[bug report][stable] xfstests:generic/538 failed on xfs

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Hi,
  I was using kernel v4.19.48 and found that it cannot pass the generic/538 on xfs. The error output is as follows:

  FSTYP         -- xfs (non-debug)
  PLATFORM      -- Linux/x86_64 alinux2-6 4.19.48
  MKFS_OPTIONS  -- -f -bsize=4096 /dev/vdc
  MOUNT_OPTIONS -- /dev/vdc /mnt/testarea/scra
  generic/538 0s ... - output mismatch (see /root/usr/local/src/xfstests/results//generic/538.out.bad)
      --- tests/generic/538.out   2019-05-27 13:57:06.505666465 +0800
      +++ /root/usr/local/src/xfstests/results//generic/538.out.bad       2019-06-05 16:43:14.702002326 +0800
      @@ -1,2 +1,10 @@
       QA output created by 538
      +Data verification fails
      +Find corruption
      +00000000  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
      +*
      +00000200  5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
      +00002000
      ...
      (Run 'diff -u /root/usr/local/src/xfstests/tests/generic/538.out /root/usr/local/src/xfstests/results//generic/538.out.bad'  to see the entire diff)
  Ran: generic/538
  Failures: generic/538
  Failed 1 of 1 tests
  
I also found that the latest kernel (v5.2.0-rc2) of upstream can pass the generic/538 test. Therefore, I bisected and found the first good commit is 3110fc79606. This commit adds the hardware queue into the sort function. Besides, the sort function returns a negative value when the offset and queue (software and hardware) of two I/O requests are same. I think the second part of the change make senses. The kernel should not change the relative position of two I/O requests when their offset and queue are same. So I made the following changes and merged it into the kernel 4.19.48. After the modification, we can pass the generic/538 test on xfs. The same case can be passed on ext4, since ext4 has corresponding fix 0db24122bd7f ("ext4: fix data corruption caused by overlapping unaligned and aligned IO"). Though I think xfs should be responsible for this issue, the block layer code below is also problematic. Any ideas?

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4e563ee..a7309cd 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1610,7 +1610,7 @@ static int plug_ctx_cmp(void *priv, struct list_head *a, struct list_head *b)

        return !(rqa->mq_ctx < rqb->mq_ctx ||
                 (rqa->mq_ctx == rqb->mq_ctx &&
-                 blk_rq_pos(rqa) < blk_rq_pos(rqb)));
+                 blk_rq_pos(rqa) <= blk_rq_pos(rqb)));
 }

 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)

Best regards,
Alvin




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