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

[David Hildenbrand <david@redhat.com>]

On 24.11.22 14:22, David Hildenbrand wrote:
> 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.
> 

After some unsuccessful attempts and many head-scratches, I realized 
that it's quite simple why we don't have to worry about swapcache pages 
here:

page_mapping() is != NULL for pages in the swapcache: folio_mapping() 
makes this rather obvious:

if (unlikely(folio_test_swapcache(folio))
	return swap_address_space(folio_swap_entry(folio));


I think the get_page_unless_zero() might also be a fix for the 
page_mapping() call, smells like something could blow up on concurrent 
page freeing. (what about concurrent removal from the swapcache? nobody 
knows :) )


Thanks Gavin!

Acked-by: David Hildenbrand <david@redhat.com>


-- 
Thanks,

David / dhildenb