Re: [PATCH v4 4/8] mm/rmap: Split migration into its own function

[Ralph Campbell <rcampbell@nvidia.com>]

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>