Re: [Xen-devel] [PATCH] drivers/block/xen-blkback/common.h: use DIV_ROUND_UP instead of reimplementing its function

[Julien Grall]

Hi Roger,

On 09/12/2018 11:29 AM, Roger Pau Monnà wrote:
> On Wed, Sep 12, 2018 at 10:48:42AM +0100, Julien Grall wrote:
> > Hi,
> >
> > On 09/12/2018 10:16 AM, Roger Pau Monnà wrote:
> > > On Wed, Sep 12, 2018 at 11:13:50AM +0200, Roger Pau Monnà wrote:
> > > > Adding Julien how did the work to support XEN_PAGE_SIZE != PAGE_SIZE.
> > > >
> > > > On Wed, Sep 12, 2018 at 02:14:26AM -0600, Jan Beulich wrote:
> > > > > > > > On 12.09.18 at 07:45, <zhongjiang@xxxxxxxxxx> wrote:
> > > > > > --- a/drivers/block/xen-blkback/common.h
> > > > > > +++ b/drivers/block/xen-blkback/common.h
> > > > > > @@ -65,7 +65,7 @@
> > > > > >    	(XEN_PAGES_PER_INDIRECT_FRAME / XEN_PAGES_PER_SEGMENT)
> > > > > >    #define MAX_INDIRECT_PAGES \
> > > > > > -	((MAX_INDIRECT_SEGMENTS + SEGS_PER_INDIRECT_FRAME - 1)/SEGS_PER_INDIRECT_FRAME)
> > > > > > +		DIV_ROUND_UP(MAX_INDIRECT_SEGMENTS, SEGS_PER_INDIRECT_FRAME)
> > > > > >    #define INDIRECT_PAGES(_segs) DIV_ROUND_UP(_segs, XEN_PAGES_PER_INDIRECT_FRAME)
> > > > >
> > > > > My first reaction was to suggest
> > > > >
> > > > > #define MAX_INDIRECT_PAGES INDIRECT_PAGES(MAX_INDIRECT_SEGMENTS)
> > > > >
> > > > > but that wouldn't match what's there currently (note the two different
> > > > > divisors). I can't really decide whether that's just unfortunate naming
> > > > > of the two macros, or an actual bug.
> > > >
> > > > I think there's indeed a bug here.
> > > >
> > > > AFAICT, MAX_INDIRECT_PAGES should use XEN_PAGES_PER_INDIRECT_FRAME and
> > > > then it could be changed as Jan suggested.
> >
> > The problem is SEGS_PER_INDIRECT_FRAME has been miscalculated. So I think it
> > would be fine to use XEN_PAGES_PER_INDIRECT_FRAME in MAX_INDIRECT_PAGES.
> >
> > However the naming for XEN_PAGES_PER_INDIRECT_FRAME is misnamed. We return
> > number of a for segments per indirect frame. So I would rename to
> > SEGS_PER_INDIRECT_FRAME.
>
> I don't think I agree with this last part, SEGS_PER_INDIRECT_FRAME
> would have to take into account XEN_PAGES_PER_SEGMENT, and
> XEN_PAGES_PER_INDIRECT_FRAME doesn't.
>
> XEN_PAGES_PER_INDIRECT_FRAME currently returns the number of grant
> references per indirect page, but as I understand it a segment can use
> more than one grant reference, if for example the guest page size is
> 64KB.

I am a bit confused. By segment, do you refer to the backend or frontend 
segment?

In any case, it would be possible to remove SEGS_PER_INDIRECT_FRAME if 
we rework MAX_INDIRECT_PAGES(...). This should improve the readability 
as well.

Cheers,

--
Julien Grall