From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4DB88C43217 for ; Mon, 17 Oct 2022 02:38:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230309AbiJQCir (ORCPT ); Sun, 16 Oct 2022 22:38:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48716 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230517AbiJQCia (ORCPT ); Sun, 16 Oct 2022 22:38:30 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A74F145F49 for ; Sun, 16 Oct 2022 19:38:29 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 44F6860F00 for ; Mon, 17 Oct 2022 02:38:29 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 61B85C433C1; Mon, 17 Oct 2022 02:38:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1665974308; bh=m863YB6o4rswhZYdI5CDz0jbWRvS7MOt8KB2hFC4S3M=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Ylz297VEVHewx6YBQIegnEyi0kHKHm9Tnbe3TPLJcYCPGyNF/cPkAi6Qh5W/kXtaU sthS7gz4wpyfPyJi1TDyta+xd6DYFC5+OGbzrl52bElFQ3SUAcrFphkc2R6GPEYat3 DKyXGzk3D0UHL2N/8FcgMV4dkOxODDBA4R0aWomQ= Date: Sun, 16 Oct 2022 19:38:27 -0700 From: Andrew Morton To: Mel Gorman Cc: Michal Hocko , NeilBrown , Thierry Reding , Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman , Vlastimil Babka Subject: Re: [PATCH] MM: discard __GFP_ATOMIC Message-Id: <20221016193827.37c8c2165b22ae4401c269b4@linux-foundation.org> In-Reply-To: <20220907094724.5lanecgcjg75vxv3@techsingularity.net> References: <163712397076.13692.4727608274002939094@noble.neil.brown.name> <163764199967.7248.2528204111227925210@noble.neil.brown.name> <20220430113028.9daeebeedf679aa384da5945@linux-foundation.org> <20220907094724.5lanecgcjg75vxv3@techsingularity.net> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 7 Sep 2022 10:47:24 +0100 Mel Gorman wrote: > On Tue, Sep 06, 2022 at 09:35:41AM +0200, Michal Hocko wrote: > > > From: "NeilBrown" > > > Subject: mm: discard __GFP_ATOMIC > > > > > > __GFP_ATOMIC serves little purpose. Its main effect is to set > > > ALLOC_HARDER which adds a few little boosts to increase the chance of an > > > allocation succeeding, one of which is to lower the water-mark at which it > > > will succeed. > > > > > > It is *always* paired with __GFP_HIGH which sets ALLOC_HIGH which also > > > adjusts this watermark. It is probable that other users of __GFP_HIGH > > > should benefit from the other little bonuses that __GFP_ATOMIC gets. > > > > > > __GFP_ATOMIC also gives a warning if used with __GFP_DIRECT_RECLAIM. > > > There is little point to this. We already get a might_sleep() warning if > > > __GFP_DIRECT_RECLAIM is set. > > > > > > __GFP_ATOMIC allows the "watermark_boost" to be side-stepped. It is > > > probable that testing ALLOC_HARDER is a better fit here. > > > > > > __GFP_ATOMIC is used by tegra-smmu.c to check if the allocation might > > > sleep. This should test __GFP_DIRECT_RECLAIM instead. > > > > > > This patch: > > > - removes __GFP_ATOMIC > > > - causes __GFP_HIGH to set ALLOC_HARDER unless __GFP_NOMEMALLOC is set > > > (as well as ALLOC_HIGH). > > > - makes other adjustments as suggested by the above. > > > > > > The net result is not change to GFP_ATOMIC allocations. Other > > > allocations that use __GFP_HIGH will benefit from a few different extra > > > privileges. This affects: > > > xen, dm, md, ntfs3 > > > the vermillion frame buffer > > > hibernation > > > ksm > > > swap > > > all of which likely produce more benefit than cost if these selected > > > allocation are more likely to succeed quickly. > > > > This is a good summary of the current usage and existing issues. It also > > shows that the naming is tricky and allows people to make wrong calls > > (tegra-smmu.c). I also thing that it is wrong to couple memory reserves > > access to the reclaim constrains/expectations of the caller. > > > > I think it's worth trying to get rid of __GFP_ATOMIC although this patch > needs to be rebased. Without rebasing it, I suspect there is a corner case > for reserving high order atomic blocks. A high-order atomic allocation > might get confused with a __GFP_HIGH high-order allocation that can sleep. > It would not be completely irrational to have such a caller if it was in a > path that can tolerate a stall but stalling might have visible consequences. > I'm also worried that the patch might allow __GFP_HIGH to ignore cpusets > which is probably not intended by direct users like ksm. Unclear what you mean by "rebased". You're saying the patch might have issues - doesn't that mean it needs to be "fixed"? Anyway, I've been maintaining this change for nearly a year - if nothing happens soon I guess I'll drop it so it doesn't get in people's way.