On Wed, Nov 25, 2020 at 10:35:50AM -0500, Sasha Levin wrote:
From: Dave Chinner dchinner@redhat.com
[ Upstream commit 883a790a84401f6f55992887fd7263d808d4d05d ]
Jens has reported a situation where partial direct IOs can be issued and completed yet still return -EAGAIN. We don't want this to report a short IO as we want XFS to complete user DIO entirely or not at all.
This partial IO situation can occur on a write IO that is split across an allocated extent and a hole, and the second mapping is returning EAGAIN because allocation would be required.
The trivial reproducer:
$ sudo xfs_io -fdt -c "pwrite 0 4k" -c "pwrite -V 1 -b 8k -N 0 8k" /mnt/scr/foo wrote 4096/4096 bytes at offset 0 4 KiB, 1 ops; 0.0001 sec (27.509 MiB/sec and 7042.2535 ops/sec) pwrite: Resource temporarily unavailable $
The pwritev2(0, 8kB, RWF_NOWAIT) call returns EAGAIN having done the first 4kB write:
xfs_file_direct_write: dev 259:1 ino 0x83 size 0x1000 offset 0x0 count 0x2000 iomap_apply: dev 259:1 ino 0x83 pos 0 length 8192 flags WRITE|DIRECT|NOWAIT (0x31) ops xfs_direct_write_iomap_ops caller iomap_dio_rw actor iomap_dio_actor xfs_ilock_nowait: dev 259:1 ino 0x83 flags ILOCK_SHARED caller xfs_ilock_for_iomap xfs_iunlock: dev 259:1 ino 0x83 flags ILOCK_SHARED caller xfs_direct_write_iomap_begin xfs_iomap_found: dev 259:1 ino 0x83 size 0x1000 offset 0x0 count 8192 fork data startoff 0x0 startblock 24 blockcount 0x1 iomap_apply_dstmap: dev 259:1 ino 0x83 bdev 259:1 addr 102400 offset 0 length 4096 type MAPPED flags DIRTY
Here the first iomap loop has mapped the first 4kB of the file and issued the IO, and we enter the second iomap_apply loop:
iomap_apply: dev 259:1 ino 0x83 pos 4096 length 4096 flags WRITE|DIRECT|NOWAIT (0x31) ops xfs_direct_write_iomap_ops caller iomap_dio_rw actor iomap_dio_actor xfs_ilock_nowait: dev 259:1 ino 0x83 flags ILOCK_SHARED caller xfs_ilock_for_iomap xfs_iunlock: dev 259:1 ino 0x83 flags ILOCK_SHARED caller xfs_direct_write_iomap_begin
And we exit with -EAGAIN out because we hit the allocate case trying to make the second 4kB block.
Then IO completes on the first 4kB and the original IO context completes and unlocks the inode, returning -EAGAIN to userspace:
xfs_end_io_direct_write: dev 259:1 ino 0x83 isize 0x1000 disize 0x1000 offset 0x0 count 4096 xfs_iunlock: dev 259:1 ino 0x83 flags IOLOCK_SHARED caller xfs_file_dio_aio_write
There are other vectors to the same problem when we re-enter the mapping code if we have to make multiple mappinfs under NOWAIT conditions. e.g. failing trylocks, COW extents being found, allocation being required, and so on.
Avoid all these potential problems by only allowing IOMAP_NOWAIT IO to go ahead if the mapping we retrieve for the IO spans an entire allocated extent. This avoids the possibility of subsequent mappings to complete the IO from triggering NOWAIT semantics by any means as NOWAIT IO will now only enter the mapping code once per NOWAIT IO.
Reported-and-tested-by: Jens Axboe axboe@kernel.dk Signed-off-by: Dave Chinner dchinner@redhat.com Reviewed-by: Darrick J. Wong darrick.wong@oracle.com Signed-off-by: Darrick J. Wong darrick.wong@oracle.com Signed-off-by: Sasha Levin sashal@kernel.org
fs/xfs/xfs_iomap.c | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+)
No, please don't pick this up for stable kernels until at least 5.10 is released. This still needs some time integration testing time to ensure that we haven't introduced any other performance regressions with this change.
We've already had one XFS upstream kernel regression in this -rc cycle propagated to the stable kernels in 5.9.9 because the stable process picked up a bunch of random XFS fixes within hours of them being merged by Linus. One of those commits was a result of a thinko, and despite the fact we found it and reverted it within a few days, users of stable kernels have been exposed to it for a couple of weeks. That *should never have happened*.
This has happened before, and *again* we were lucky this wasn't worse than it was. We were saved by the flaw being caught by own internal pre-write corruption verifiers (which exist because we don't trust our code to be bug-free, let alone the collections of random, poorly tested backports) so that it only resulted in corruption shutdowns rather than permanent on-disk damage and data loss.
Put simply: the stable process is flawed because it shortcuts the necessary stabilisation testing for new code. It doesn't matter if the merged commits have a "fixes" tag in them, that tag doesn't mean the change is ready to be exposed to production systems. We need the *-rc stabilisation process* to weed out thinkos, brown paper bag bugs, etc, because we all make mistakes, and bugs in filesystem code can *lose user data permanently*.
Hence I ask that the stable maintainers only do automated pulls of iomap and XFS changes from upstream kernels when Linus officially releases them rather than at random points in time in the -rc cycle. If there is a critical fix we need to go back to stable kernels immediately, we will let stable@kernel.org know directly that we want this done.
Cheers,
Dave.