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

From: Julien Grall
Date: Wed Sep 12 2018 - 05:48:48 EST


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.


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