[PATCH 5.7 048/112] btrfs: fix RWF_NOWAIT writes blocking on extent locks and waiting for IO

Greg Kroah-Hartman
2020-07-07T15:24:24Z

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
--
2.25.1