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=-7.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no 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 87E5FC433E2 for ; Thu, 3 Sep 2020 06:38:10 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 52079206EB for ; Thu, 3 Sep 2020 06:38:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 52079206EB 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 CED216B0002; Thu, 3 Sep 2020 02:38:09 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C9D176B0003; Thu, 3 Sep 2020 02:38:09 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BB2466B0037; Thu, 3 Sep 2020 02:38:09 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0137.hostedemail.com [216.40.44.137]) by kanga.kvack.org (Postfix) with ESMTP id A15E16B0002 for ; Thu, 3 Sep 2020 02:38:09 -0400 (EDT) Received: from smtpin05.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 5A155181AEF09 for ; Thu, 3 Sep 2020 06:38:09 +0000 (UTC) X-FDA: 77220795498.05.rule44_360ca04270a7 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin05.hostedemail.com (Postfix) with ESMTP id 4322B18001BEE for ; Thu, 3 Sep 2020 06:38:09 +0000 (UTC) X-HE-Tag: rule44_360ca04270a7 X-Filterd-Recvd-Size: 5455 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf41.hostedemail.com (Postfix) with ESMTP for ; Thu, 3 Sep 2020 06:38:08 +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 6C722AB91; Thu, 3 Sep 2020 06:38:08 +0000 (UTC) Date: Thu, 3 Sep 2020 08:38:06 +0200 From: Michal Hocko To: Vlastimil Babka Cc: Pavel Tatashin , LKML , Andrew Morton , linux-mm Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline Message-ID: <20200903063806.GM4617@dhcp22.suse.cz> References: <20200901124615.137200-1-pasha.tatashin@soleen.com> <20200902140851.GJ4617@dhcp22.suse.cz> <74f2341a-7834-3e37-0346-7fbc48d74df3@suse.cz> <20200902151306.GL4617@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 4322B18001BEE X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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 Wed 02-09-20 19:51:45, Vlastimil Babka wrote: > On 9/2/20 5:13 PM, Michal Hocko wrote: > > On Wed 02-09-20 16:55:05, Vlastimil Babka wrote: > >> On 9/2/20 4:26 PM, Pavel Tatashin wrote: > >> > On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko wrote: > >> >> > >> >> > > >> >> > Thread#1 - continue > >> >> > free_unref_page_commit > >> >> > migratetype = get_pcppage_migratetype(page); > >> >> > // get old migration type > >> >> > list_add(&page->lru, &pcp->lists[migratetype]); > >> >> > // add new page to already drained pcp list > >> >> > > >> >> > Thread#2 > >> >> > Never drains pcp again, and therefore gets stuck in the loop. > >> >> > > >> >> > The fix is to try to drain per-cpu lists again after > >> >> > check_pages_isolated_cb() fails. > >> >> > >> >> But this means that the page is not isolated and so it could be reused > >> >> for something else. No? > >> > > >> > The page is in a movable zone, has zero references, and the section is > >> > isolated (i.e. set_pageblock_migratetype(page, MIGRATE_ISOLATE);) is > >> > set. The page should be offlinable, but it is lost in a pcp list as > >> > that list is never drained again after the first failure to migrate > >> > all pages in the range. > >> > >> Yeah. To answer Michal's "it could be reused for something else" - yes, somebody > >> could allocate it from the pcplist before we do the extra drain. But then it > >> becomes "visible again" and the loop in __offline_pages() should catch it by > >> scan_movable_pages() - do_migrate_range(). And this time the pageblock is > >> already marked as isolated, so the page (freed by migration) won't end up on the > >> pcplist again. > > > > So the page block is marked MIGRATE_ISOLATE but the allocation itself > > could be used for non migrateable objects. Or does anything prevent that > > from happening? > > In a movable zone, the allocation should not be used for non migrateable > objects. E.g. if the zone was not ZONE_MOVABLE, the offlining could fail > regardless of this race (analogically for migrating away from CMA pageblocks). > > > We really do depend on isolation to not allow reuse when offlining. > > This is not really different than if the page on pcplist was allocated just a > moment before the offlining, thus isolation started. We ultimately rely on being > able to migrate any allocated pages away during the isolation. This "freeing to > pcplists" race doesn't fundamentally change anything in this regard. We just > have to guarantee that pages on pcplists will be eventually flushed, to make > forward progress, and there was a bug in this aspect. You are right. I managed to confuse myself yesterday. The race is impossible for !ZONE_MOVABLE because we do PageBuddy check there. And on the movable zone we are not losing the migrateability property. Pavel I think this will be a useful information to add to the changelog. We should also document this in the code to prevent from further confusion. I would suggest something like the following: diff --git a/mm/page_isolation.c b/mm/page_isolation.c index 242c03121d73..56d4892bceb8 100644 --- a/mm/page_isolation.c +++ b/mm/page_isolation.c @@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages) * pageblocks we may have modified and return -EBUSY to caller. This * prevents two threads from simultaneously working on overlapping ranges. * + * Please note that there is no strong synchronization with the page allocator + * either. Pages might be freed while their page blocks are marked ISOLATED. + * In some cases pages might still end up on pcp lists and that would allow + * for their allocation even when they are in fact isolated already. Depending on + * how strong of a guarantee the caller needs drain_all_pages might be needed + * (e.g. __offline_pages will need to call it after check for isolated range for + * a next retry). + * * Return: the number of isolated pageblocks on success and -EBUSY if any part * of range cannot be isolated. */ -- Michal Hocko SUSE Labs