Re: [PATCH v3 2/2] block/io: skip head/tail requests on EINVAL

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Apr 17, 2025 at 10:49:55AM +0200, Kevin Wolf wrote:
> Am 14.04.2025 um 22:12 hat Stefan Hajnoczi geschrieben:
> > When guests send misaligned discard requests, the block layer breaks
> > them up into a misaligned head, an aligned main body, and a misaligned
> > tail.
> > 
> > The file-posix block driver on Linux returns -EINVAL on misaligned
> > discard requests. This causes bdrv_co_pdiscard() to fail and guests
> > configured with werror=stop will pause.
> > 
> > Add a special case for misaligned head/tail requests. Simply continue
> > when EINVAL is encountered so that the aligned main body of the request
> > can be completed and the guest is not paused. This is the best we can do
> > when guest discard limits do not match the host discard limits.
> > 
> > Fixes: https://issues.redhat.com/browse/RHEL-86032
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
> 
> It would be good to also update the comment a bit further up:
> 
>     /* Discard is advisory, but some devices track and coalesce
>      * unaligned requests, so we must pass everything down rather than
>      * round here.  Still, most devices will just silently ignore
>      * unaligned requests (by returning -ENOTSUP), so we must fragment
>      * the request accordingly.  */
> 
> I'm not sure where the -ENOTSUP came from (Eric, do you remember?), but
> we should at least mention this -EINVAL case separately.

Sounds good.

Stefan

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