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

[Zi Yan <ziy@nvidia.com>]

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On 24 Nov 2022, at 5:43, 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

Added the original THP swapout code author, Ying.

At this step, the THP will be split, right?

https://elixir.bootlin.com/linux/latest/source/mm/vmscan.c#L1786

Even if a THP has PMD mapping, IIRC, it is split in the add_to_swap()
then swapped out. But I cannot find that split code now.


>
> 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,
>
> David / dhildenb

--
Best Regards,
Yan Zi

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