btrfs: use correct count in btrfs_file_write_iter()

BugLink: https://bugs.launchpad.net/bugs/1863019
[ 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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 8abb00d215627be3d22125f45a2f2f7096ecb743..73c1647680fef437c5c6fbd3c292728474c903f1 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1912,7 +1912,7 @@ 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;
 
@@ -1934,6 +1934,7 @@ 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,