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

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Message-ID: <20121025124013.GB25640@localhost.localdomain>
Date:	Thu, 25 Oct 2012 08:40:14 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	roger.pau@...rix.com, Konrad Rzeszutek Wilk <konrad@...nel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH RFC] Persistent grant maps for xen blk drivers

On Wed, Oct 24, 2012 at 08:40:10AM +0100, Jan Beulich wrote:
> >>> 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?

I haven't been able (time-wise) to look at making blkif2 a possibility
and I don't want to hinder this work - which provides great
performance benefits.
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