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

From: Julien Grall
Date: Mon Sep 24 2018 - 09:28:33 EST


Hi Roger,

On 09/12/2018 11:29 AM, Roger Pau Monnà wrote:
On Wed, Sep 12, 2018 at 10:48:42AM +0100, Julien Grall wrote:
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.

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