linux-kernel - Re: [PATCH] mm: add a "struct page_frag" type containing a page, offset and length

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Message-ID: <1318575363.11016.8.camel@dagon.hellion.org.uk>
Date:	Fri, 14 Oct 2011 07:56:02 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	David Miller <davem@...emloft.net>,
	"rientjes@...gle.com" <rientjes@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: add a "struct page_frag" type containing a page,
 offset and length

On Thu, 2011-10-13 at 22:22 +0100, Andrew Morton wrote:
> Looks OK to me.  I'm surprised we don't already have such a thing.
> 
> Review comments:
> 
> 
> > +struct page_frag {
> > +	struct page *page;
> > +#if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)
> 
> It does add risk that people will add compile warnings and bugs by
> failing to consider or test the other case.
>
> We could reduce that risk by doing
> 
>    #if (PAGE_SIZE >= 65536)
> 
> but then the 32-bit version would hardly ever be tested at all.

Indeed. The first variant has the benefit that most 32-bit arches will
test one case and most 64-bit ones the other.

Perhaps the need to keep this struct small is not so acute as it is for
the skb_frag_t I nicked it from and just using __u32 unconditionally is
sufficient?

> 
> > +	__u32 page_offset;
> 
> I suggest this be called simply "offset".

ACK.

Thanks,

Ian.


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