From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764148AbYF3Uzh (ORCPT ); Mon, 30 Jun 2008 16:55:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753739AbYF3Uz3 (ORCPT ); Mon, 30 Jun 2008 16:55:29 -0400 Received: from extu-mxob-2.symantec.com ([216.10.194.135]:38946 "EHLO extu-mxob-2.symantec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752486AbYF3Uz2 (ORCPT ); Mon, 30 Jun 2008 16:55:28 -0400 Date: Mon, 30 Jun 2008 21:55:13 +0100 (BST) From: Hugh Dickins X-X-Sender: hugh@blonde.site To: Rik van Riel cc: KAMEZAWA Hiroyuki , KOSAKI Motohiro , Andrew Morton , Lee Schermerhorn , Nick Piggin , linux-kernel@vger.kernel.org Subject: Re: [PATCH] splitlru: BDI_CAP_SWAP_BACKED In-Reply-To: <20080630154912.15f03b97@bree.surriel.com> Message-ID: References: <20080629173841.37CD.KOSAKI.MOTOHIRO@jp.fujitsu.com> <20080630115354.3a75edd3.kamezawa.hiroyu@jp.fujitsu.com> <20080630154912.15f03b97@bree.surriel.com> 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 Mon, 30 Jun 2008, Rik van Riel wrote: > > Tmpfs is often in the same boat as anonymous memory. > Used for shared memory segments, or for files that > are temporary and will be gone soon. Anonymous memory, and temporary files, are often soon gone, okay. But I don't find that generalization compelling; and if they're soon gone, does it matter which lru they go on? > If swap space runs out, tmpfs pages should not be > scanned. That point I like. But I hope they'd go to the Unevictable on systems with no swap at all (of course, as with mlocking, that can change soon after). > To me, this suggests they should probably continue > to live on the *_ANON LRUs. Worst case we make > tmpfs pages in files that are not mmaped (/tmp use) > start out on the inactive list, so they get evicted > first. Tweaking in/active I'll gladly leave to you! Whatever proves best. What's worrying me is that we have always treated shmem/tmpfs pages as file pages (e.g. in /proc/meminfo as Cached not as SwapCached), up until the point that we retire them to swap; but in splitlru you're sending them down another path; then mem cgroups seem to want them as something else again. Your SwapBacked may indeed turn out to be the only implementable distinction, but it does worry me. A more useful distinction, my gut tells me, would be separate LRUs for page_mapped() and !page_mapped(), which reflects the existing swappiness notion. But that immediately hits the difficulty we have in switching LRU midstream, which your SwapBacked-throughout tmpfs neatly sidesteps. I'd really like to be able to try page_mapped/!page_mapped versus swap-backed/file-backed, but it would need some LRU-switching infrastructure (which might come at a prohibitive performance cost, since it's the batching that poses the problem). Hugh