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

[Ian Campbell <Ian.Campbell@citrix.com>]

On Fri, 2011-10-14 at 07:56 +0100, Ian Campbell wrote:
> 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?

I wasn't sure what to do here so I left it as it was, so the only change
here is s/page_offset/offset/. Switching to a u32 only version is
something we can easily do in the future if this scheme turns out to be
problematic.

Ian.

8<---------------------------------------------------------------

>From 806b74572ad63e2ed3ca69bb5640a55dc4475e73 Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@citrix.com>
Date: Mon, 3 Oct 2011 16:46:54 +0100
Subject: [PATCH] mm: add a "struct page_frag" type containing a page, offset and length

A few network drivers currently use skb_frag_struct for this purpose but I have
patches which add additional fields and semantics there which these other uses
do not want.

A structure for reference sub-page regions seems like a generally useful thing
so do so instead of adding a network subsystem specific structure.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jens Axboe <jaxboe@fusionio.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Miller <davem@davemloft.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
[since v1: s/struct subpage/struct page_frag/ on advice from Christoph]
[since v2: s/page_offset/offset/ on advice from Andrew]
---
 include/linux/mm_types.h |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 774b895..29971a5 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -135,6 +135,17 @@ struct page {
 #endif
 ;
 
+struct page_frag {
+	struct page *page;
+#if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)
+	__u32 offset;
+	__u32 size;
+#else
+	__u16 offset;
+	__u16 size;
+#endif
+};
+
 typedef unsigned long __nocast vm_flags_t;
 
 /*
-- 
1.7.2.5