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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>
 <20200619125923.22602-2-david@redhat.com>
 <20200622082635.GA93552@L-31X9LVDL-1304.local>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <2185539f-b210-5d3f-5da2-a497b354eebb@redhat.com>
Date:   Mon, 22 Jun 2020 10:43:11 +0200
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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.

This is, however, a different problem to solve and affects most other
pfn walkers as well. The "if (page_zone(page) != zone)" checks guards us
from most harm, as the devmem zone won't match.

Thanks!

-- 
Thanks,

David / dhildenb