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Thu, 10 Sep 2020 08:42:07 -0400 X-MC-Unique: E2iP_pULPrGCke5S3vSm8g-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 161578015DB; Thu, 10 Sep 2020 12:42:05 +0000 (UTC) Received: from [10.36.113.88] (ovpn-113-88.ams2.redhat.com [10.36.113.88]) by smtp.corp.redhat.com (Postfix) with ESMTP id 313C55C5AF; Thu, 10 Sep 2020 12:42:01 +0000 (UTC) Subject: Re: [RFC 5/5] mm, page_alloc: disable pcplists during page isolation To: Vlastimil Babka , Michal Hocko Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pavel Tatashin , Oscar Salvador , Joonsoo Kim , Mel Gorman References: <20200907163628.26495-1-vbabka@suse.cz> <20200907163628.26495-6-vbabka@suse.cz> <20200909113647.GG7348@dhcp22.suse.cz> <5c5753b4-8cb9-fa02-a0fa-d5ca22731cbb@redhat.com> <3d3b53db-aeaa-ff24-260b-36427fac9b1c@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: <3d3b53db-aeaa-ff24-260b-36427fac9b1c@suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10.09.20 13:05, Vlastimil Babka wrote: > On 9/10/20 12:29 PM, David Hildenbrand wrote: >> On 09.09.20 13:55, Vlastimil Babka wrote: >>> On 9/9/20 1:36 PM, Michal Hocko wrote: >>>> On Wed 09-09-20 12:48:54, Vlastimil Babka wrote: >>>>> Here's a version that will apply on top of next-20200908. The first 4 patches need no change. >>>>> For callers that pair start_isolate_page_range() with >>>>> undo_isolated_page_range() properly, this is transparent. Currently that's >>>>> alloc_contig_range(). __offline_pages() doesn't call undo_isolated_page_range() >>>>> in the succes case, so it has to be carful to handle restoring pcp->high and batch >>>>> and unlocking pcp_batch_high_lock. >>>> >>>> I was hoping that it would be possible to have this completely hidden >>>> inside start_isolate_page_range code path. >>> >>> I hoped so too, but we can't know the moment when all processes that were in the >>> critical part of freeing pages to pcplists have moved on (they might have been >>> rescheduled). >>> We could change free_unref_page() to disable IRQs sooner, before >>> free_unref_page_prepare(), or at least the get_pfnblock_migratetype() part. Then >>> after the single drain, we should be safe, AFAICS? >> >> At least moving it before getting the migratetype should not be that severe? >> >>> RT guys might not be happy though, but it's much simpler than this patch. I >>> still like some of the cleanups in 1-4 though tbh :) >> >> It would certainly make this patch much simpler. Do you have a prototype >> lying around? > > Below is the critical part, while writing it I realized that there's also > free_unref_page_list() where it's more ugly. > > So free_unref_page() simply moves the "determine migratetype" part under > disabled interrupts. For free_unref_page_list() we optimistically determine them > without disabling interrupts, and with disabled interrupts we check if zone has > isolated pageblocks and thus we should not trust that value anymore. That's same > pattern as free_pcppages_bulk uses. > > But unfortunately it means an extra page_zone() plus a test for each page on the > list :/ That function is mostly called from mm/vmscan.c AFAIKS. The fist thing free_unref_page_commit() does is doing page_zone(). That's easy to optimize if needed. > > ----8<---- > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index defefed79cfb..57e2a341c95c 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -3103,18 +3103,21 @@ void mark_free_pages(struct zone *zone) > } > #endif /* CONFIG_PM */ > > -static bool free_unref_page_prepare(struct page *page, unsigned long pfn) > +static bool free_unref_page_prepare(struct page *page) > { > - int migratetype; > - > if (!free_pcp_prepare(page)) > return false; > > - migratetype = get_pfnblock_migratetype(page, pfn); > - set_pcppage_migratetype(page, migratetype); > return true; > } > > +static inline void free_unref_page_set_migratetype(struct page *page, unsigned long pfn) > +{ > + int migratetype = get_pfnblock_migratetype(page, pfn); > + > + set_pcppage_migratetype(page, migratetype); > +} > + > static void free_unref_page_commit(struct page *page, unsigned long pfn) > { > struct zone *zone = page_zone(page); > @@ -3156,10 +3159,17 @@ void free_unref_page(struct page *page) > unsigned long flags; > unsigned long pfn = page_to_pfn(page); > > - if (!free_unref_page_prepare(page, pfn)) > + if (!free_unref_page_prepare(page)) > return; > > + /* > + * by disabling interrupts before reading pageblock's migratetype, > + * we can guarantee that after changing a pageblock to MIGRATE_ISOLATE > + * and drain_all_pages(), there's nobody who would read the old > + * migratetype and put a page from isoalted pageblock to pcplists > + */ > local_irq_save(flags); > + free_unref_page_set_migratetype(page, pfn); > free_unref_page_commit(page, pfn); > local_irq_restore(flags); > } > @@ -3176,9 +3186,10 @@ void free_unref_page_list(struct list_head *list) > /* Prepare pages for freeing */ > list_for_each_entry_safe(page, next, list, lru) { > pfn = page_to_pfn(page); > - if (!free_unref_page_prepare(page, pfn)) > + if (!free_unref_page_prepare(page)) > list_del(&page->lru); > set_page_private(page, pfn); > + free_unref_page_set_migratetype(page, pfn); > } > > local_irq_save(flags); > @@ -3187,6 +3198,8 @@ void free_unref_page_list(struct list_head *list) > > set_page_private(page, 0); > trace_mm_page_free_batched(page); > + if (has_isolate_pageblock(page_zone(page))) > + free_unref_page_set_migratetype(page, pfn); > free_unref_page_commit(page, pfn); > FWIW: You can safely add unlikely() for the has_isolate_pageblock(), as for most other users of that function I was able to spot. -- Thanks, David / dhildenb