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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Sep 12, 2018 at 10:48:42AM +0100, Julien Grall wrote:
> Hi,
> 
> On 09/12/2018 10:16 AM, Roger Pau Monné wrote:
> > On Wed, Sep 12, 2018 at 11:13:50AM +0200, Roger Pau Monné 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.

> 
> > > 
> > > Current MAX_INDIRECT_PAGES is misnamed and should instead be
> > > MAX_INDIRECT_SEGS (which on x86 is exactly the same because PAGE_SIZE
> > > == XEN_PAGE_SIZE).
> 
> Looking at the usage:
> 
> j = min(MAX_INDIRECT_PAGES, INDIRECT_PAGES(nr_segments))
> 
> Where j is used as the number of grant ref. So I don't think the variable is
> misnamed here.

Right, I agree that MAX_INDIRECT_PAGE/INDIRECT_PAGES is correct.

Thanks, Roger.

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