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

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Date:   Tue, 2 Oct 2018 09:43:44 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Julien Grall <julien.grall@....com>
CC:     Jan Beulich <JBeulich@...e.com>, Jens Axboe <axboe@...nel.dk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        <linux-kernel@...r.kernel.org>, <linux-block@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        zhong jiang <zhongjiang@...wei.com>
Subject: Re: [Xen-devel] [PATCH] drivers/block/xen-blkback/common.h: use
 DIV_ROUND_UP instead of reimplementing its function

On Mon, Sep 24, 2018 at 02:28:26PM +0100, Julien Grall wrote:
> Hi Roger,
> 
> On 09/12/2018 11:29 AM, Roger Pau Monné wrote:
> > 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@...wei.com> 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?

Backend segment. I guess it's quite messy to have both frontend
segment size and backend segment size which can be different.

> 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.

Yes, I think this should improve the code.

Roger.