From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) by kanga.kvack.org (Postfix) with ESMTP id 30E546B0008 for ; Mon, 22 Oct 2018 09:27:57 -0400 (EDT) Received: by mail-ed1-f71.google.com with SMTP id x10-v6so24964897edx.9 for ; Mon, 22 Oct 2018 06:27:57 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id k26-v6si1317185edq.215.2018.10.22.06.27.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 22 Oct 2018 06:27:55 -0700 (PDT) Subject: Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask References: <20180925120326.24392-1-mhocko@kernel.org> <20180925120326.24392-3-mhocko@kernel.org> <20180926133039.y7o5x4nafovxzh2s@kshutemo-mobl1> <20180926141708.GX6278@dhcp22.suse.cz> <20180926142227.GZ6278@dhcp22.suse.cz> <20181018191147.33e8d5e1ebd785c06aab7b30@linux-foundation.org> <20181019080657.GJ18839@dhcp22.suse.cz> From: Vlastimil Babka Message-ID: <583b20e5-4925-e175-1533-5c2d2bab9192@suse.cz> Date: Mon, 22 Oct 2018 15:27:54 +0200 MIME-Version: 1.0 In-Reply-To: <20181019080657.GJ18839@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko , Andrew Morton Cc: "Kirill A. Shutemov" , Mel Gorman , David Rientjes , Andrea Argangeli , Zi Yan , Stefan Priebe - Profihost AG , linux-mm@kvack.org, LKML On 10/19/18 10:06 AM, Michal Hocko wrote: > On Thu 18-10-18 19:11:47, Andrew Morton wrote: >> On Wed, 26 Sep 2018 16:22:27 +0200 Michal Hocko wrote: >> >>>> MPOL_PREFERRED is handled by policy_node() before we call __alloc_pages_nodemask. >>>> __GFP_THISNODE is applied only when we are not using >>>> __GFP_DIRECT_RECLAIM which is handled in alloc_hugepage_direct_gfpmask >>>> now. >>>> Lastly MPOL_BIND wasn't handled explicitly but in the end the removed >>>> late check would remove __GFP_THISNODE for it as well. So in the end we >>>> are doing the same thing unless I miss something >>> >>> Forgot to add. One notable exception would be that the previous code >>> would allow to hit >>> WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE)); >>> in policy_node if the requested node (e.g. cpu local one) was outside of >>> the mbind nodemask. This is not possible now. We haven't heard about any >>> such warning yet so it is unlikely that it happens though. >> >> Perhaps a changelog addition is needed to cover the above? > > : THP allocation mode is quite complex and it depends on the defrag > : mode. This complexity is hidden in alloc_hugepage_direct_gfpmask from a > : large part currently. The NUMA special casing (namely __GFP_THISNODE) is > : however independent and placed in alloc_pages_vma currently. This both > : adds an unnecessary branch to all vma based page allocation requests and > : it makes the code more complex unnecessarily as well. Not to mention > : that e.g. shmem THP used to do the node reclaiming unconditionally > : regardless of the defrag mode until recently. This was not only > : unexpected behavior but it was also hardly a good default behavior and I > : strongly suspect it was just a side effect of the code sharing more than > : a deliberate decision which suggests that such a layering is wrong. > : > : Moreover the oriinal code allowed to trigger > : WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE)); > : in policy_node if the requested node (e.g. cpu local one) was outside of > : the mbind nodemask. This is not possible now. We haven't heard about any > : such warning yet so it is unlikely that it happens but still a signal of > : a wrong code layering. Ah, as I said in the other mail, I think it's inaccurate, the warning was not possible to hit. There's also a slight difference wrt MPOL_BIND. The previous code would avoid using __GFP_THISNODE if the local node was outside of policy_nodemask(). After your patch __GFP_THISNODE is avoided for all MPOL_BIND policies. So there's a difference that if local node is actually allowed by the bind policy's nodemask, previously __GFP_THISNODE would be added, but now it won't be. I don't think it matters that much though, but maybe the changelog could say that (instead of the inaccurate note about warning). Note the other policy where nodemask is relevant is MPOL_INTERLEAVE, and that's unchanged by this patch. When that's addressed, you can add Acked-by: Vlastimil Babka (Note I also agree with patch 1/2 but didn't think it was useful to formally ack it on top of Mel's ack supported by actual measurements, as we're all from the same company). > : Get rid of the thp special casing from alloc_pages_vma and move the logic > : to alloc_hugepage_direct_gfpmask. __GFP_THISNODE is applied to > : the resulting gfp mask only when the direct reclaim is not requested and > : when there is no explicit numa binding to preserve the current logic. > : > : This allows for removing alloc_hugepage_vma as well. > > Better? > >> I assume that David's mbind() concern has gone away. > > Either I've misunderstood it or it was not really a real issue. >