Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and don't read garbage memmaps

[Wei Yang <richard.weiyang@linux.alibaba.com>]

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"?

>
>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.
>

Yes, a different problem, just jump into my mind. Hope this won't affect this
patch.

>Thanks!
>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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