block: use bio_add_page in bio_iov_iter_get_pages

Replace a nasty hack with a different nasty hack to prepare for multipage
bio_vecs.  By moving the temporary page array as far up as possible in
the space allocated for the bio_vec array we can iterate forward over it
and thus use bio_add_page.  Using bio_add_page means we'll be able to
merge physically contiguous pages once support for multipath bio_vecs is
merged.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bio.c b/block/bio.c
index f685e762809ddf5d25539d94e7500d301fcd2384..81c05ee51d6ced51ac08a6a37f7727d82eedfde0 100644 (file)
--- a/block/bio.c
+++ b/block/bio.c
@@ -827,6 +827,8 @@ int bio_add_page(struct bio *bio, struct page *page,
 }
 EXPORT_SYMBOL(bio_add_page);
 
+#define PAGE_PTRS_PER_BVEC     (sizeof(struct bio_vec) / sizeof(struct page *))
+
 /**
  * __bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
  * @bio: bio to add pages to
@@ -839,38 +841,35 @@ EXPORT_SYMBOL(bio_add_page);
  */
 static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {
-       unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt, idx;
+       unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
+       unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
        struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
        struct page **pages = (struct page **)bv;
+       ssize_t size, left;
+       unsigned len, i;
        size_t offset;
-       ssize_t size;
+
+       /*
+        * Move page array up in the allocated memory for the bio vecs as far as
+        * possible so that we can start filling biovecs from the beginning
+        * without overwriting the temporary page array.
+       */
+       BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
+       pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
 
        size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
        if (unlikely(size <= 0))
                return size ? size : -EFAULT;
-       idx = nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
 
-       /*
-        * Deep magic below:  We need to walk the pinned pages backwards
-        * because we are abusing the space allocated for the bio_vecs
-        * for the page array.  Because the bio_vecs are larger than the
-        * page pointers by definition this will always work.  But it also
-        * means we can't use bio_add_page, so any changes to it's semantics
-        * need to be reflected here as well.
-        */
-       bio->bi_iter.bi_size += size;
-       bio->bi_vcnt += nr_pages;
+       for (left = size, i = 0; left > 0; left -= len, i++) {
+               struct page *page = pages[i];
 
-       while (idx--) {
-               bv[idx].bv_page = pages[idx];
-               bv[idx].bv_len = PAGE_SIZE;
-               bv[idx].bv_offset = 0;
+               len = min_t(size_t, PAGE_SIZE - offset, left);
+               if (WARN_ON_ONCE(bio_add_page(bio, page, len, offset) != len))
+                       return -EINVAL;
+               offset = 0;
        }
 
-       bv[0].bv_offset += offset;
-       bv[0].bv_len -= offset;
-       bv[nr_pages - 1].bv_len -= nr_pages * PAGE_SIZE - offset - size;
-
        iov_iter_advance(iter, size);
        return 0;
 }