netdev - Re: [net-next PATCH 1/6] net: Add skb_free_frag to replace use of put

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Message-Id: <20150506123840.312f41000e8d46f1ef9ce046@linux-foundation.org>
Date:	Wed, 6 May 2015 12:38:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	linux-mm@...ck.org, netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [net-next PATCH 1/6] net: Add skb_free_frag to replace use of
 put_page in freeing skb->head

On Mon, 04 May 2015 16:14:48 -0700 Alexander Duyck <alexander.h.duyck@...hat.com> wrote:

> +/**
> + * skb_free_frag - free a page fragment
> + * @head: virtual address of page fragment
> + *
> + * Frees a page fragment allocated out of either a compound or order 0 page.
> + * The function itself is a hybrid between free_pages and free_compound_page
> + * which can be found in mm/page_alloc.c
> + */
> +void skb_free_frag(void *head)
> +{
> +	struct page *page = virt_to_head_page(head);
> +
> +	if (unlikely(put_page_testzero(page))) {
> +		if (likely(PageHead(page)))
> +			__free_pages_ok(page, compound_order(page));
> +		else
> +			free_hot_cold_page(page, false);
> +	}
> +}

Why are we testing for PageHead in here?  If the code were to simply do

	if (unlikely(put_page_testzero(page)))
		__free_pages_ok(page, compound_order(page));

that would still work?

There's nothing networking-specific in here.  I suggest the function be
renamed and moved to page_alloc.c.  Add an inlined skb_free_frag() in a
net header which calls it.  This way the mm developers know about it
and will hopefully maintain it.  It would need a comment explaining
when and why people should and shouldn't use it.

The term "page fragment" is a net thing and isn't something we know
about.  What is it?  From context I'm thinking a definition would look
something like

  An arbitrary-length arbitrary-offset area of memory which resides
  within a 0 or higher order page.  Multiple fragments within that page
  are individually refcounted, in the page's reference counter.

Is that correct and complete?

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