From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752394AbdKEA22 (ORCPT ); Sat, 4 Nov 2017 20:28:28 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:55720 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751777AbdKEA21 (ORCPT ); Sat, 4 Nov 2017 20:28:27 -0400 X-Google-Smtp-Source: ABhQp+ToKUqUsb/dkdrgD4kAydhUEfyHyQt7W38qFjbMnJ6VKP75rd13jGwlR7m1+02gehlMg+NbIg== Date: Sat, 4 Nov 2017 17:28:05 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Michal Hocko cc: Hugh Dickins , linux-mm@kvack.org, Peter Zijlstra , Thomas Gleixner , Johannes Weiner , Mel Gorman , Tejun Heo , LKML , David Herrmann Subject: Re: [PATCH 1/2] shmem: drop lru_add_drain_all from shmem_wait_for_pins In-Reply-To: <20171103082417.7rwns74txzzoyzyv@dhcp22.suse.cz> Message-ID: References: <20171102093613.3616-1-mhocko@kernel.org> <20171102093613.3616-2-mhocko@kernel.org> <20171103082417.7rwns74txzzoyzyv@dhcp22.suse.cz> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 3 Nov 2017, Michal Hocko wrote: > On Fri 03-11-17 00:46:18, Hugh Dickins wrote: > > > > NAK. shmem_wait_for_pins() is waiting for temporary pins on the pages > > to go away, and using lru_add_drain_all() in the usual way, to lower > > the refcount of pages temporarily pinned in a pagevec somewhere. Page > > count is touched by draining pagevecs: I'm surprised to see you say > > that it isn't - or have pagevec page references been eliminated by > > a recent commit that I missed? > > I must be missing something here. __pagevec_lru_add_fn merely about > moving the page into the appropriate LRU list, pagevec_move_tail only > rotates, lru_deactivate_file_fn moves from active to inactive LRUs, > lru_lazyfree_fn moves from anon to file LRUs and activate_page_drain > just moves to the active list. None of those operations touch the page > count AFAICS. So I would agree that some pages might be pinned outside > of the LRU (lru_add_pvec) and thus unreclaimable but does this really > matter. Or what else I am missing? Line 213 of mm/swap.c? Where pagevec_lru_move_fn() calls release_pages() to release the extra references (which each page came in with when added). Think about it, the mayhem that would follow from a page being freed while on pagevec: of course it must hold a reference. The only surprise is that the extra reference is not needed while on LRU: one can think of PageLRU as an extension of the page count. Hugh