From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754028Ab1KXIyM (ORCPT ); Thu, 24 Nov 2011 03:54:12 -0500 Received: from zene.cmpxchg.org ([85.214.230.12]:38282 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752999Ab1KXIyL (ORCPT ); Thu, 24 Nov 2011 03:54:11 -0500 Date: Thu, 24 Nov 2011 09:53:57 +0100 From: Johannes Weiner To: Hugh Dickins Cc: Andrew Morton , KAMEZAWA Hiroyuki , Michal Hocko , Balbir Singh , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [patch 7/8] mm: memcg: modify PageCgroupAcctLRU non-atomically Message-ID: <20111124085357.GA6843@cmpxchg.org> References: <1322062951-1756-1-git-send-email-hannes@cmpxchg.org> <1322062951-1756-8-git-send-email-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 23, 2011 at 10:52:39AM -0800, Hugh Dickins wrote: > On Wed, 23 Nov 2011, Johannes Weiner wrote: > > > From: Johannes Weiner > > > > This bit is protected by zone->lru_lock, there is no need for locked > > operations when setting and clearing it. > > > > Signed-off-by: Johannes Weiner > > Unless there are special considerations which you have not mentioned at > all in the description above, this 7/8 and the similar 8/8 are mistaken. > > The atomic operation is not for guaranteeing the setting and clearing > of the bit in question: it's for guaranteeing that you don't accidentally > set or clear any of the other bits in the same word when you're doing so, > if another task is updating them at the same time as you're doing this. > > There are circumstances when non-atomic shortcuts can be taken, when > you're sure the field cannot yet be visible to other tasks (we do that > when setting PageLocked on a freshly allocated page, for example - but > even then have to rely on others using get_page_unless_zero properly). > But I don't think that's the case here. I have no idea how I could oversee this. You are, of course, right. That said, I *think* that it is safe for PageCgroupCache because nobody else should be modifying any pc->flags concurrently: PCG_LOCK: by definition exclusive and held during setting and clearing PCG_CACHE PCG_CACHE: serialized by PCG_LOCK PCG_USED: serialized by PCG_LOCK PCG_MIGRATION: serialized by PCG_LOCK PCG_MOVE_LOCK: 1. update_page_stat() is only called against file pages, and the page lock serializes charging against mapping. the page is also charged before establishing a pte mapping, so an unmap can not race with a charge. 2. split_huge_fixup runs against already charged pages. 3. move_account is serialized both by LRU-isolation during charging and PCG_LOCK PCG_FILE_MAPPED: same as PCG_MOVE_LOCK's 1. PCG_ACCT_LRU: pages are isolated from the LRU during charging But all this is obviously anything but robust, and so I retract the broken 7/8 and the might-actually-work 8/8.