[PATCH] fuse: invalidate the correct range after O_APPEND direct write

[Baokun Li <libaokun@linux.alibaba.com>]

fuse_direct_write_iter() captures pos before generic_write_checks(),
which moves ki_pos to EOF for O_APPEND writes:

  fuse_direct_write_iter()
  {
      pos = iocb->ki_pos;           /* 0 (user-supplied)       */
      generic_write_checks();       /* ki_pos -> EOF           */
      fuse_direct_io();             /* writes at EOF, correct  */
      invalidate(pos, pos + res);   /* [0, res) -- wrong       */
  }

The post-write invalidation targets a stale range instead of the
actual written range at EOF.

This can cause data inconsistency when the file size is not
page-aligned.  The tail page straddling EOF has a valid portion
before EOF that concurrent readers can fault back in during the
DIO write window:

  Tail page (file size X not page-aligned):

    page_start         X (EOF)   page_end
    |--- valid data ----|-- stale --|

  CPU0 (O_APPEND DIO writer)    CPU1 (buffered reader)
  --------------------------    ----------------------
  invalidate [X, X+len)
    tail page evicted
  FUSE_WRITE in flight ...
                                read [page_start, X)
                                  tail page re-faulted
                                  [X, page_end) = stale
  FUSE_WRITE completes
  i_size = X + len
  invalidate [0, len)  <- WRONG
    tail page still cached
                                read [X, X+len)
                                  hits stale tail page
                                  returns old data

Fix by reading pos back from iocb->ki_pos after generic_write_checks(),
as generic_file_direct_write() does.

Also fix a typo in the comment ("may have" -> "may have competed").

Fixes: 2b0408d0284f ("fuse: invalidate page cache after DIO and async DIO writes")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fuse/file.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 45ebd1b15874..d12a9fdf770e 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1789,13 +1789,14 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
 {
 	struct inode *inode = file_inode(iocb->ki_filp);
 	struct address_space *mapping = inode->i_mapping;
-	loff_t pos = iocb->ki_pos;
 	ssize_t res;
 	bool exclusive;
 
 	fuse_dio_lock(iocb, from, &exclusive);
 	res = generic_write_checks(iocb, from);
 	if (res > 0) {
+		loff_t pos = iocb->ki_pos;
+
 		task_io_account_write(res);
 		if (!is_sync_kiocb(iocb)) {
 			res = fuse_direct_IO(iocb, from);
@@ -1810,7 +1811,7 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
 			/*
 			 * As in generic_file_direct_write(), invalidate after
 			 * write, to invalidate read-ahead cache that may have
-			 * with the write.
+			 * competed with the write.
 			 */
 			invalidate_inode_pages2_range(mapping,
 				pos >> PAGE_SHIFT,
-- 
2.43.7