Date:   Mon, 22 Jun 2020 21:55:20 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>, stable@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Minchan Kim <minchan@...nel.org>,
        Huang Ying <ying.huang@...el.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and
 don't read garbage memmaps

On Mon, Jun 22, 2020 at 04:06:15PM +0200, David Hildenbrand wrote:
>On 22.06.20 15:10, Wei Yang wrote:
>> 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@...e.com>
>>>>>>> Cc: stable@...r.kernel.org # v5.2+
>>>>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>>>>> Cc: Johannes Weiner <hannes@...xchg.org>
>>>>>>> Cc: Michal Hocko <mhocko@...e.com>
>>>>>>> Cc: Minchan Kim <minchan@...nel.org>
>>>>>>> Cc: Huang Ying <ying.huang@...el.com>
>>>>>>> Cc: Wei Yang <richard.weiyang@...il.com>
>>>>>>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>>>>>>> Signed-off-by: David Hildenbrand <david@...hat.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.
>
>Let me clarify, sub-section hotremove works differently when overlying
>with (online) boot memory within a section.
>
>Early sections (IOW, boot memory) are never partially removed. See

Thanks for your time and patience. 

Looked into the comment of section_deactivate():

 * 1. deactivation of a partial hot-added section (only possible in
 *    the SPARSEMEM_VMEMMAP=y case).
 *      a) section was present at memory init.
 *      b) section was hot-added post memory init.

Case a) seems do partial remove for an early section?

>mm/sparse.c:section_deactivate(). We only free a early memmap when the
>section is completely empty. Also see how

Hmm.. I thought this is the behavior for early section, while it looks current
code doesn't work like this:

       if (section_is_early && memmap)
               free_map_bootmem(memmap);
       else
	       depopulate_section_memmap(pfn, nr_pages, altmap);

section_is_early is always "true" for early section, while memmap is not-NULL
only when sub-section map is empty.

If my understanding is correct, when we remove a sub-section in early section,
the code would call depopulate_section_memmap(), which in turn free related
memmap. By removing the memmap, the return value from pfn_to_online_page() is
not a valid one.

Maybe we want to write the code like this:

       if (section_is_early)
               if (memmap)
                       free_map_bootmem(memmap);
       else
	       depopulate_section_memmap(pfn, nr_pages, altmap);

This makes sure we only free memmap for early section only when the whole
section is removed.

>include/linux/mmzone.h:pfn_valid() handles early sections.
>
>So when we have a partially present section with boot memory, we
>a) marked the whole section present and online (there is only a single
>   bit)
>b) allocated the memmap for the whole section
>c) Only exposed the relevant pages to the buddy. The memmap of non-
>   present parts in a section were initialized and are reserved.
>
>pfn_valid() will return for all non-present pfns valid, because there is
>a memmap. pfn_to_online_page() will return for all pfns "true", because
>we only have a single bit for the whole section. This has been the case
>before sub-section hotplug and is still the case. It simply looks like
>just another memory hole for which we have a memmap.
>
>Now, with devmem it is possible to suddenly change these sub-section
>holes (memmaps) to become ZONE_DEVICE memory. pfn_to_online_page() would
>have to detect that and report a "false". Possible fixes were already
>discussed (e.g., sub-section online map instead of a single bit).
>
>Again, the zone check safes us from the worst, just as in the case of
>all other pfn walkers that use (as documented) pfn_to_online_page(). It
>still needs a fix as dicussed, but it seems to work reasonably fine like
>that for now.
>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me

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