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:   Mon, 24 Sep 2018 14:28:26 +0100
From:   Julien Grall <julien.grall@....com>
To:     Roger Pau Monné <roger.pau@...rix.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

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?

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