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Wed, 25 Nov 2020 08:41:14 -0500 X-MC-Unique: 06i0NvPvMl2IWapNuFFuAw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 42D049A234; Wed, 25 Nov 2020 13:41:13 +0000 (UTC) Received: from [10.36.112.131] (ovpn-112-131.ams2.redhat.com [10.36.112.131]) by smtp.corp.redhat.com (Postfix) with ESMTP id 65D6C60854; Wed, 25 Nov 2020 13:41:11 +0000 (UTC) Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages To: Mel Gorman Cc: Andrea Arcangeli , Vlastimil Babka , Andrew Morton , linux-mm@kvack.org, Qian Cai , Michal Hocko , linux-kernel@vger.kernel.org, Mike Rapoport , Baoquan He References: <35F8AADA-6CAA-4BD6-A4CF-6F29B3F402A4@redhat.com> <20201125103933.GM3306@suse.de> <5f01bde6-fe31-9b0e-f288-06b82598a8b3@redhat.com> <20201125133346.GN3306@suse.de> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Wed, 25 Nov 2020 14:41:10 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: <20201125133346.GN3306@suse.de> Content-Type: text/plain; charset=iso-8859-15 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 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 25.11.20 14:33, Mel Gorman wrote: > On Wed, Nov 25, 2020 at 12:04:15PM +0100, David Hildenbrand wrote: >> On 25.11.20 11:39, Mel Gorman wrote: >>> On Wed, Nov 25, 2020 at 07:45:30AM +0100, David Hildenbrand wrote: >>>>> Something must have changed more recently than v5.1 that caused the >>>>> zoneid of reserved pages to be wrong, a possible candidate for the >>>>> real would be this change below: >>>>> >>>>> + __init_single_page(pfn_to_page(pfn), pfn, 0, 0); >>>>> >>>> >>>> Before that change, the memmap of memory holes were only zeroed out. So the zones/nid was 0, however, pages were not reserved and had a refcount of zero - resulting in other issues. >>>> >>>> Most pfn walkers shouldn???t mess with reserved pages and simply skip them. That would be the right fix here. >>>> >>> >>> Ordinarily yes, pfn walkers should not care about reserved pages but it's >>> still surprising that the node/zone linkages would be wrong for memory >>> holes. If they are in the middle of a zone, it means that a hole with >>> valid struct pages could be mistaken for overlapping nodes (if the hole >>> was in node 1 for example) or overlapping zones which is just broken. >> >> I agree within zones - but AFAIU, the issue is reserved memory between >> zones, right? >> > > It can also occur in the middle of the zone. > >> Assume your end of memory falls within a section - what would be the >> right node/zone for such a memory hole at the end of the section? > > Assuming a hole is not MAX_ORDER-aligned but there is real memory within > the page block, then the node/zone for the struct pages backing the hole > should match the real memorys node and zone. > > As it stands, with the uninitialised node/zone, certain checks like > page_is_buddy(): page_zone_id(page) != page_zone_id(buddy) may only > work by co-incidence. page_is_buddy() happens to work anyway because > PageBuddy(buddy) would never be true for a PageReserved page. > >> With >> memory hotplug after such a hole, we can easily have multiple >> nodes/zones spanning such a hole, unknown before hotplug. >> > > When hotplugged, the same logic would apply. Where the hole is not aligned, > the struct page linkages should match the "real" memory". > >>> It would partially paper over the issue that setting the pageblock type >>> based on a reserved page. I agree that compaction should not be returning >>> pfns that are outside of the zone range because that is buggy in itself >>> but valid struct pages should have valid information. I don't think we >>> want to paper over that with unnecessary PageReserved checks. >> >> Agreed as long as we can handle that issue using range checks. >> > > I think it'll be ok as long as the struct pages within a 1<<(MAX_ORDER-1) > range have proper linkages. Agreed. -- Thanks, David / dhildenb