[PATCH v2] xfs: truncate pagecache before writeback in xfs_setattr_size()

[Eryu Guan <eguan@redhat.com>]

On truncate down, if new size is not block size aligned, we zero the
rest of block to avoid exposing stale data to user, and
iomap_truncate_page() skips zeroing if the range is already in
unwritten state or a hole. Then we writeback from on-disk i_size to
the new size if this range hasn't been written to disk yet, and
truncate page cache beyond new EOF and set in-core i_size.

The problem is that we could write data between di_size and newsize
before removing the page cache beyond newsize, as the extents may
still be in unwritten state right after a buffer write. As such, the
page of data that newsize lies in has not been zeroed by page cache
invalidation before it is written, and xfs_do_writepage() hasn't
triggered it's "zero data beyond EOF" case because we haven't
updated in-core i_size yet. Then a subsequent mmap read could see
non-zeros past EOF.

I occasionally see this in fsx runs in fstests generic/112, a
simplified fsx operation sequence is like (assuming 4k block size
xfs):

  fallocate 0x0 0x1000 0x0 keep_size
  write 0x0 0x1000 0x0
  truncate 0x0 0x800 0x1000
  punch_hole 0x0 0x800 0x800
  mapread 0x0 0x800 0x800

where fallocate allocates unwritten extent but doesn't update
i_size, buffer write populates the page cache and extent is still
unwritten, truncate skips zeroing page past new EOF and writes the
page to disk, punch_hole invalidates the page cache, at last mapread
reads the block back and sees non-zero beyond EOF.

Fix it by moving truncate_setsize() to before writeback so the page
cache invalidation zeros the partial page at the new EOF. This also
triggers "zero data beyond EOF" in xfs_do_writepage() at writeback
time, because newsize has been set and page straddles the newsize.

Also fixed the wrong 'end' param of filemap_write_and_wait_range()
call while we're at it, the 'end' is inclusive and should be
'newsize - 1'.

Suggested-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
---

v2:
- fix the bug by moving truncate_setsize() before writeback as suggested
  by Dave
- update summary and commit log accordingly, with some words stolen from
  Dave :)

v1: https://www.spinics.net/lists/linux-xfs/msg12321.html

 fs/xfs/xfs_iops.c | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 17081c77ef86..f24e5b6cfc86 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -886,22 +886,6 @@ xfs_setattr_size(
 		return error;
 
 	/*
-	 * We are going to log the inode size change in this transaction so
-	 * any previous writes that are beyond the on disk EOF and the new
-	 * EOF that have not been written out need to be written here.  If we
-	 * do not write the data out, we expose ourselves to the null files
-	 * problem. Note that this includes any block zeroing we did above;
-	 * otherwise those blocks may not be zeroed after a crash.
-	 */
-	if (did_zeroing ||
-	    (newsize > ip->i_d.di_size && oldsize != ip->i_d.di_size)) {
-		error = filemap_write_and_wait_range(VFS_I(ip)->i_mapping,
-						      ip->i_d.di_size, newsize);
-		if (error)
-			return error;
-	}
-
-	/*
 	 * We've already locked out new page faults, so now we can safely remove
 	 * pages from the page cache knowing they won't get refaulted until we
 	 * drop the XFS_MMAP_EXCL lock after the extent manipulations are
@@ -917,9 +901,29 @@ xfs_setattr_size(
 	 * user visible changes). There's not much we can do about this, except
 	 * to hope that the caller sees ENOMEM and retries the truncate
 	 * operation.
+	 *
+	 * And we update in-core i_size and truncate page cache beyond newsize
+	 * before writeback the [di_size, newsize] range, so we're guaranteed
+	 * not to write stale data past the new EOF on truncate down.
 	 */
 	truncate_setsize(inode, newsize);
 
+	/*
+	 * We are going to log the inode size change in this transaction so
+	 * any previous writes that are beyond the on disk EOF and the new
+	 * EOF that have not been written out need to be written here.  If we
+	 * do not write the data out, we expose ourselves to the null files
+	 * problem. Note that this includes any block zeroing we did above;
+	 * otherwise those blocks may not be zeroed after a crash.
+	 */
+	if (did_zeroing ||
+	    (newsize > ip->i_d.di_size && oldsize != ip->i_d.di_size)) {
+		error = filemap_write_and_wait_range(VFS_I(ip)->i_mapping,
+						ip->i_d.di_size, newsize - 1);
+		if (error)
+			return error;
+	}
+
 	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_itruncate, 0, 0, 0, &tp);
 	if (error)
 		return error;
-- 
2.13.6