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Mon, 22 Jun 2020 10:12:10 -0400 X-MC-Unique: PhNvIBVRM1a0F2SVErre5w-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9E7EA100CCC5; Mon, 22 Jun 2020 14:12:08 +0000 (UTC) Received: from [10.36.113.213] (ovpn-113-213.ams2.redhat.com [10.36.113.213]) by smtp.corp.redhat.com (Postfix) with ESMTP id 763D360C47; Mon, 22 Jun 2020 14:12:01 +0000 (UTC) Subject: Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and don't read garbage memmaps To: Wei Yang Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michal Hocko , stable@vger.kernel.org, Andrew Morton , Johannes Weiner , Minchan Kim , Huang Ying , Wei Yang , Mel Gorman References: <20200619125923.22602-1-david@redhat.com> <20200619125923.22602-2-david@redhat.com> <20200622082635.GA93552@L-31X9LVDL-1304.local> <2185539f-b210-5d3f-5da2-a497b354eebb@redhat.com> <20200622092221.GA96699@L-31X9LVDL-1304.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200622092221.GA96699@L-31X9LVDL-1304.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Rspamd-Queue-Id: 09D2B1803AF01 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 22.06.20 11:22, Wei Yang wrote: > On Mon, Jun 22, 2020 at 10:43:11AM +0200, David Hildenbrand wrote: >> On 22.06.20 10:26, Wei Yang wrote: >>> On Fri, Jun 19, 2020 at 02:59:20PM +0200, David Hildenbrand wrote: >>>> Especially with memory hotplug, we can have offline sections (with a >>>> garbage memmap) and overlapping zones. We have to make sure to only >>>> touch initialized memmaps (online sections managed by the buddy) and that >>>> the zone matches, to not move pages between zones. >>>> >>>> To test if this can actually happen, I added a simple >>>> BUG_ON(page_zone(page_i) != page_zone(page_j)); >>>> right before the swap. When hotplugging a 256M DIMM to a 4G x86-64 VM and >>>> onlining the first memory block "online_movable" and the second memory >>>> block "online_kernel", it will trigger the BUG, as both zones (NORMAL >>>> and MOVABLE) overlap. >>>> >>>> This might result in all kinds of weird situations (e.g., double >>>> allocations, list corruptions, unmovable allocations ending up in the >>>> movable zone). >>>> >>>> Fixes: e900a918b098 ("mm: shuffle initial free memory to improve memory-side-cache utilization") >>>> Acked-by: Michal Hocko >>>> Cc: stable@vger.kernel.org # v5.2+ >>>> Cc: Andrew Morton >>>> Cc: Johannes Weiner >>>> Cc: Michal Hocko >>>> Cc: Minchan Kim >>>> Cc: Huang Ying >>>> Cc: Wei Yang >>>> Cc: Mel Gorman >>>> Signed-off-by: David Hildenbrand >>>> --- >>>> mm/shuffle.c | 18 +++++++++--------- >>>> 1 file changed, 9 insertions(+), 9 deletions(-) >>>> >>>> diff --git a/mm/shuffle.c b/mm/shuffle.c >>>> index 44406d9977c77..dd13ab851b3ee 100644 >>>> --- a/mm/shuffle.c >>>> +++ b/mm/shuffle.c >>>> @@ -58,25 +58,25 @@ module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400); >>>> * For two pages to be swapped in the shuffle, they must be free (on a >>>> * 'free_area' lru), have the same order, and have the same migratetype. >>>> */ >>>> -static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order) >>>> +static struct page * __meminit shuffle_valid_page(struct zone *zone, >>>> + unsigned long pfn, int order) >>>> { >>>> - struct page *page; >>>> + struct page *page = pfn_to_online_page(pfn); >>> >>> Hi, David and Dan, >>> >>> One thing I want to confirm here is we won't have partially online section, >>> right? We can add a sub-section to system, but we won't manage it by buddy. >> >> Hi, >> >> there is still a BUG with sub-section hot-add (devmem), which broke >> pfn_to_online_page() in corner cases (especially, see the description in >> include/linux/mmzone.h). We can have a boot-memory section partially >> populated and marked online. Then, we can hot-add devmem, marking the >> remaining pfns valid - and as the section is maked online, also as online. > > Oh, yes, I see this description. > > This means we could have section marked as online, but with a sub-section even > not added. > > While the good news is even the sub-section is not added, but its memmap is > populated for an early section. So the page returned from pfn_to_online_page() > is a valid one. > > But what would happen, if the sub-section is removed after added? Would > section_deactivate() release related memmap to this "struct page"? Just to clarify now that I get your point: No it would not, as it is an early section, and the early section is not completely empty. -- Thanks, David / dhildenb