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 11:51:34AM +0200, David Hildenbrand wrote:
>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.
>

Sorry, I may not catch your point. If my understanding is correct, the
above behavior happens in function section_deactivate().

Let me draw my understanding of function section_deactivate():

    section_deactivate(pfn, nr_pages)
        clear_subsection_map(pfn, nr_pages)
	depopulate_section_memmap(pfn, nr_pages)

Since we just remove a sub-section, I skipped some un-related codes. These two
functions would:

  * clear bitmap in ms->usage->subsection_map
  * free memmap for the sub-section

While since the section is not empty, ms->section_mem_map is not set no null.

Per my understanding, the section present state is set in ms->section_mem_map
with SECTION_MARKED_PRESENT. It looks we don't clear it when just remote a
sub-section.

Do I miss something?

>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me