On 05/04/2015 05:16 PM, Eric Dumazet wrote:
On Mon, 2015-05-04 at 16:14 -0700, Alexander Duyck wrote:
This change adds a function called skb_free_frag which is meant to
compliment the function __alloc_page_frag. The general idea is to enable a
more lightweight version of page freeing since we don't actually need all
the overhead of a put_page, and we don't quite fit the model of __free_pages.
Could you describe what are the things that put_page() handle that we
don't need for skb frags ?
A large part of it is just all the extra code flow with each level
having to retest flags. So if you follow the calls for put_page w/ a
page frag you end up with something like:
skb_free_head - virt_to_head_page
put_page() - (Head | Tail)
put_compound_page() - Tail, Head, _count
__put_compound_page() - compound_dtor
__page_cache_release() - LRU, mem_cgroup
free_compound_page() - Head, compound_order
__free_pages_ok()
If I free the same page frag in skb_frag_free the path ends up looking like:
skb_free_head - inlined by compiler
skb_free_frag - virt_to_head_page, count, head, order
__free_pages_ok()
It looks the change could benefit to other users (outside of networking)
It could, but there are also other mechanisms already in place for
freeing pages. For example in the Intel drivers I had gotten into the
habit of using __free_pages for Rx pages since it does exactly the same
thing but you need to know the order of the pages you are freeing before
you can use it. In the case of head frags we don't really know that
since it can be a page fragment given to us by any of the drivers using
alloc_pages.
The motivation behind creating a centralized function was to take care
of the virt_to_head_page and compound_order portions of this in one
centralized spot. It avoids bloating things as I'm able to get away
with little tricks like combining the compound_order Head flag check
with the one to determine if I call the compound freeing function or the
order 0 one.
- Alex
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