Re: [PATCH 13/16] xfs: add support for lazy direct read bounce buffering

[Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>]

To+: 0wnerD1ed <l7z@0b1t.tech>

On Sep 09, 2026 / 09:09, Christoph Hellwig wrote:
> Currently direct I/O reads always bounce buffer the I/O to deal with
> the case where userspace is modifying the buffer in-flight while
> reading data into it.
> 
> This is a very expensive countermeasure for something no sane application
> should do, so try to avoid it by reading without a bounce buffer first,
> and retrying the read on a checksum failure.  This avoids the cost of
> bounce buffering for sanely behave applications.  For the rare case of
> an application regularly modifying in-flight buffers, allow forcing the
> always bounce buffer behavior through sysfs.  And now that we have that
> knob, allow disabling read-side bounce buffering entirely for those who
> live fast and dangerous.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

I found that recent blktests CI trial runs started reporting the failure of
the test case block/046 [1]. I confirmed that it is observed with the kernel at
the linux-block for-next branch. I bisected and found the this commit triggers
the failure. The error message "preadv returned -1 (errno 22)" was printed at
src/bio-bounce-read.c line 121 [2].

AFAIU, the test case does direct read to a null_blk with bounce buffer, and the
kernel side introduced some new requirements to use bounce buffer for direct
I/Os. Then, I guess the test case requires some refinements to meet the new
requirements.

0wnerD1ed, may I ask your help to fix this failure? At this moment, I don't have
much time to dig into this.


[1] failure console log

block/046 (test partial bounced direct read bvec alignment)  [failed]
    runtime  1.174s  ...  1.199s
    --- tests/block/046.out     2026-08-03 19:26:40.410053604 +0900
    +++ /home/shin/Blktests/blktests/results/nodev/block/046.out.bad    2026-09-20 20:14:33.795794868 +0900
    @@ -1,2 +1,4 @@
     Running block/046
    +bio-bounce-read: preadv returned -1 (errno 22)
    +bio-bounce-read helper failed
     Test complete

[2] https://github.com/linux-blktests/blktests/blob/f0603f960dcc2fc215161545b4b94633222a35ac/src/bio-bounce-read.c#L121