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

Darrick J. Wong
2021-03-23T20:58:44Z

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