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

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Date:   Thu, 24 Nov 2022 11:43:07 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Gavin Shan <gshan@...hat.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        william.kucharski@...cle.com, ziy@...dia.com,
        kirill.shutemov@...ux.intel.com, zhenyzha@...hat.com,
        apopple@...dia.com, hughd@...gle.com, willy@...radead.org,
        shan.gavin@...il.com
Subject: Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

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,

David / dhildenb