linux-kernel - Re: [Xen-devel] [PATCH RFC 2/4] xen-blkfront: improve aproximation of required grants per request

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Message-ID: <51DEC59B.2040501@citrix.com>
Date:	Thu, 11 Jul 2013 16:47:55 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	<xen-devel@...ts.xen.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH RFC 2/4] xen-blkfront: improve aproximation
 of	required grants per request

On 11/07/13 15:20, David Vrabel wrote:
> On 08/07/13 14:03, Roger Pau Monne wrote:
>> Improve the calculation of required grants to process a request by
>> using nr_phys_segments instead of always assuming a request is going
>> to use all posible segments.
> 
> This isn't obviously correct to me.  Why is this correct value for the
> number of pages that need to be granted?

This contains the number of scatter-gather DMA addr+len pairs, which is
basically what we put at every granted page. for_each_sg iterates over
the DMA addr+len pairs and uses a grant page for each of them.
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