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Thu, 01 Oct 2020 04:47:20 -0400 X-MC-Unique: AiKM0gHyPWWc-Ae5ViC31Q-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5C7E19CC03; Thu, 1 Oct 2020 08:47:18 +0000 (UTC) Received: from [10.36.114.102] (ovpn-114-102.ams2.redhat.com [10.36.114.102]) by smtp.corp.redhat.com (Postfix) with ESMTP id 93B107515B; Thu, 1 Oct 2020 08:47:15 +0000 (UTC) Subject: Re: [PATCH 9/9] mm, page_alloc: optionally disable pcplists during page isolation To: Vlastimil Babka , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Michal Hocko , Pavel Tatashin , Oscar Salvador , Joonsoo Kim , Michal Hocko References: <20200922143712.12048-1-vbabka@suse.cz> <20200922143712.12048-10-vbabka@suse.cz> <10cdae53-c64b-e371-1b83-01d1af7a131e@redhat.com> <2ce92f9a-eaa2-45b2-207c-46a79d6a2bde@suse.cz> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <2ce92f9a-eaa2-45b2-207c-46a79d6a2bde@suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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 25.09.20 13:10, Vlastimil Babka wrote: > On 9/25/20 12:54 PM, David Hildenbrand wrote: >>>> --- a/mm/page_isolation.c >>>> +++ b/mm/page_isolation.c >>>> @@ -15,6 +15,22 @@ >>>> #define CREATE_TRACE_POINTS >>>> #include >>>> >>>> +void zone_pcplist_disable(struct zone *zone) >>>> +{ >>>> + down_read(&pcp_batch_high_lock); >>>> + if (atomic_inc_return(&zone->pcplist_disabled) == 1) { >>>> + zone_update_pageset_high_and_batch(zone, 0, 1); >>>> + __drain_all_pages(zone, true); >>>> + } >>> Hm, if one CPU is still inside the if-clause, the other one would >>> continue, however pcp wpould not be disabled and zones not drained when >>> returning. > > Ah, well spotted, thanks! > >>> (while we only allow a single Offline_pages() call, it will be different >>> when we use the function in other context - especially, >>> alloc_contig_range() for some users) >>> >>> Can't we use down_write() here? So it's serialized and everybody has to >>> properly wait. (and we would not have to rely on an atomic_t) >> Sorry, I meant down_write only temporarily in this code path. Not >> keeping it locked in write when returning (I remember there is a way to >> downgrade). > > Hmm that temporary write lock would still block new callers until previous > finish with the downgraded-to-read lock. > > But I guess something like this would work: > > retry: > if (atomic_read(...) == 0) { > // zone_update... + drain > atomic_inc(...); > else if (atomic_inc_return == 1) > // atomic_cmpxchg from 0 to 1; if that fails, goto retry > > Tricky, but races could only read to unnecessary duplicated updates + flushing > but nothing worse? > > Or add another spinlock to cover this part instead of the temp write lock... My gut feeling is, that that would be the cleanest approach. -- Thanks, David / dhildenb