Re: [PATCH v2 3/6] block: Clarify that @bytes is no limit on *pnum

[Vladimir Sementsov-Ogievskiy]

24.06.2021 13:16, Max Reitz wrote:
> On 24.06.21 11:15, Vladimir Sementsov-Ogievskiy wrote:
> > 23.06.2021 18:01, Max Reitz wrote:
> > > .bdrv_co_block_status() implementations are free to return a *pnum that
> > > exceeds @bytes, because bdrv_co_block_status() in block/io.c will clamp
> > > *pnum as necessary.
> > >
> > > On the other hand, if drivers' implementations return values for *pnum
> > > that are as large as possible, our recently introduced block-status
> > > cache will become more effective.
> > >
> > > So, make a note in block_int.h that @bytes is no upper limit for *pnum.
> > >
> > > Suggested-by: Eric Blake <eblake@redhat.com>
> > > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > > ---
> > >   include/block/block_int.h | 5 +++++
> > >   1 file changed, 5 insertions(+)
> > >
> > > diff --git a/include/block/block_int.h b/include/block/block_int.h
> > > index fcb599dd1c..f85b96ed99 100644
> > > --- a/include/block/block_int.h
> > > +++ b/include/block/block_int.h
> > > @@ -347,6 +347,11 @@ struct BlockDriver {
> > >        * clamped to bdrv_getlength() and aligned to request_alignment,
> > >        * as well as non-NULL pnum, map, and file; in turn, the driver
> > >        * must return an error or set pnum to an aligned non-zero value.
> > > +     *
> > > +     * Note that @bytes is just a hint on how big of a region the
> > > +     * caller wants to inspect.  It is not a limit on *pnum.
> > > +     * Implementations are free to return larger values of *pnum if
> > > +     * doing so does not incur a performance penalty.
> >
> > Worth mention that the cache will benefit of it?
>
> Oh, right, absolutely.  Like so:
>
> "block/io.c's bdrv_co_block_status() will clamp *pnum before returning it to its 
> caller, but it itself can still make use of the unclamped *pnum value.  Specifically, the 
> block-status cache for protocol nodes will benefit from storing as large a region as 
> possible."

Sounds good. Do you mean this as an addition or substitution? If the latter, I'd keep 
"if doing so does not incur a performance penalty"



--
Best regards,
Vladimir