Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH] drivers/block/xen-blkback/common.h: use DIV_ROUND_UP instead of reimplementing its function | From | Julien Grall <> | Date | Wed, 12 Sep 2018 10:48:42 +0100 |
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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@huawei.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.
>> >> 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.
Cheers,
-- Julien Grall
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