From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [patch v2] vmscan: protect zone rotation stats by lru lock From: Lee Schermerhorn In-Reply-To: <49345B3B.30703@redhat.com> References: <20081201134112.24c647ff.akpm@linux-foundation.org> <49345B3B.30703@redhat.com> Content-Type: text/plain Date: Mon, 01 Dec 2008 17:09:45 -0500 Message-Id: <1228169385.18834.136.camel@lts-notebook> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Rik van Riel Cc: Andrew Morton , Johannes Weiner , torvalds@linux-foundation.org, kosaki.motohiro@jp.fujitsu.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Mon, 2008-12-01 at 16:46 -0500, Rik van Riel wrote: > Andrew Morton wrote: > > On Mon, 01 Dec 2008 03:00:35 +0100 > > Johannes Weiner wrote: > > > >> The zone's rotation statistics must not be accessed without the > >> corresponding LRU lock held. Fix an unprotected write in > >> shrink_active_list(). > >> > > > > I don't think it really matters. It's quite common in that code to do > > unlocked, racy update to statistics such as this. Because on those > > rare occasions where a race does happen, there's a small glitch in the > > reclaim logic which nobody will notice anyway. > > > > Of course, this does need to be done with some care, to ensure the > > glitch _will_ be small. > > Processing at most SWAP_CLUSTER_MAX pages at once probably > ensures that glitches will be small most of the time. > > The only way this could be a big problem is if we end up > racing with the divide-by-two logic in get_scan_ratio, > leaving the rotated pages a factor two higher than they > should be. > > Putting all the writes to the stats under the LRU lock > should ensure that never happens. And he's not actually adding a lock. Just moving the exiting one up to include the stats update. The intervening pagevec, pgmoved and lru initializations don't need to be under the lock, but that's probably not a big deal? Lee -- 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