linux-kernel - Re: [Xen-devel] [PATCH RFC] Persistent grant maps for xen blk drivers

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Message-Id: <5087B77A02000078000A3D00@nat28.tlf.novell.com>
Date:	Wed, 24 Oct 2012 08:40:10 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<roger.pau@...rix.com>, "Konrad Rzeszutek Wilk" <konrad@...nel.org>
Cc:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH RFC] Persistent grant maps for xen blk
 drivers

>>> On 23.10.12 at 20:50, Konrad Rzeszutek Wilk <konrad@...nel.org> wrote:
> On Tue, Oct 23, 2012 at 08:09:27PM +0200, Roger Pau Monné wrote:
>> On 23/10/12 19:20, Konrad Rzeszutek Wilk wrote:
>> >>>> diff --git a/drivers/block/xen-blkback/blkback.c 
> b/drivers/block/xen-blkback/blkback.c
>> >>>> index c6decb9..2b982b2 100644
>> >>>> --- a/drivers/block/xen-blkback/blkback.c
>> >>>> +++ b/drivers/block/xen-blkback/blkback.c
>> >>>> @@ -78,6 +78,7 @@ struct pending_req {
>> >>>>       unsigned short          operation;
>> >>>>       int                     status;
>> >>>>       struct list_head        free_list;
>> >>>> +     unsigned int            unmap_seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
>> 
>> Should I change this to a bool? Since we are only setting it to 0 or 1.
> 
> I would just keep it as 'int'. Eventually we can replace this with a
> bit-map, but that can be done later.

I think this should be a bitmap from the beginning - why would
you want to waste 44 bytes per request for something that fits
in a single unsigned long (and the picture would get worse with
the number-of-segments extension)?

Also - am I taking this work being done here as a silent agreement
to not invest into blkif2 to streamline the various extensions
floating around?

Jan
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