netdev - Re: [PATCH 1/2] af_packet: use vmalloc_to_page() instead for the addresss returned by vmalloc()

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Date:	Wed, 01 Dec 2010 16:13:16 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jpirko@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] af_packet: use vmalloc_to_page() instead for the
 addresss returned by vmalloc()

Le mercredi 01 décembre 2010 à 22:16 +0800, Changli Gao a écrit :

> Even then, tpacket_fill_skb() is called for every skb, and
> pgv_to_page() is used in it. We have to optimize pgv_to_page().

With the __pure trick I gave, pgv_to_page() is _not_ called for the
typical use case of af_packet : packet sniffing.

Compiler is able to remove the call completely, since

static inline void flush_dcache_page(struct page *page) { }

The only remaining pgv_to_page() call is the one done in mmap packet
send, since we have to do :

page = pgv_to_page(data);
get_page(page);

I personally dont use this path, its known to be buggy...

Optimize if you want, but make all this
ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE conditional.

Its not needed to maintain an array of 'struct page *' if its not needed
at all.

# vi +2448 block/blk-core.c

#if ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE
/**
 * rq_flush_dcache_pages - Helper function to flush all pages in a request
 * @rq: the request to be flushed
 *
 * Description:
 *     Flush all pages in @rq.
 */
void rq_flush_dcache_pages(struct request *rq)
{
        struct req_iterator iter;
        struct bio_vec *bvec;

        rq_for_each_segment(bvec, rq, iter)
                flush_dcache_page(bvec->bv_page);
}
EXPORT_SYMBOL_GPL(rq_flush_dcache_pages);
#endif



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