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

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Date:   Thu, 24 Nov 2022 20:55:39 +0800
From:   Gavin Shan <gshan@...hat.com>
To:     David Hildenbrand <david@...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 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.

    # numactl -p 1 testsuite mm swap -k
    Any key to split THP
    Any key to swap sub-pages
    Any key to read the swapped sub-pages
        Page[000]: 0xffffffffffffffff
        Page[001]: 0xffffffffffffffff
          :
        Page[255]: 0xffffffffffffffff
    Any key to exit                                // hold here and the program doesn't exit

    (qemu) qom-set vm1 requested-size 0
    [  356.005396] virtio_mem virtio1: plugged size: 0x40000000
    [  356.005996] virtio_mem virtio1: requested size: 0x0
    [  356.350299] Fallback order for Node 0: 0 1
    [  356.350810] Fallback order for Node 1: 1 0
    [  356.351260] Built 2 zonelists, mobility grouping on.  Total pages: 491343
    [  356.351998] Policy zone: DMA

Thanks,
Gavin

View attachment "swap.c" of type "text/x-csrc" (3258 bytes)