From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Duyck Subject: Re: [net-next PATCH 1/6] net: Add skb_free_frag to replace use of put_page in freeing skb->head Date: Mon, 04 May 2015 19:49:53 -0700 Message-ID: <55482FD1.5060707@redhat.com> References: <20150504231000.1538.70520.stgit@ahduyck-vm-fedora22> <20150504231448.1538.84164.stgit@ahduyck-vm-fedora22> <1430785003.27254.20.camel@edumazet-glaptop2.roam.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-mm@kvack.org, netdev@vger.kernel.org, akpm@linux-foundation.org, davem@davemloft.net To: Eric Dumazet Return-path: In-Reply-To: <1430785003.27254.20.camel@edumazet-glaptop2.roam.corp.google.com> Sender: owner-linux-mm@kvack.org List-Id: netdev.vger.kernel.org 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org