Re: [PATCH 1/3] file-posix: populate pwrite_zeroes_alignment

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Oct 03, 2025 at 11:04:38AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 02.10.25 21:39, Stefan Hajnoczi wrote:
> > Linux block devices require write zeroes alignment whereas files do not.
> > 
> > It may come as a surprise that block devices opened in buffered I/O mode
> > require the alignment although regular read/write requests do not.
> > 
> > Therefore it is necessary to populate the pwrite_zeroes_alignment field.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >   block/file-posix.c | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> > 
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index 8c738674ce..05c92c824d 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -1602,6 +1602,23 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
> >               bs->bl.pdiscard_alignment = dalign;
> >           }
> > +
> > +#ifdef __linux__
> > +        /*
> > +         * When request_alignment > 1, pwrite_zeroes_alignment does not need to
> > +         * be set explicitly. When request_alignment == 1, it must be set
> > +         * explicitly because Linux requires logical block size alignment.
> > +         */
> > +        if (bs->bl.request_alignment == 1) {
> 
> would "if (!s->needs_alignment) {" be a more visual check? This way reader will not have to analyze
> raw_probe_alignment, and understand that needs_alignment=false is the only path for block device
> to have request_alignment==1.

Yes, that would make it easier to understand. Thanks!

> 
> > +            ret = probe_logical_blocksize(s->fd,
> > +                                          &bs->bl.pwrite_zeroes_alignment);
> > +            if (ret < 0) {
> > +                error_setg_errno(errp, -ret,
> > +                                 "Failed to probe logical block size");
> > +                return;
> > +            }
> > +        }
> > +#endif /* __linux__ */
> >       }
> >       raw_refresh_zoned_limits(bs, &st, errp);
> 
> 
> -- 
> Best regards,
> Vladimir
> 

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