Commit 5dbb75ed authored by Filipe Manana's avatar Filipe Manana Committed by David Sterba
Browse files

btrfs: fix RWF_NOWAIT writes blocking on extent locks and waiting for IO 



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: edf064e7 ("btrfs: nowait aio support")
Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 260a6339

fs/btrfs/file.c

+26 −11
Original line number Diff line number Diff line
@@ -1533,7 +1533,7 @@ 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;
@@ -1541,27 +1541,43 @@ 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;



	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;

		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;
@@ -1633,7 +1649,7 @@ 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.
@@ -1912,12 +1928,11 @@ 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

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