From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx113.postini.com [74.125.245.113]) by kanga.kvack.org (Postfix) with SMTP id 52F416B0005 for ; Wed, 6 Mar 2013 20:06:03 -0500 (EST) Received: by mail-oa0-f53.google.com with SMTP id m1so13617409oag.12 for ; Wed, 06 Mar 2013 17:06:02 -0800 (PST) Message-ID: <5137E7F4.1060509@gmail.com> Date: Thu, 07 Mar 2013 09:05:56 +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> In-Reply-To: <20130306194703.GA1953@cmpxchg.org> 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 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