From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx133.postini.com [74.125.245.133]) by kanga.kvack.org (Postfix) with SMTP id 728B86B0006 for ; Thu, 7 Mar 2013 21:13:31 -0500 (EST) Received: by mail-ie0-f182.google.com with SMTP id k14so1459838iea.27 for ; Thu, 07 Mar 2013 18:13:30 -0800 (PST) Message-ID: <51394945.4070803@gmail.com> Date: Fri, 08 Mar 2013 10:13:25 +0800 From: Simon Jeons MIME-Version: 1.0 Subject: Re: [PATCH] mm: Fixup the condition whether the page cache is free References: <20130304150937.GB23767@cmpxchg.org> <51369637.6030705@gmail.com> <20130306194703.GA1953@cmpxchg.org> <5137E7F4.1060509@gmail.com> In-Reply-To: <5137E7F4.1060509@gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Li Haifeng , open@kvack.org, list@kvack.org, MEMORY MANAGEMENT , open list , linux-arm-kernel@lists.infradead.org Ping, :-) On 03/07/2013 09:05 AM, Simon Jeons wrote: > Hi Johannes, > On 03/07/2013 03:47 AM, Johannes Weiner wrote: >> On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote: >>> Hi Johannes, >>> On 03/04/2013 11:09 PM, Johannes Weiner wrote: >>>> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote: >>>>> When a page cache is to reclaim, we should to decide whether the page >>>>> cache is free. >>>>> IMO, the condition whether a page cache is free should be 3 in page >>>>> frame reclaiming. The reason lists as below. >>>>> >>>>> When page is allocated, the page->_count is 1(code fragment is >>>>> code-1 ). >>>>> And when the page is allocated for reading files from extern disk, >>>>> the >>>>> page->_count will increment 1 by page_cache_get() in >>>>> add_to_page_cache_locked()(code fragment is code-2). When the page >>>>> is to >>>>> reclaim, the isolated LRU list also increase the page->_count(code >>>>> fragment is code-3). >>>> The page count is initialized to 1, but that does not stay with the >>>> object. It's a reference that is passed to the allocating task, which >>>> drops it again when it's done with the page. I.e. the pattern is like >>>> this: >>>> >>>> instantiation: >>>> page = page_cache_alloc() /* instantiator reference -> 1 */ >>>> add_to_page_cache(page, mapping, offset) >>>> get_page(page) /* page cache reference -> 2 */ >>>> lru_cache_add(page) >>>> get_page(page) /* pagevec reference -> 3 */ >>>> /* ...initiate read, write, associate buffers, ... */ >>>> page_cache_release(page) /* drop instantiator reference -> 2 + >>>> private */ >>>> >>>> reclaim: >>>> lru_add_drain() >>>> page_cache_release(page) /* drop pagevec reference -> 1 + >>>> private */ >>> IIUC, when add page to lru will lead to add to pagevec firstly, and >>> pagevec will take one reference, so if lru will take over the >>> reference taken by pagevec when page transmit from pagevec to lru? >>> or just drop the reference and lru will not take reference for page? >> The LRU does not hold a reference, it would not make sense. The >> pagevec only needs one because it would be awkward to remove a >> concurrently freed page out of a pagevec, but unlinking a page from >> the LRU is easy. See mm/swap.c::__page_cache_release() and friends. > > Since pagevec is per cpu, when can remove a concurrently freed page > out of a pagevec happen? > > -- 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