linux-kernel - Re: [PATCH 1/2] mm/migrate_device.c: Copy pte dirty bit to page

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Date:   Tue, 16 Aug 2022 10:51:57 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, linuxram@...ibm.com,
        paulus@...abs.org
Subject: Re: [PATCH 1/2] mm/migrate_device.c: Copy pte dirty bit to page


Peter Xu <peterx@...hat.com> writes:

> Hi, Alistair,
>
> On Fri, Aug 12, 2022 at 03:22:30PM +1000, Alistair Popple wrote:
>> migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that
>> installs migration entries directly if it can lock the migrating page.
>> When removing a dirty pte the dirty bit is supposed to be carried over
>> to the underlying page to prevent it being lost.
>>
>> Currently migrate_vma_*() can only be used for private anonymous
>> mappings. That means loss of the dirty bit usually doesn't result in
>> data loss because these pages are typically not file-backed. However
>> pages may be backed by swap storage which can result in data loss if an
>> attempt is made to migrate a dirty page that doesn't yet have the
>> PageDirty flag set.
>>
>> In this case migration will fail due to unexpected references but the
>> dirty pte bit will be lost. If the page is subsequently reclaimed data
>> won't be written back to swap storage as it is considered uptodate,
>> resulting in data loss if the page is subsequently accessed.
>>
>> Prevent this by copying the dirty bit to the page when removing the pte
>> to match what try_to_migrate_one() does.
>>
>> Signed-off-by: Alistair Popple <apopple@...dia.com>
>> Reported-by: Peter Xu <peterx@...hat.com>
>
> This line should be:
>
> Reported-by: Huang Ying <ying.huang@...el.com>
>
> Please also feel free to add:
>
> Acked-by: Peter Xu <peterx@...hat.com>

Thanks Peter, my bad. I assume Andrew can fix up the tags if I don't
need to re-spin this series.

> Thanks,