Re: [PATCH V6 05/13] xfs: Check for extent overflow when growing realtime bitmap/summary inodes

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Mar 23, 2021 at 09:21:27PM +0530, Chandan Babu R wrote:
> On 22 Mar 2021 at 23:26, Darrick J. Wong wrote:
> > On Tue, Mar 09, 2021 at 10:31:16AM +0530, Chandan Babu R wrote:
> >> Verify that XFS does not cause realtime bitmap/summary inode fork's
> >> extent count to overflow when growing the realtime volume associated
> >> with a filesystem.
> >>
> >> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> >> Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
> >
> > Soo... I discovered that this test doesn't pass with multiblock
> > directories:
> 
> Thanks for the bug report and the description of the corresponding solution. I
> am fixing the tests and will soon post corresponding patches to the mailing
> list.

Also, I found a problem with xfs/534 when it does the direct write tests
to a pmem volume with DAX enabled:

--- /tmp/fstests/tests/xfs/534.out      2021-03-21 11:44:09.384407426 -0700
+++ /var/tmp/fstests/xfs/534.out.bad    2021-03-23 13:32:15.898301839 -0700
@@ -5,7 +5,4 @@
 Fallocate 15 blocks
 Buffered write to every other block of fallocated space
 Verify $testfile's extent count
-* Direct write to unwritten extent
-Fallocate 15 blocks
-Direct write to every other block of fallocated space
-Verify $testfile's extent count
+Extent count overflow check failed: nextents = 11

looking at the xfs_bmap output for $testfile shows:

/opt/testfile:
 EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL FLAGS
   0: [0..7]:          208..215          0 (208..215)           8 010000
   1: [8..15]:         216..223          0 (216..223)           8 000000
   2: [16..23]:        224..231          0 (224..231)           8 010000
   3: [24..31]:        232..239          0 (232..239)           8 000000
   4: [32..39]:        240..247          0 (240..247)           8 010000
   5: [40..47]:        248..255          0 (248..255)           8 000000
   6: [48..55]:        256..263          0 (256..263)           8 010000
   7: [56..63]:        264..271          0 (264..271)           8 000000
   8: [64..71]:        272..279          0 (272..279)           8 010000
   9: [72..79]:        280..287          0 (280..287)           8 000000
  10: [80..119]:       288..327          0 (288..327)          40 010000

Which is ... odd since the same direct write gets cut off after writing
to block 7 (like you'd expect since it's the same function) when DAX
isn't enabled...

...OH, I see the problem.  For a non-DAX direct write,
xfs_iomap_write_direct will allocate an unwritten block into a hole, but
if the block was already mapped (written or unwritten) it won't do
anything at all.  For that case, XFS_IEXT_ADD_NOSPLIT_CNT is sufficient,
because in the worst case we add one extent to the data fork.

For DAX writes, however, the behavior is different:

	if (IS_DAX(VFS_I(ip))) {
		bmapi_flags = XFS_BMAPI_CONVERT | XFS_BMAPI_ZERO;
		if (imap->br_state == XFS_EXT_UNWRITTEN) {
			force = true;
			dblocks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;
		}
	}

This tells xfs_bmapi_write that we want to /convert/ an unwritten extent
to written, and we want to zero the blocks.  If we're dax-writing into
the middle of an unwritten range, this will cause a split.  The correct
parameter there would be XFS_IEXT_WRITE_UNWRITTEN_CNT.  Would you mind
sending a kernel patch to fix that?

--D

> --
> chandan