From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id 72A916B0313 for ; Wed, 16 May 2018 06:28:18 -0400 (EDT) Received: by mail-wr0-f197.google.com with SMTP id q67-v6so172764wrb.12 for ; Wed, 16 May 2018 03:28:18 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id g16-v6si2506275edg.21.2018.05.16.03.28.17 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 16 May 2018 03:28:17 -0700 (PDT) Date: Wed, 16 May 2018 11:28:12 +0100 From: Mel Gorman Subject: Re: [PATCH] mm/page_alloc: use ac->high_zoneidx for classzone_idx Message-ID: <20180516102811.huem4rg3mfmp2v5d@suse.de> References: <1525408246-14768-1-git-send-email-iamjoonsoo.kim@lge.com> <8b06973c-ef82-17d2-a83d-454368de75e6@suse.cz> <20180504103322.2nbadmnehwdxxaso@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Joonsoo Kim , Andrew Morton , Michal Hocko , Linux Memory Management List , LKML , Johannes Weiner , Minchan Kim , Ye Xiaolong , Joonsoo Kim On Wed, May 16, 2018 at 11:35:55AM +0200, Vlastimil Babka wrote: > On 05/08/2018 03:00 AM, Joonsoo Kim wrote: > >> classzone predates my involvement with Linux but I would be less concerneed > >> about what the original intent was and instead ensure that classzone index > >> is consistent, sane and potentially renamed while preserving the intent of > >> "reserve pages in lower zones when an allocation request can use higher > >> zones". While historically the critical intent was to preserve Normal and > >> to a lesser extent DMA on 32-bit systems, there still should be some care > >> of DMA32 so we should not lose that. > > > > Agreed! > > > >> With the patch, the allocator looks like it would be fine as just > >> reservations change. I think it's unlikely that CMA usage will result > >> in lowmem starvation. Compaction becomes a bit weird as classzone index > >> has no special meaning versis highmem and I think it'll be very easy to > >> forget. > > I don't understand this point, what do you mean about highmem here? I mean it has no special meaning as compaction is not primarily concerned with lowmem protections as it compacts within a zone. It preserves watermarks but it does not have the same degree of criticality as the page allocator and reclaim is concerned with. > I've > checked and compaction seems to use classzone_idx 1) to pass it to > watermark checks as part of compaction suitability checks, i.e. the > usual lowmem protection, and 2) to limit compaction of higher zones in > kcompactd if the direct compactor can't use them anyway - seems this > part has currently the same zone imbalance problem as reclaim. > Originally the watermark check for compaction was primarily about not depleting a single zone but the checks were duplicated anyway. It's not actually super critical for it to preserve lowmem zones as any memory usage by compaction is transient. > > Agreed! > > I will update this patch to reflect your comment. If someone have an idea > > on renaming this variable, please let me know. > > Pehaps max_zone_idx? Seems a bit more clear than "high_zoneidx". And I > have no idea what was actually meant by "class". > I don't have a better suggestion. -- Mel Gorman SUSE Labs