From: Filipe Manana fdmanana@suse.com
[ Upstream commit 5dbb75ed6900048e146247b6325742d92c892548 ]
A RWF_NOWAIT write is not supposed to wait on filesystem locks that can be held for a long time or for ongoing IO to complete.
However when calling check_can_nocow(), if the inode has prealloc extents or has the NOCOW flag set, we can block on extent (file range) locks through the call to btrfs_lock_and_flush_ordered_range(). Such lock can take a significant amount of time to be available. For example, a fiemap task may be running, and iterating through the entire file range checking all extents and doing backref walking to determine if they are shared, or a readpage operation may be in progress.
Also at btrfs_lock_and_flush_ordered_range(), called by check_can_nocow(), after locking the file range we wait for any existing ordered extent that is in progress to complete. Another operation that can take a significant amount of time and defeat the purpose of RWF_NOWAIT.
So fix this by trying to lock the file range and if it's currently locked return -EAGAIN to user space. If we are able to lock the file range without waiting and there is an ordered extent in the range, return -EAGAIN as well, instead of waiting for it to complete. Finally, don't bother trying to lock the snapshot lock of the root when attempting a RWF_NOWAIT write, as that is only important for buffered writes.
Fixes: edf064e7c6fec3 ("btrfs: nowait aio support") Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- fs/btrfs/file.c | 37 ++++++++++++++++++++++++++----------- 1 file changed, 26 insertions(+), 11 deletions(-)
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 52d565ff66e2d..93244934d4f92 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -1541,7 +1541,7 @@ lock_and_cleanup_extent_if_need(struct btrfs_inode *inode, struct page **pages, }
static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos, - size_t *write_bytes) + size_t *write_bytes, bool nowait) { struct btrfs_fs_info *fs_info = inode->root->fs_info; struct btrfs_root *root = inode->root; @@ -1549,27 +1549,43 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos, u64 num_bytes; int ret;
- if (!btrfs_drew_try_write_lock(&root->snapshot_lock)) + if (!nowait && !btrfs_drew_try_write_lock(&root->snapshot_lock)) return -EAGAIN;
lockstart = round_down(pos, fs_info->sectorsize); lockend = round_up(pos + *write_bytes, fs_info->sectorsize) - 1; + num_bytes = lockend - lockstart + 1;
- btrfs_lock_and_flush_ordered_range(inode, lockstart, - lockend, NULL); + if (nowait) { + struct btrfs_ordered_extent *ordered; + + if (!try_lock_extent(&inode->io_tree, lockstart, lockend)) + return -EAGAIN; + + ordered = btrfs_lookup_ordered_range(inode, lockstart, + num_bytes); + if (ordered) { + btrfs_put_ordered_extent(ordered); + ret = -EAGAIN; + goto out_unlock; + } + } else { + btrfs_lock_and_flush_ordered_range(inode, lockstart, + lockend, NULL); + }
- num_bytes = lockend - lockstart + 1; ret = can_nocow_extent(&inode->vfs_inode, lockstart, &num_bytes, NULL, NULL, NULL); if (ret <= 0) { ret = 0; - btrfs_drew_write_unlock(&root->snapshot_lock); + if (!nowait) + btrfs_drew_write_unlock(&root->snapshot_lock); } else { *write_bytes = min_t(size_t, *write_bytes , num_bytes - pos + lockstart); } - +out_unlock: unlock_extent(&inode->io_tree, lockstart, lockend);
return ret; @@ -1641,7 +1657,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb, if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW | BTRFS_INODE_PREALLOC)) && check_can_nocow(BTRFS_I(inode), pos, - &write_bytes) > 0) { + &write_bytes, false) > 0) { /* * For nodata cow case, no need to reserve * data space. @@ -1920,12 +1936,11 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb, */ if (!(BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW | BTRFS_INODE_PREALLOC)) || - check_can_nocow(BTRFS_I(inode), pos, &nocow_bytes) <= 0) { + check_can_nocow(BTRFS_I(inode), pos, &nocow_bytes, + true) <= 0) { inode_unlock(inode); return -EAGAIN; } - /* check_can_nocow() locks the snapshot lock on success */ - btrfs_drew_write_unlock(&root->snapshot_lock); /* * There are holes in the range or parts of the range that must * be COWed (shared extents, RO block groups, etc), so just bail