linux-kernel - Partial direct-io loop regression in 5.17-rc

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Date:   Fri, 4 Feb 2022 10:22:06 +0100
From:   Milan Broz <gmazyland@...il.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ondrej Kozina <okozina@...hat.com>
Subject: Partial direct-io loop regression in 5.17-rc

Hi Jens,

It seems that there is a regression in direct-io over loop for partial
direct-io reads (or perhaps even for other situations).

If I run this code (loop over 6M file, dd direct-io read with 4M blocks)

IMG=tst.img
LOOP=/dev/loop66

truncate -s 6M $IMG
losetup $LOOP $IMG
dd if=$LOOP of=/dev/null bs=4M iflag=direct
losetup -d $LOOP

on older kernel (<=5.16) it reads the whole file
   6291456 bytes (6.3 MB, 6.0 MiB) copied, 0.201591 s, 31.2 MB/s

while on 5.17-rc (tested on today/s Linus' git) it reads only the full blocks:
   4194304 bytes (4.2 MB, 4.0 MiB) copied, 0.201904 s, 20.8 MB/s

No error reported, exit code is 0.

I am not sure if the reproducer is intended behavior (there was
some discussion, though), but it is a major regression IMO.
If you do not want to support partial direct-io, it should return
an error then and not EOF (the last read returns 0)!

It was detected in our cryptsetup testsuite and we always use
direct aligned IO, but for the reported device sector size only (like 4k).
But here it breaks dd without any chance to detect error.

Bisect ends on your patch (and clean revert indeed fixes the issue):

ceaa762527f41a431b552bc000de4b626d2d8cb7 is the first bad commit
commit ceaa762527f41a431b552bc000de4b626d2d8cb7
Author: Jens Axboe <axboe@...nel.dk>
Date:   Thu Oct 28 08:57:09 2021 -0600

      block: move direct_IO into our own read_iter handler

      Don't call into generic_file_read_iter() if we know it's O_DIRECT, just
      set it up ourselves and call our own handler. This avoids an indirect call
      for O_DIRECT.

      Fall back to filemap_read() if we fail.

      Signed-off-by: Jens Axboe <axboe@...nel.dk>

   block/fops.c | 37 ++++++++++++++++++++++++++++++++-----
   1 file changed, 32 insertions(+), 5 deletions(-)

Please could you check what's wrong here?

Thanks,
Milan