From mboxrd@z Thu Jan 1 00:00:00 1970 From: Minchan Kim Subject: Re: [PATCH v1 01/19] mm: use put_page to free page instead of putback_lru_page Date: Tue, 15 Mar 2016 10:16:56 +0900 Message-ID: <20160315011656.GD19514@bbox> References: <1457681423-26664-1-git-send-email-minchan@kernel.org> <1457681423-26664-2-git-send-email-minchan@kernel.org> <56E67AE1.60700@suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <56E67AE1.60700@suse.cz> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Vlastimil Babka Cc: jlayton@poochiereds.net, Rik van Riel , aquini@redhat.com, rknize@motorola.com, Sergey Senozhatsky , Hugh Dickins , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, bfields@fieldses.org, linux-mm@kvack.org, Gioh Kim , Mel Gorman , Andrew Morton , Naoya Horiguchi , Joonsoo Kim , koct9i@gmail.com List-Id: virtualization@lists.linuxfoundation.org On Mon, Mar 14, 2016 at 09:48:33AM +0100, Vlastimil Babka wrote: > On 03/11/2016 08:30 AM, Minchan Kim wrote: > >Procedure of page migration is as follows: > > > >First of all, it should isolate a page from LRU and try to > >migrate the page. If it is successful, it releases the page > >for freeing. Otherwise, it should put the page back to LRU > >list. > > > >For LRU pages, we have used putback_lru_page for both freeing > >and putback to LRU list. It's okay because put_page is aware of > >LRU list so if it releases last refcount of the page, it removes > >the page from LRU list. However, It makes unnecessary operations > >(e.g., lru_cache_add, pagevec and flags operations. > > Yeah, and compaction (perhaps also other migration users) has to > drain the lru pvec... Getting rid of this stuff is worth even by > itself. Good note. Although we cannot remove lru pvec draining completely, at least, this patch removes a case which should drain pvec for returning freed page to buddy. Thanks for the notice. > > >It would be > >not significant but no worth to do) and harder to support new > >non-lru page migration because put_page isn't aware of non-lru > >page's data structure. > > > >To solve the problem, we can add new hook in put_page with > >PageMovable flags check but it can increase overhead in > >hot path and needs new locking scheme to stabilize the flag check > >with put_page. > > > >So, this patch cleans it up to divide two semantic(ie, put and putback). > >If migration is successful, use put_page instead of putback_lru_page and > >use putback_lru_page only on failure. That makes code more readable > >and doesn't add overhead in put_page. > > I had an idea of checking for count==1 in putback_lru_page() which > would take the put_page() shortcut from there. But maybe it can't be > done nicely without races. I thought about it and we might do it via page_freeze_refs but what I want at this moment is to separte two semantic put and putback. ;-) > > >Cc: Vlastimil Babka > >Cc: Mel Gorman > >Cc: Hugh Dickins > >Cc: Naoya Horiguchi > >Signed-off-by: Minchan Kim > > Acked-by: Vlastimil Babka > > Note in -next/after 4.6-rc1 this will need some rebasing though. Thanks for the review!