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Thu, 03 Dec 2020 12:48:39 -0500 X-MC-Unique: lMO4LyFNPOGAZZ-MajlGIw-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0F5731800D42; Thu, 3 Dec 2020 17:48:38 +0000 (UTC) Received: from [10.36.113.250] (ovpn-113-250.ams2.redhat.com [10.36.113.250]) by smtp.corp.redhat.com (Postfix) with ESMTP id A9051100239A; Thu, 3 Dec 2020 17:48:36 +0000 (UTC) Subject: Re: [PATCH] mm/page_isolation: do not isolate the max order page To: Vlastimil Babka , Muchun Song , akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joonsoo Kim References: <20201202122114.75316-1-songmuchun@bytedance.com> <1505b548-968b-2053-fd17-0cc4ae240a2f@suse.cz> <29022300-6d8e-0532-7abc-6d11ab1db04a@redhat.com> <92e54bf2-adc5-d51b-3b78-b881567335dc@suse.cz> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <4acd86de-e5a7-7fbe-5cdc-939e12f7fb3a@redhat.com> Date: Thu, 3 Dec 2020 18:48:35 +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: <92e54bf2-adc5-d51b-3b78-b881567335dc@suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 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 03.12.20 18:15, Vlastimil Babka wrote: > On 12/3/20 5:26 PM, David Hildenbrand wrote: >> On 03.12.20 01:03, Vlastimil Babka wrote: >>> On 12/2/20 1:21 PM, Muchun Song wrote: >>>> The max order page has no buddy page and never merge to other order. >>>> So isolating and then freeing it is pointless. >>>> >>>> Signed-off-by: Muchun Song >>> >>> Acked-by: Vlastimil Babka >>> >>>> --- >>>> mm/page_isolation.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c >>>> index a254e1f370a3..bddf788f45bf 100644 >>>> --- a/mm/page_isolation.c >>>> +++ b/mm/page_isolation.c >>>> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype) >>>> */ >>>> if (PageBuddy(page)) { >>>> order = buddy_order(page); >>>> - if (order >= pageblock_order) { >>>> + if (order >= pageblock_order && order < MAX_ORDER - 1) { >>>> pfn = page_to_pfn(page); >>>> buddy_pfn = __find_buddy_pfn(pfn, order); >>>> buddy = page + (buddy_pfn - pfn); >>> >>> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a >>> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would >>> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy) >>> might access an invalid buddy. So this might be actually a bug fix and not just >>> optimization, just the bug hasn't been observed in practice. >> >> I think we have no users that isolate/unisolate close to holes. >> >> CMA regions are properly aligned (to max of page_order / >> max_order_nr_pages) and don't contain holes. > > The problem as I see it, is that buddy_order(page) might be already MAX_ORDER - > 1 (e.g. two pageblocks on x86), and then finding buddy of that one is beyond the > guaranteed alignment (if they merged, which they can't, it would be four Oh, I see. I would have assume that __find_buddy_pfn() would not hand out invalid buddies. But you're right, it's generic: pfn = 1024 (4M) order = MAX_ORDER - 1 = 10 buddy_pfn = __find_buddy_pfn(pfn, order) -> pfn ^ (1 << order) = 0 If that page has no struct page (!pfn_valid), we're doomed, I agree. It would be problematic if we have alloc_contig_range() users with ranges not aligned/multiples of to 8 MB (MAX_ORDER) I guess. virtio-mem and gigantic pages should be fine. CMA might be problematic, though? Do we have such small CMA ranges or with such alignment? COuld be I guess. cma_init_reserved_mem() only checks alignment = PAGE_SIZE << max_t(unsigned long, MAX_ORDER - 1, pageblock_order); > pageblocks). Might not be just a hole within zone, but also across zone boundary? > While being isolated and used pages migrated away, the freed pages shouldn't > merge to MAX_ORDER-1, but if the MAX_ORDER-1 free page was already there before > the isolation? -- Thanks, David / dhildenb