[PATCH AUTOSEL 4.19 542/671] btrfs: use correct count in btrfs_file_write_iter()

Sasha Levin
2020-01-16T18:41:57Z

From: Omar Sandoval <osandov@fb.com>

[ Upstream commit c09767a8960ca0500fb636bf73686723337debf4 ]

generic_write_checks() may modify iov_iter_count(), so we must get the
count after the call, not before. Using the wrong one has a couple of
consequences:

1. We check a longer range in check_can_nocow() for nowait than we're
actually writing.
2. We create extra hole extent maps in btrfs_cont_expand(). As far as I
can tell, this is harmless, but I might be missing something.

These issues are pretty minor, but let's fix it before something more
important trips on it.

Fixes: edf064e7c6fe ("btrfs: nowait aio support")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/file.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index d9d90f0b66d2..f4a7dac76356 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1895,7 +1895,7 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
bool sync = (file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host);
ssize_t err;
loff_t pos;
- size_t count = iov_iter_count(from);
+ size_t count;
loff_t oldsize;
int clean_page = 0;

@@ -1916,6 +1916,7 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
}

pos = iocb->ki_pos;
+ count = iov_iter_count(from);
if (iocb->ki_flags & IOCB_NOWAIT) {
/*
* We will allocate space in case nodatacow is not set,
--
2.20.1