From 135da9766eea6134229146f6f67fdcbba41120e1 Mon Sep 17 00:00:00 2001 From: Qu Wenruo Date: Fri, 19 Jul 2019 14:51:42 +0800 Subject: [PATCH] btrfs: volumes: Add comment for find_free_dev_extent_start() Since commit 6df9a95e6339 ("Btrfs: make the chunk allocator completely tree lockless") we search commit root of device tree to avoid deadlock. This introduced a safety feature, find_free_dev_extent_start() won't use dev extents which just get freed in current transaction. This safety feature makes sure we won't allocate new block group using just freed dev extents to break CoW. However, this feature also makes find_free_dev_extent_start() not reliable reporting free device space. Just add such comment to make later viewer careful about this behavior. This behavior makes one caller, btrfs_can_relocate() unreliable determining the device free space. Reviewed-by: Johannes Thumshirn Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/volumes.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 8d64279d117c..ac16734c0f44 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -1551,6 +1551,12 @@ static bool contains_pending_extent(struct btrfs_device *device, u64 *start, * @len is used to store the size of the free space that we find. * But if we don't find suitable free space, it is used to store the size of * the max free space. + * + * NOTE: This function will search *commit* root of device tree, and does extra + * check to ensure dev extents are not double allocated. + * This makes the function safe to allocate dev extents but may not report + * correct usable device space, as device extent freed in current transaction + * is not reported as avaiable. */ static int find_free_dev_extent_start(struct btrfs_device *device, u64 num_bytes, u64 search_start, u64 *start, -- 2.30.2