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Subject: Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and
 don't read garbage memmaps
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal Hocko <mhocko@suse.com>, stable@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Minchan Kim <minchan@kernel.org>,
        Huang Ying <ying.huang@intel.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>
References: <20200619125923.22602-1-david@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <34f36733-805e-cc61-38da-2ee578ae096c@redhat.com>
Date:   Mon, 22 Jun 2020 11:51:34 +0200
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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 <mhocko@suse.com>
>>>> Cc: stable@vger.kernel.org # v5.2+
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Cc: Minchan Kim <minchan@kernel.org>
>>>> Cc: Huang Ying <ying.huang@intel.com>
>>>> Cc: Wei Yang <richard.weiyang@gmail.com>
>>>> Cc: Mel Gorman <mgorman@techsingularity.net>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>> 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"?

If devmem is removed, the memmap will be freed and the sub-sections are
marked as non-present. So this works as expected.

-- 
Thanks,

David / dhildenb