+
+ There are some situations when pages from high memory may need to
+ be kmapped, even if bounce buffers are not necessary. For example a device
+ may need to abort DMA operations and revert to PIO for the transfer, in
+ which case a virtual mapping of the page is required. For SCSI it is also
+ done in some scenarios where the low level driver cannot be trusted to
+ handle a single sg entry correctly. The driver is expected to perform the
+ kmaps as needed on such occasions as appropriate. A driver could also use
+ the blk_queue_bounce() routine on its own to bounce highmem i/o to low
+ memory for specific requests if so desired.
+
+ iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
+
+ As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
+ queue or pick from (copy) existing generic schedulers and replace/override
+ certain portions of it. The 2.5 rewrite provides improved modularization
+ of the i/o scheduler. There are more pluggable callbacks, e.g for init,
+ add request, extract request, which makes it possible to abstract specific
+ i/o scheduling algorithm aspects and details outside of the generic loop.
+ It also makes it possible to completely hide the implementation details of
+ the i/o scheduler from block drivers.
+
+ I/O scheduler wrappers are to be used instead of accessing the queue directly.
+ See section 4. The I/O scheduler for details.
+
+ 1.2 Tuning Based on High level code capabilities
+ ------------------------------------------------
+
+ i. Application capabilities for raw i/o
+
+ This comes from some of the high-performance database/middleware
+ requirements where an application prefers to make its own i/o scheduling
+ decisions based on an understanding of the access patterns and i/o
+ characteristics
+
+ ii. High performance filesystems or other higher level kernel code's
+ capabilities
+
+ Kernel components like filesystems could also take their own i/o scheduling
+ decisions for optimizing performance. Journalling filesystems may need
+ some control over i/o ordering.
+
+ What kind of support exists at the generic block layer for this ?
+
+ The flags and rw fields in the bio structure can be used for some tuning
+ from above e.g indicating that an i/o is just a readahead request, or priority
+ settings (currently unused). As far as user applications are concerned they
+ would need an additional mechanism either via open flags or ioctls, or some
+ other upper level mechanism to communicate such settings to block.
+
+ 1.2.1 Request Priority/Latency
+ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ Todo/Under discussion::
+
+ Arjan's proposed request priority scheme allows higher levels some broad
+ control (high/med/low) over the priority of an i/o request vs other pending
+ requests in the queue. For example it allows reads for bringing in an
+ executable page on demand to be given a higher priority over pending write
+ requests which haven't aged too much on the queue. Potentially this priority
+ could even be exposed to applications in some manner, providing higher level
+ tunability. Time based aging avoids starvation of lower priority
+ requests. Some bits in the bi_opf flags field in the bio structure are
+ intended to be used for this priority information.
+
+
+ 1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
+ -----------------------------------------------------------------------
+
+ (e.g Diagnostics, Systems Management)
+
+ There are situations where high-level code needs to have direct access to
+ the low level device capabilities or requires the ability to issue commands
+ to the device bypassing some of the intermediate i/o layers.
+ These could, for example, be special control commands issued through ioctl
+ interfaces, or could be raw read/write commands that stress the drive's
+ capabilities for certain kinds of fitness tests. Having direct interfaces at
+ multiple levels without having to pass through upper layers makes
+ it possible to perform bottom up validation of the i/o path, layer by
+ layer, starting from the media.
+
+ The normal i/o submission interfaces, e.g submit_bio, could be bypassed
+ for specially crafted requests which such ioctl or diagnostics
+ interfaces would typically use, and the elevator add_request routine
+ can instead be used to directly insert such requests in the queue or preferably
+ the blk_do_rq routine can be used to place the request on the queue and
+ wait for completion. Alternatively, sometimes the caller might just
+ invoke a lower level driver specific interface with the request as a
+ parameter.
+
+ If the request is a means for passing on special information associated with
+ the command, then such information is associated with the request->special
+ field (rather than misuse the request->buffer field which is meant for the
+ request data buffer's virtual mapping).
+
+ For passing request data, the caller must build up a bio descriptor
+ representing the concerned memory buffer if the underlying driver interprets
+ bio segments or uses the block layer end*request* functions for i/o
+ completion. Alternatively one could directly use the request->buffer field to
+ specify the virtual address of the buffer, if the driver expects buffer
+ addresses passed in this way and ignores bio entries for the request type
+ involved. In the latter case, the driver would modify and manage the
+ request->buffer, request->sector and request->nr_sectors or
+ request->current_nr_sectors fields itself rather than using the block layer
+ end_request or end_that_request_first completion interfaces.
+ (See 2.3 or Documentation/block/request.rst for a brief explanation of
+ the request structure fields)
+
+ ::
+
+ [TBD: end_that_request_last should be usable even in this case;
+ Perhaps an end_that_direct_request_first routine could be implemented to make
+ handling direct requests easier for such drivers; Also for drivers that
+ expect bios, a helper function could be provided for setting up a bio
+ corresponding to a data buffer]
+
+ <JENS: I dont understand the above, why is end_that_request_first() not
+ usable? Or _last for that matter. I must be missing something>
+
+ <SUP: What I meant here was that if the request doesn't have a bio, then
+ end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
+ and hence can't be used for advancing request state settings on the
+ completion of partial transfers. The driver has to modify these fields
+ directly by hand.
+ This is because end_that_request_first only iterates over the bio list,
+ and always returns 0 if there are none associated with the request.
+ _last works OK in this case, and is not a problem, as I mentioned earlier
+ >
+
+ 1.3.1 Pre-built Commands
+ ^^^^^^^^^^^^^^^^^^^^^^^^
+
+ A request can be created with a pre-built custom command to be sent directly
+ to the device. The cmd block in the request structure has room for filling
+ in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
+ command pre-building, and the type of the request is now indicated
+ through rq->flags instead of via rq->cmd)
+
+ The request structure flags can be set up to indicate the type of request
+ in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
+ packet command issued via blk_do_rq, REQ_SPECIAL: special request).
+
+ It can help to pre-build device commands for requests in advance.
+ Drivers can now specify a request prepare function (q->prep_rq_fn) that the
+ block layer would invoke to pre-build device commands for a given request,
+ or perform other preparatory processing for the request. This is routine is
+ called by elv_next_request(), i.e. typically just before servicing a request.
+ (The prepare function would not be called for requests that have RQF_DONTPREP
+ enabled)
+
+ Aside:
+ Pre-building could possibly even be done early, i.e before placing the
+ request on the queue, rather than construct the command on the fly in the
+ driver while servicing the request queue when it may affect latencies in
+ interrupt context or responsiveness in general. One way to add early
+ pre-building would be to do it whenever we fail to merge on a request.
+ Now REQ_NOMERGE is set in the request flags to skip this one in the future,
+ which means that it will not change before we feed it to the device. So
+ the pre-builder hook can be invoked there.
+
+
+ 2. Flexible and generic but minimalist i/o structure/descriptor
+ ===============================================================
+
+ 2.1 Reason for a new structure and requirements addressed
+ ---------------------------------------------------------
+
+ Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
+ layer, and the low level request structure was associated with a chain of
+ buffer heads for a contiguous i/o request. This led to certain inefficiencies
+ when it came to large i/o requests and readv/writev style operations, as it
+ forced such requests to be broken up into small chunks before being passed
+ on to the generic block layer, only to be merged by the i/o scheduler
+ when the underlying device was capable of handling the i/o in one shot.
+ Also, using the buffer head as an i/o structure for i/os that didn't originate
+ from the buffer cache unnecessarily added to the weight of the descriptors
+ which were generated for each such chunk.
+
+ The following were some of the goals and expectations considered in the
+ redesign of the block i/o data structure in 2.5.
+
+ 1. Should be appropriate as a descriptor for both raw and buffered i/o -
+ avoid cache related fields which are irrelevant in the direct/page i/o path,
+ or filesystem block size alignment restrictions which may not be relevant
+ for raw i/o.
+ 2. Ability to represent high-memory buffers (which do not have a virtual
+ address mapping in kernel address space).
+ 3. Ability to represent large i/os w/o unnecessarily breaking them up (i.e
+ greater than PAGE_SIZE chunks in one shot)
+ 4. At the same time, ability to retain independent identity of i/os from
+ different sources or i/o units requiring individual completion (e.g. for
+ latency reasons)
+ 5. Ability to represent an i/o involving multiple physical memory segments
+ (including non-page aligned page fragments, as specified via readv/writev)
+ without unnecessarily breaking it up, if the underlying device is capable of
+ handling it.
+ 6. Preferably should be based on a memory descriptor structure that can be
+ passed around different types of subsystems or layers, maybe even
+ networking, without duplication or extra copies of data/descriptor fields
+ themselves in the process
+ 7. Ability to handle the possibility of splits/merges as the structure passes
+ through layered drivers (lvm, md, evms), with minimal overhead.
+
+ The solution was to define a new structure (bio) for the block layer,
+ instead of using the buffer head structure (bh) directly, the idea being
+ avoidance of some associated baggage and limitations. The bio structure
+ is uniformly used for all i/o at the block layer ; it forms a part of the
+ bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
+ mapped to bio structures.
+
+ 2.2 The bio struct
+ ------------------
+
+ The bio structure uses a vector representation pointing to an array of tuples
+ of <page, offset, len> to describe the i/o buffer, and has various other
+ fields describing i/o parameters and state that needs to be maintained for
+ performing the i/o.
+
+ Notice that this representation means that a bio has no virtual address
+ mapping at all (unlike buffer heads).
+
+ ::
+
+ struct bio_vec {
+ struct page *bv_page;
+ unsigned short bv_len;
+ unsigned short bv_offset;
+ };
+
+ /*
+ * main unit of I/O for the block layer and lower layers (ie drivers)
+ */
+ struct bio {
+ struct bio *bi_next; /* request queue link */
+ struct block_device *bi_bdev; /* target device */
+ unsigned long bi_flags; /* status, command, etc */
+ unsigned long bi_opf; /* low bits: r/w, high: priority */
+
+ unsigned int bi_vcnt; /* how may bio_vec's */
+ struct bvec_iter bi_iter; /* current index into bio_vec array */
+
+ unsigned int bi_size; /* total size in bytes */
+ unsigned short bi_hw_segments; /* segments after DMA remapping */
+ unsigned int bi_max; /* max bio_vecs we can hold
+ used as index into pool */
+ struct bio_vec *bi_io_vec; /* the actual vec list */
+ bio_end_io_t *bi_end_io; /* bi_end_io (bio) */
+ atomic_t bi_cnt; /* pin count: free when it hits zero */
+ void *bi_private;
+ };
+
+ With this multipage bio design:
+
+ - Large i/os can be sent down in one go using a bio_vec list consisting
+ of an array of <page, offset, len> fragments (similar to the way fragments
+ are represented in the zero-copy network code)
+ - Splitting of an i/o request across multiple devices (as in the case of
+ lvm or raid) is achieved by cloning the bio (where the clone points to
+ the same bi_io_vec array, but with the index and size accordingly modified)
+ - A linked list of bios is used as before for unrelated merges [#]_ - this
+ avoids reallocs and makes independent completions easier to handle.
+ - Code that traverses the req list can find all the segments of a bio
+ by using rq_for_each_segment. This handles the fact that a request
+ has multiple bios, each of which can have multiple segments.
+ - Drivers which can't process a large bio in one shot can use the bi_iter
+ field to keep track of the next bio_vec entry to process.
+ (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
+ [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
+ bi_offset an len fields]
+
+ .. [#]
+
+ unrelated merges -- a request ends up containing two or more bios that
+ didn't originate from the same place.
+
+ bi_end_io() i/o callback gets called on i/o completion of the entire bio.
+
+ At a lower level, drivers build a scatter gather list from the merged bios.
+ The scatter gather list is in the form of an array of <page, offset, len>
+ entries with their corresponding dma address mappings filled in at the
+ appropriate time. As an optimization, contiguous physical pages can be
+ covered by a single entry where <page> refers to the first page and <len>
+ covers the range of pages (up to 16 contiguous pages could be covered this
+ way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
+ the sg list.
+
+ Note: Right now the only user of bios with more than one page is ll_rw_kio,
+ which in turn means that only raw I/O uses it (direct i/o may not work
+ right now). The intent however is to enable clustering of pages etc to
+ become possible. The pagebuf abstraction layer from SGI also uses multi-page
+ bios, but that is currently not included in the stock development kernels.
+ The same is true of Andrew Morton's work-in-progress multipage bio writeout
+ and readahead patches.
+
+ 2.3 Changes in the Request Structure
+ ------------------------------------
+
+ The request structure is the structure that gets passed down to low level
+ drivers. The block layer make_request function builds up a request structure,
+ places it on the queue and invokes the drivers request_fn. The driver makes
+ use of block layer helper routine elv_next_request to pull the next request
+ off the queue. Control or diagnostic functions might bypass block and directly
+ invoke underlying driver entry points passing in a specially constructed
+ request structure.
+
+ Only some relevant fields (mainly those which changed or may be referred
+ to in some of the discussion here) are listed below, not necessarily in
+ the order in which they occur in the structure (see include/linux/blkdev.h)
+ Refer to Documentation/block/request.rst for details about all the request
+ structure fields and a quick reference about the layers which are
+ supposed to use or modify those fields::
+
+ struct request {
+ struct list_head queuelist; /* Not meant to be directly accessed by
+ the driver.
+ Used by q->elv_next_request_fn
+ rq->queue is gone
+ */
+ .
+ .
+ unsigned char cmd[16]; /* prebuilt command data block */
+ unsigned long flags; /* also includes earlier rq->cmd settings */
+ .
+ .
+ sector_t sector; /* this field is now of type sector_t instead of int
+ preparation for 64 bit sectors */
+ .
+ .
+
+ /* Number of scatter-gather DMA addr+len pairs after
+ * physical address coalescing is performed.
+ */
+ unsigned short nr_phys_segments;
+
+ /* Number of scatter-gather addr+len pairs after
+ * physical and DMA remapping hardware coalescing is performed.
+ * This is the number of scatter-gather entries the driver
+ * will actually have to deal with after DMA mapping is done.
+ */
+ unsigned short nr_hw_segments;
+
+ /* Various sector counts */
+ unsigned long nr_sectors; /* no. of sectors left: driver modifiable */
+ unsigned long hard_nr_sectors; /* block internal copy of above */
+ unsigned int current_nr_sectors; /* no. of sectors left in the
+ current segment:driver modifiable */
+ unsigned long hard_cur_sectors; /* block internal copy of the above */
+ .
+ .
+ int tag; /* command tag associated with request */
+ void *special; /* same as before */
+ char *buffer; /* valid only for low memory buffers up to
+ current_nr_sectors */
+ .
+ .
+ struct bio *bio, *biotail; /* bio list instead of bh */
+ struct request_list *rl;
+ }
+
+ See the req_ops and req_flag_bits definitions for an explanation of the various
+ flags available. Some bits are used by the block layer or i/o scheduler.
+
+ The behaviour of the various sector counts are almost the same as before,
+ except that since we have multi-segment bios, current_nr_sectors refers
+ to the numbers of sectors in the current segment being processed which could
+ be one of the many segments in the current bio (i.e i/o completion unit).
+ The nr_sectors value refers to the total number of sectors in the whole
+ request that remain to be transferred (no change). The purpose of the
+ hard_xxx values is for block to remember these counts every time it hands
+ over the request to the driver. These values are updated by block on
+ end_that_request_first, i.e. every time the driver completes a part of the
+ transfer and invokes block end*request helpers to mark this. The
+ driver should not modify these values. The block layer sets up the
+ nr_sectors and current_nr_sectors fields (based on the corresponding
+ hard_xxx values and the number of bytes transferred) and updates it on
+ every transfer that invokes end_that_request_first. It does the same for the
+ buffer, bio, bio->bi_iter fields too.
+
+ The buffer field is just a virtual address mapping of the current segment
+ of the i/o buffer in cases where the buffer resides in low-memory. For high
+ memory i/o, this field is not valid and must not be used by drivers.
+
+ Code that sets up its own request structures and passes them down to
+ a driver needs to be careful about interoperation with the block layer helper
+ functions which the driver uses. (Section 1.3)
+
+ 3. Using bios
+ =============
+
+ 3.1 Setup/Teardown
+ ------------------
+
+ There are routines for managing the allocation, and reference counting, and
+ freeing of bios (bio_alloc, bio_get, bio_put).
+
+ This makes use of Ingo Molnar's mempool implementation, which enables
+ subsystems like bio to maintain their own reserve memory pools for guaranteed
+ deadlock-free allocations during extreme VM load. For example, the VM
+ subsystem makes use of the block layer to writeout dirty pages in order to be
+ able to free up memory space, a case which needs careful handling. The
+ allocation logic draws from the preallocated emergency reserve in situations
+ where it cannot allocate through normal means. If the pool is empty and it
+ can wait, then it would trigger action that would help free up memory or
+ replenish the pool (without deadlocking) and wait for availability in the pool.
+ If it is in IRQ context, and hence not in a position to do this, allocation
+ could fail if the pool is empty. In general mempool always first tries to
+ perform allocation without having to wait, even if it means digging into the
+ pool as long it is not less that 50% full.
+
+ On a free, memory is released to the pool or directly freed depending on
+ the current availability in the pool. The mempool interface lets the
+ subsystem specify the routines to be used for normal alloc and free. In the
+ case of bio, these routines make use of the standard slab allocator.
+
+ The caller of bio_alloc is expected to taken certain steps to avoid
+ deadlocks, e.g. avoid trying to allocate more memory from the pool while
+ already holding memory obtained from the pool.
+
+ ::
+
+ [TBD: This is a potential issue, though a rare possibility
+ in the bounce bio allocation that happens in the current code, since
+ it ends up allocating a second bio from the same pool while
+ holding the original bio ]
+
+ Memory allocated from the pool should be released back within a limited
+ amount of time (in the case of bio, that would be after the i/o is completed).
+ This ensures that if part of the pool has been used up, some work (in this
+ case i/o) must already be in progress and memory would be available when it
+ is over. If allocating from multiple pools in the same code path, the order
+ or hierarchy of allocation needs to be consistent, just the way one deals
+ with multiple locks.
+
+ The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
+ for a non-clone bio. There are the 6 pools setup for different size biovecs,
+ so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
+ given size from these slabs.
+
+ The bio_get() routine may be used to hold an extra reference on a bio prior
+ to i/o submission, if the bio fields are likely to be accessed after the
+ i/o is issued (since the bio may otherwise get freed in case i/o completion
+ happens in the meantime).
+
+ The bio_clone_fast() routine may be used to duplicate a bio, where the clone
+ shares the bio_vec_list with the original bio (i.e. both point to the
+ same bio_vec_list). This would typically be used for splitting i/o requests
+ in lvm or md.
+
+ 3.2 Generic bio helper Routines
+ -------------------------------
+
+ 3.2.1 Traversing segments and completion units in a request
+ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ The macro rq_for_each_segment() should be used for traversing the bios
+ in the request list (drivers should avoid directly trying to do it
+ themselves). Using these helpers should also make it easier to cope
+ with block changes in the future.
+
+ ::
+
+ struct req_iterator iter;
+ rq_for_each_segment(bio_vec, rq, iter)
+ /* bio_vec is now current segment */
+
+ I/O completion callbacks are per-bio rather than per-segment, so drivers
+ that traverse bio chains on completion need to keep that in mind. Drivers
+ which don't make a distinction between segments and completion units would
+ need to be reorganized to support multi-segment bios.
+
+ 3.2.2 Setting up DMA scatterlists
+ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ The blk_rq_map_sg() helper routine would be used for setting up scatter
+ gather lists from a request, so a driver need not do it on its own.
+
+ nr_segments = blk_rq_map_sg(q, rq, scatterlist);
+
+ The helper routine provides a level of abstraction which makes it easier
+ to modify the internals of request to scatterlist conversion down the line
+ without breaking drivers. The blk_rq_map_sg routine takes care of several
+ things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
+ is set) and correct segment accounting to avoid exceeding the limits which
+ the i/o hardware can handle, based on various queue properties.
+
+ - Prevents a clustered segment from crossing a 4GB mem boundary
+ - Avoids building segments that would exceed the number of physical
+ memory segments that the driver can handle (phys_segments) and the
+ number that the underlying hardware can handle at once, accounting for
+ DMA remapping (hw_segments) (i.e. IOMMU aware limits).
+
+ Routines which the low level driver can use to set up the segment limits:
+
+ blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
+ hw data segments in a request (i.e. the maximum number of address/length
+ pairs the host adapter can actually hand to the device at once)
+
+ blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
+ of physical data segments in a request (i.e. the largest sized scatter list
+ a driver could handle)
+
+ 3.2.3 I/O completion
+ ^^^^^^^^^^^^^^^^^^^^
+
+ The existing generic block layer helper routines end_request,
+ end_that_request_first and end_that_request_last can be used for i/o
+ completion (and setting things up so the rest of the i/o or the next
+ request can be kicked of) as before. With the introduction of multi-page
+ bio support, end_that_request_first requires an additional argument indicating
+ the number of sectors completed.
+
+ 3.2.4 Implications for drivers that do not interpret bios
+ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ (don't handle multiple segments)
+
+ Drivers that do not interpret bios e.g those which do not handle multiple
+ segments and do not support i/o into high memory addresses (require bounce
+ buffers) and expect only virtually mapped buffers, can access the rq->buffer
+ field. As before the driver should use current_nr_sectors to determine the
+ size of remaining data in the current segment (that is the maximum it can
+ transfer in one go unless it interprets segments), and rely on the block layer
+ end_request, or end_that_request_first/last to take care of all accounting
+ and transparent mapping of the next bio segment when a segment boundary
+ is crossed on completion of a transfer. (The end*request* functions should
+ be used if only if the request has come down from block/bio path, not for
+ direct access requests which only specify rq->buffer without a valid rq->bio)
+
+ 3.3 I/O Submission
+ ------------------
+
+ The routine submit_bio() is used to submit a single io. Higher level i/o
+ routines make use of this:
+
+ (a) Buffered i/o:
+
+ The routine submit_bh() invokes submit_bio() on a bio corresponding to the
+ bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
+
+ (b) Kiobuf i/o (for raw/direct i/o):
+
+ The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
+ maps the array to one or more multi-page bios, issuing submit_bio() to
+ perform the i/o on each of these.
+
+ The embedded bh array in the kiobuf structure has been removed and no
+ preallocation of bios is done for kiobufs. [The intent is to remove the
+ blocks array as well, but it's currently in there to kludge around direct i/o.]
+ Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
+
+ Todo/Observation:
+
+ A single kiobuf structure is assumed to correspond to a contiguous range
+ of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
+ So right now it wouldn't work for direct i/o on non-contiguous blocks.
+ This is to be resolved. The eventual direction is to replace kiobuf
+ by kvec's.
+
+ Badari Pulavarty has a patch to implement direct i/o correctly using
+ bio and kvec.
+
+
+ (c) Page i/o:
+
+ Todo/Under discussion:
+
+ Andrew Morton's multi-page bio patches attempt to issue multi-page
+ writeouts (and reads) from the page cache, by directly building up
+ large bios for submission completely bypassing the usage of buffer
+ heads. This work is still in progress.
+
+ Christoph Hellwig had some code that uses bios for page-io (rather than
+ bh). This isn't included in bio as yet. Christoph was also working on a
+ design for representing virtual/real extents as an entity and modifying
+ some of the address space ops interfaces to utilize this abstraction rather
+ than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
+ abstraction, but intended to be as lightweight as possible).
+
+ (d) Direct access i/o:
+
+ Direct access requests that do not contain bios would be submitted differently
+ as discussed earlier in section 1.3.
+
+ Aside:
+
+ Kvec i/o:
+
+ Ben LaHaise's aio code uses a slightly different structure instead
+ of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
+ tuples (very much like the networking code), together with a callback function
+ and data pointer. This is embedded into a brw_cb structure when passed
+ to brw_kvec_async().
+
+ Now it should be possible to directly map these kvecs to a bio. Just as while
+ cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
+ array pointer to point to the veclet array in kvecs.
+
+ TBD: In order for this to work, some changes are needed in the way multi-page
+ bios are handled today. The values of the tuples in such a vector passed in
+ from higher level code should not be modified by the block layer in the course
+ of its request processing, since that would make it hard for the higher layer
+ to continue to use the vector descriptor (kvec) after i/o completes. Instead,
+ all such transient state should either be maintained in the request structure,
+ and passed on in some way to the endio completion routine.
+
+
+ 4. The I/O scheduler
+ ====================
+
+ I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch
+ queue and specific I/O schedulers. Unless stated otherwise, elevator is used
+ to refer to both parts and I/O scheduler to specific I/O schedulers.
+
+ Block layer implements generic dispatch queue in `block/*.c`.
+ The generic dispatch queue is responsible for requeueing, handling non-fs
+ requests and all other subtleties.
+
+ Specific I/O schedulers are responsible for ordering normal filesystem
+ requests. They can also choose to delay certain requests to improve
+ throughput or whatever purpose. As the plural form indicates, there are
+ multiple I/O schedulers. They can be built as modules but at least one should
+ be built inside the kernel. Each queue can choose different one and can also
+ change to another one dynamically.
+
+ A block layer call to the i/o scheduler follows the convention elv_xxx(). This
+ calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx
+ and xxx might not match exactly, but use your imagination. If an elevator
+ doesn't implement a function, the switch does nothing or some minimal house
+ keeping work.
+
+ 4.1. I/O scheduler API
+ ----------------------
+
+ The functions an elevator may implement are: (* are mandatory)
+
+ =============================== ================================================
+ elevator_merge_fn called to query requests for merge with a bio
+
+ elevator_merge_req_fn called when two requests get merged. the one
+ which gets merged into the other one will be
+ never seen by I/O scheduler again. IOW, after
+ being merged, the request is gone.
+
+ elevator_merged_fn called when a request in the scheduler has been
+ involved in a merge. It is used in the deadline
+ scheduler for example, to reposition the request
+ if its sorting order has changed.
+
+ elevator_allow_merge_fn called whenever the block layer determines
+ that a bio can be merged into an existing
+ request safely. The io scheduler may still
+ want to stop a merge at this point if it
+ results in some sort of conflict internally,
+ this hook allows it to do that. Note however
+ that two *requests* can still be merged at later
+ time. Currently the io scheduler has no way to
+ prevent that. It can only learn about the fact
+ from elevator_merge_req_fn callback.
+
+ elevator_dispatch_fn* fills the dispatch queue with ready requests.
+ I/O schedulers are free to postpone requests by
+ not filling the dispatch queue unless @force
+ is non-zero. Once dispatched, I/O schedulers
+ are not allowed to manipulate the requests -
+ they belong to generic dispatch queue.
+
+ elevator_add_req_fn* called to add a new request into the scheduler
+
+ elevator_former_req_fn
+ elevator_latter_req_fn These return the request before or after the
+ one specified in disk sort order. Used by the
+ block layer to find merge possibilities.
+
+ elevator_completed_req_fn called when a request is completed.
+
+ elevator_set_req_fn
+ elevator_put_req_fn Must be used to allocate and free any elevator
+ specific storage for a request.
+
+ elevator_activate_req_fn Called when device driver first sees a request.
+ I/O schedulers can use this callback to
+ determine when actual execution of a request
+ starts.
+ elevator_deactivate_req_fn Called when device driver decides to delay
+ a request by requeueing it.
+
+ elevator_init_fn*
+ elevator_exit_fn Allocate and free any elevator specific storage
+ for a queue.
+ =============================== ================================================
+
+ 4.2 Request flows seen by I/O schedulers
+ ----------------------------------------
+
+ All requests seen by I/O schedulers strictly follow one of the following three
+ flows.
+
+ set_req_fn ->
+
+ i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
+ (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
+ ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
+ iii. [none]
+
+ -> put_req_fn
+
+ 4.3 I/O scheduler implementation
+ --------------------------------
+
+ The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
+ optimal disk scan and request servicing performance (based on generic
+ principles and device capabilities), optimized for:
+
+ i. improved throughput
+ ii. improved latency
+ iii. better utilization of h/w & CPU time
+
+ Characteristics:
+
+ i. Binary tree
+ AS and deadline i/o schedulers use red black binary trees for disk position
+ sorting and searching, and a fifo linked list for time-based searching. This
+ gives good scalability and good availability of information. Requests are
+ almost always dispatched in disk sort order, so a cache is kept of the next
+ request in sort order to prevent binary tree lookups.
+
+ This arrangement is not a generic block layer characteristic however, so
+ elevators may implement queues as they please.
+
+ ii. Merge hash
+ AS and deadline use a hash table indexed by the last sector of a request. This
+ enables merging code to quickly look up "back merge" candidates, even when
+ multiple I/O streams are being performed at once on one disk.
+
+ "Front merges", a new request being merged at the front of an existing request,
+ are far less common than "back merges" due to the nature of most I/O patterns.
+ Front merges are handled by the binary trees in AS and deadline schedulers.
+
+ iii. Plugging the queue to batch requests in anticipation of opportunities for
+ merge/sort optimizations
+
+ Plugging is an approach that the current i/o scheduling algorithm resorts to so
+ that it collects up enough requests in the queue to be able to take
+ advantage of the sorting/merging logic in the elevator. If the
+ queue is empty when a request comes in, then it plugs the request queue
+ (sort of like plugging the bath tub of a vessel to get fluid to build up)
+ till it fills up with a few more requests, before starting to service
+ the requests. This provides an opportunity to merge/sort the requests before
+ passing them down to the device. There are various conditions when the queue is
+ unplugged (to open up the flow again), either through a scheduled task or
+ could be on demand. For example wait_on_buffer sets the unplugging going
+ through sync_buffer() running blk_run_address_space(mapping). Or the caller
+ can do it explicity through blk_unplug(bdev). So in the read case,
+ the queue gets explicitly unplugged as part of waiting for completion on that
+ buffer.
+
+ Aside:
+ This is kind of controversial territory, as it's not clear if plugging is
+ always the right thing to do. Devices typically have their own queues,
+ and allowing a big queue to build up in software, while letting the device be
+ idle for a while may not always make sense. The trick is to handle the fine
+ balance between when to plug and when to open up. Also now that we have
+ multi-page bios being queued in one shot, we may not need to wait to merge
+ a big request from the broken up pieces coming by.
+
+ 4.4 I/O contexts
+ ----------------
+
+ I/O contexts provide a dynamically allocated per process data area. They may
+ be used in I/O schedulers, and in the block layer (could be used for IO statis,
+ priorities for example). See `*io_context` in block/ll_rw_blk.c, and as-iosched.c
+ for an example of usage in an i/o scheduler.
+
+
+ 5. Scalability related changes
+ ==============================
+
+ 5.1 Granular Locking: io_request_lock replaced by a per-queue lock
+ ------------------------------------------------------------------
+
+ The global io_request_lock has been removed as of 2.5, to avoid
+ the scalability bottleneck it was causing, and has been replaced by more
+ granular locking. The request queue structure has a pointer to the
+ lock to be used for that queue. As a result, locking can now be
+ per-queue, with a provision for sharing a lock across queues if
+ necessary (e.g the scsi layer sets the queue lock pointers to the
+ corresponding adapter lock, which results in a per host locking
+ granularity). The locking semantics are the same, i.e. locking is
+ still imposed by the block layer, grabbing the lock before
+ request_fn execution which it means that lots of older drivers
+ should still be SMP safe. Drivers are free to drop the queue
+ lock themselves, if required. Drivers that explicitly used the
+ io_request_lock for serialization need to be modified accordingly.
+ Usually it's as easy as adding a global lock::
+
+ static DEFINE_SPINLOCK(my_driver_lock);
+
+ and passing the address to that lock to blk_init_queue().
+
+ 5.2 64 bit sector numbers (sector_t prepares for 64 bit support)
+ ----------------------------------------------------------------
+
+ The sector number used in the bio structure has been changed to sector_t,
+ which could be defined as 64 bit in preparation for 64 bit sector support.
+
+ 6. Other Changes/Implications
+ =============================
+
+ 6.1 Partition re-mapping handled by the generic block layer
+ -----------------------------------------------------------
+
+ In 2.5 some of the gendisk/partition related code has been reorganized.
+ Now the generic block layer performs partition-remapping early and thus
+ provides drivers with a sector number relative to whole device, rather than
+ having to take partition number into account in order to arrive at the true
+ sector number. The routine blk_partition_remap() is invoked by
+ generic_make_request even before invoking the queue specific make_request_fn,
+ so the i/o scheduler also gets to operate on whole disk sector numbers. This
+ should typically not require changes to block drivers, it just never gets
+ to invoke its own partition sector offset calculations since all bios
+ sent are offset from the beginning of the device.
+
+
+ 7. A Few Tips on Migration of older drivers
+ ===========================================
+
+ Old-style drivers that just use CURRENT and ignores clustered requests,
+ may not need much change. The generic layer will automatically handle
+ clustered requests, multi-page bios, etc for the driver.
+
+ For a low performance driver or hardware that is PIO driven or just doesn't
+ support scatter-gather changes should be minimal too.
+
+ The following are some points to keep in mind when converting old drivers
+ to bio.
+
+ Drivers should use elv_next_request to pick up requests and are no longer
+ supposed to handle looping directly over the request list.
+ (struct request->queue has been removed)
+
+ Now end_that_request_first takes an additional number_of_sectors argument.
+ It used to handle always just the first buffer_head in a request, now
+ it will loop and handle as many sectors (on a bio-segment granularity)
+ as specified.
+
+ Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
+ right thing to use is bio_endio(bio) instead.
+
+ If the driver is dropping the io_request_lock from its request_fn strategy,
+ then it just needs to replace that with q->queue_lock instead.
+
+ As described in Sec 1.1, drivers can set max sector size, max segment size
+ etc per queue now. Drivers that used to define their own merge functions i
+ to handle things like this can now just use the blk_queue_* functions at
+ blk_init_queue time.
+
+ Drivers no longer have to map a {partition, sector offset} into the
+ correct absolute location anymore, this is done by the block layer, so
+ where a driver received a request ala this before::
+
+ rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */
+ rq->sector = 0; /* first sector on hda5 */
+
+ it will now see::
+
+ rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */
+ rq->sector = 123128; /* offset from start of disk */
+
+ As mentioned, there is no virtual mapping of a bio. For DMA, this is
+ not a problem as the driver probably never will need a virtual mapping.
+ Instead it needs a bus mapping (dma_map_page for a single segment or
+ use dma_map_sg for scatter gather) to be able to ship it to the driver. For
+ PIO drivers (or drivers that need to revert to PIO transfer once in a
+ while (IDE for example)), where the CPU is doing the actual data
+ transfer a virtual mapping is needed. If the driver supports highmem I/O,
+ (Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map
+ a bio into the virtual address space.
+
+
+ 8. Prior/Related/Impacted patches
+ =================================
+
+ 8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
+ -----------------------------------------------------
+
+ - orig kiobuf & raw i/o patches (now in 2.4 tree)
+ - direct kiobuf based i/o to devices (no intermediate bh's)
+ - page i/o using kiobuf
+ - kiobuf splitting for lvm (mkp)
+ - elevator support for kiobuf request merging (axboe)
+
+ 8.2. Zero-copy networking (Dave Miller)
+ ---------------------------------------
+
+ 8.3. SGI XFS - pagebuf patches - use of kiobufs
+ -----------------------------------------------
+ 8.4. Multi-page pioent patch for bio (Christoph Hellwig)
+ --------------------------------------------------------
+ 8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
+ --------------------------------------------------------------------
+ 8.6. Async i/o implementation patch (Ben LaHaise)
+ -------------------------------------------------
+ 8.7. EVMS layering design (IBM EVMS team)
+ -----------------------------------------
+ 8.8. Larger page cache size patch (Ben LaHaise) and Large page size (Daniel Phillips)
+ -------------------------------------------------------------------------------------
+
+ => larger contiguous physical memory buffers
+
+ 8.9. VM reservations patch (Ben LaHaise)
+ ----------------------------------------
+ 8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
+ ----------------------------------------------------------
+ 8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
+ ---------------------------------------------------------------------------
+ 8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, Badari)
+ -------------------------------------------------------------------------------
+ 8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven)
+ ------------------------------------------------------------------
+ 8.14 IDE Taskfile i/o patch (Andre Hedrick)
+ --------------------------------------------
+ 8.15 Multi-page writeout and readahead patches (Andrew Morton)
+ ---------------------------------------------------------------
+ 8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
+ -----------------------------------------------------------------------
+
+ 9. Other References
+ ===================
+
+ 9.1 The Splice I/O Model
+ ------------------------
+
+ Larry McVoy (and subsequent discussions on lkml, and Linus' comments - Jan 2001
+
+ 9.2 Discussions about kiobuf and bh design
+ ------------------------------------------
+
+ On lkml between sct, linus, alan et al - Feb-March 2001 (many of the
+ initial thoughts that led to bio were brought up in this discussion thread)
+
+ 9.3 Discussions on mempool on lkml - Dec 2001.
+ ----------------------------------------------
projects / openwrt / staging / blogic.git / commitdiff