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Date:   Mon, 15 Aug 2022 16:29:20 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Alistair Popple <apopple@nvidia.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        John Hubbard <jhubbard@nvidia.com>,
        David Hildenbrand <david@redhat.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Matthew Wilcox <willy@infradead.org>,
        Karol Herbst <kherbst@redhat.com>,
        Lyude Paul <lyude@redhat.com>, Ben Skeggs <bskeggs@redhat.com>,
        Logan Gunthorpe <logang@deltatee.com>, linuxram@us.ibm.com,
        paulus@ozlabs.org
Subject: Re: [PATCH 1/2] mm/migrate_device.c: Copy pte dirty bit to page
Message-ID: <YvqsoL7fKJzEqTA3@xz-m1.local>
References: <a9daea363991c023d0364be22a762405b6c6f5c4.1660281458.git-series.apopple@nvidia.com>
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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@nvidia.com>
> Reported-by: Peter Xu <peterx@redhat.com>

This line should be:

Reported-by: Huang Ying <ying.huang@intel.com>

Please also feel free to add:

Acked-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu