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Subject: Re: [PATCH v4 4/8] mm/rmap: Split migration into its own function
To:     Alistair Popple <apopple@nvidia.com>, <linux-mm@kvack.org>,
        <nouveau@lists.freedesktop.org>, <bskeggs@redhat.com>,
        <akpm@linux-foundation.org>
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        <kvm-ppc@vger.kernel.org>, <dri-devel@lists.freedesktop.org>,
        <jhubbard@nvidia.com>, <jglisse@redhat.com>,
        Christoph Hellwig <hch@lst.de>
References: <20210304061645.29747-1-apopple@nvidia.com>
 <20210304061645.29747-5-apopple@nvidia.com>
From:   Ralph Campbell <rcampbell@nvidia.com>
Message-ID: <9fa77684-149a-e6ed-296d-dc852aecea97@nvidia.com>
Date:   Mon, 8 Mar 2021 10:58:12 -0800
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On 3/3/21 10:16 PM, Alistair Popple wrote:
> Migration is currently implemented as a mode of operation for
> try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag
> or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE.
> 
> However it does not have much in common with the rest of the unmap
> functionality of try_to_unmap_one() and thus splitting it into a
> separate function reduces the complexity of try_to_unmap_one() making it
> more readable.
> 
> Several simplifications can also be made in try_to_migrate_one() based
> on the following observations:
> 
>   - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK.
>   - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON.
>   - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH.
> 
> TTU_SPLIT_FREEZE is a special case of migration used when splitting an
> anonymous page. This is most easily dealt with by calling the correct
> function from unmap_page() in mm/huge_memory.c  - either
> try_to_migrate() for PageAnon or try_to_unmap().
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Looks reasonable to me. I do worry a bit about code duplication.
At some point in the future, it might be discovered that other combinations
of TTU_XXX flags are needed in which case a careful check of try_to_migrate()
and try_to_unmap() will be needed.

Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>