Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

[David Hildenbrand <david@redhat.com>]

On 24.11.22 13:55, Gavin Shan wrote:
> On 11/24/22 6:43 PM, David Hildenbrand wrote:
>> On 24.11.22 11:21, Gavin Shan wrote:
>>> On 11/24/22 6:09 PM, David Hildenbrand wrote:
>>>> On 24.11.22 10:55, Gavin Shan wrote:
>>>>> The issue is reported when removing memory through virtio_mem device.
>>>>> The transparent huge page, experienced copy-on-write fault, is wrongly
>>>>> regarded as pinned. The transparent huge page is escaped from being
>>>>> isolated in isolate_migratepages_block(). The transparent huge page
>>>>> can't be migrated and the corresponding memory block can't be put
>>>>> into offline state.
>>>>>
>>>>> Fix it by replacing page_mapcount() with total_mapcount(). With this,
>>>>> the transparent huge page can be isolated and migrated, and the memory
>>>>> block can be put into offline state. Besides, The page's refcount is
>>>>> increased a bit earlier to avoid the page is released when the check
>>>>> is executed.
>>>>
>>>> Did you look into handling pages that are in the swapcache case as well?
>>>>
>>>> See is_refcount_suitable() in mm/khugepaged.c.
>>>>
>>>> Should be easy to reproduce, let me know if you need inspiration.
>>>>
>>>
>>> Nope, I didn't look into the case. Please elaborate the details so that
>>> I can reproduce it firstly.
>>
>>
>> A simple reproducer would be (on a system with ordinary swap (not zram))
>>
>> 1) mmap a region (MAP_ANON|MAP_PRIVATE) that can hold a THP
>>
>> 2) Enable THP for that region (MADV_HUGEPAGE)
>>
>> 3) Populate a THP (e.g., write access)
>>
>> 4) PTE-map the THP, for example, using MADV_FREE on the last subpage
>>
>> 5) Trigger swapout of the THP, for example, using MADV_PAGEOUT
>>
>> 6) Read-access to some subpages to fault them in from the swapcache
>>
>>
>> Now you'd have a THP, which
>>
>> 1) Is partially PTE-mapped into the page table
>> 2) Is in the swapcache (each subpage should have one reference from the swapache)
>>
>>
>> Now we could test, if alloc_contig_range() will still succeed (e.g., using virtio-mem).
>>
> 
> Thanks for the details. Step (4) and (5) can be actually combined. To swap part of
> the THP (e.g. one sub-page) will force the THP to be split.
> 
> I followed your steps in the attached program, there is no issue to do memory hot-remove
> through virtio-mem with or without this patch.

Interesting. But I don't really see how we could pass this check with a 
page that's in the swapcache, maybe I'm missing something else.

I'll try to see if I can reproduce it.

-- 
Thanks,

David / dhildenb