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Thu, 03 Dec 2020 14:24:04 -0500 X-MC-Unique: nSGgsFzVPm6kO0YBJNaOag-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 04B328581A4; Thu, 3 Dec 2020 19:24:03 +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 7E456189BB; Thu, 3 Dec 2020 19:24:01 +0000 (UTC) Subject: Re: [PATCH] mm/page_isolation: do not isolate the max order page From: David Hildenbrand 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> <4acd86de-e5a7-7fbe-5cdc-939e12f7fb3a@redhat.com> Organization: Red Hat GmbH Message-ID: Date: Thu, 3 Dec 2020 20:24:00 +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: <4acd86de-e5a7-7fbe-5cdc-939e12f7fb3a@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 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 18:48, David Hildenbrand wrote: > 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); > Thinking again (SPARSE), we always end up in a single memory section. Usually, all pfns within a single section are valid. The only exception is with HAVE_ARCH_PFN_VALID - arm and arm6. arm64 also has HOLES_IN_ZONE - so we always check for pfn_valid() in this code. arm only has HAVE_ARCH_PFN_VALID with SPARSE on ARCH_OMAP1. So only in that combination, we might run into that issue if I am not wrong. Not sure about !SPARSE and mips. -- Thanks, David / dhildenb