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 X-Spam-Level: X-Spam-Status: No, score=-10.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E354C433E0 for ; Tue, 11 Aug 2020 06:54:51 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 11579208B3 for ; Tue, 11 Aug 2020 06:54:50 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 11579208B3 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 7D7926B0005; Tue, 11 Aug 2020 02:54:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 789056B0007; Tue, 11 Aug 2020 02:54:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 69D926B0008; Tue, 11 Aug 2020 02:54:50 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0100.hostedemail.com [216.40.44.100]) by kanga.kvack.org (Postfix) with ESMTP id 5411E6B0005 for ; Tue, 11 Aug 2020 02:54:50 -0400 (EDT) Received: from smtpin26.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 0686B8248047 for ; Tue, 11 Aug 2020 06:54:50 +0000 (UTC) X-FDA: 77137375140.26.gate55_131766f26fe0 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin26.hostedemail.com (Postfix) with ESMTP id C70F01804B669 for ; Tue, 11 Aug 2020 06:54:49 +0000 (UTC) X-HE-Tag: gate55_131766f26fe0 X-Filterd-Recvd-Size: 4970 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf07.hostedemail.com (Postfix) with ESMTP for ; Tue, 11 Aug 2020 06:54:49 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A8E87AC98; Tue, 11 Aug 2020 06:55:08 +0000 (UTC) Date: Tue, 11 Aug 2020 08:54:46 +0200 From: Michal Hocko To: Baoquan He Cc: Mike Kravetz , Wei Yang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 10/10] mm/hugetlb: not necessary to abuse temporary page to workaround the nasty free_huge_page Message-ID: <20200811065406.GC4793@dhcp22.suse.cz> References: <20200807091251.12129-1-richard.weiyang@linux.alibaba.com> <20200807091251.12129-11-richard.weiyang@linux.alibaba.com> <20200810021737.GV14854@MiWiFi-R3L-srv> <129cc03e-c6d5-24f8-2f3c-f5a3cc821e76@oracle.com> <20200811015148.GA10792@MiWiFi-R3L-srv> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200811015148.GA10792@MiWiFi-R3L-srv> X-Rspamd-Queue-Id: C70F01804B669 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue 11-08-20 09:51:48, Baoquan He wrote: > On 08/10/20 at 05:19pm, Mike Kravetz wrote: > > On 8/9/20 7:17 PM, Baoquan He wrote: > > > On 08/07/20 at 05:12pm, Wei Yang wrote: > > >> Let's always increase surplus_huge_pages and so that free_huge_page > > >> could decrease it at free time. > > >> > > >> Signed-off-by: Wei Yang > > >> --- > > >> mm/hugetlb.c | 14 ++++++-------- > > >> 1 file changed, 6 insertions(+), 8 deletions(-) > > >> > > >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c > > >> index 1f2010c9dd8d..a0eb81e0e4c5 100644 > > >> --- a/mm/hugetlb.c > > >> +++ b/mm/hugetlb.c > > >> @@ -1913,21 +1913,19 @@ static struct page *alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask, > > >> return NULL; > > >> > > >> spin_lock(&hugetlb_lock); > > >> + > > >> + h->surplus_huge_pages++; > > >> + h->surplus_huge_pages_node[page_to_nid(page)]++; > > >> + > > >> /* > > >> * We could have raced with the pool size change. > > >> * Double check that and simply deallocate the new page > > >> - * if we would end up overcommiting the surpluses. Abuse > > >> - * temporary page to workaround the nasty free_huge_page > > >> - * codeflow > > >> + * if we would end up overcommiting the surpluses. > > >> */ > > >> - if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) { > > >> - SetPageHugeTemporary(page); > > > > > > Hmm, the temporary page way is taken intentionally in > > > commit 9980d744a0428 ("mm, hugetlb: get rid of surplus page accounting tricks"). > > > From code, this is done inside hugetlb_lock holding, and the code flow > > > is straightforward, should be safe. Adding Michal to CC. But the lock is not held during the migration, right? > > I remember when the temporary page code was added for page migration. > > The use of temporary page here was added at about the same time. Temporary > > page does have one advantage in that it will not CAUSE surplus count to > > exceed overcommit. This patch could cause surplus to exceed overcommit > > for a very short period of time. However, do note that for this to happen > > the code needs to race with a pool resize which itself could cause surplus > > to exceed overcommit. Correct. > > IMO both approaches are valid. > > - Advantage of temporary page is that it can not cause surplus to exceed > > overcommit. Disadvantage is as mentioned in the comment 'abuse of temporary > > page'. > > - Advantage of this patch is that it uses existing counters. Disadvantage > > is that it can momentarily cause surplus to exceed overcommit. Do I remember correctly that this can cause an allocation failure due to overcommit check? In other words it would be user space visible thing? > Yeah, since it's all done inside hugetlb_lock, should be OK even > though it may cause surplus to exceed overcommit. > > > > Unless someone has a strong opinion, I prefer the changes in this patch. > > Agree, I also prefer the code change in this patch, to remove the > unnecessary confusion about the temporary page. I have managed to forgot all the juicy details since I have made that change. All that remains is that the surplus pages accounting was quite tricky and back then I didn't figure out a simpler method that would achieve the consistent look at those counters. As mentioned above I suspect this could lead to pre-mature allocation failures while the migration is ongoing. Sure quite unlikely to happen and the race window is likely very small. Maybe this is even acceptable but I would strongly recommend to have all this thinking documented in the changelog. -- Michal Hocko SUSE Labs