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

[Kevin Wolf <kwolf@redhat.com>]

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.

Kevin