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

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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@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>

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@xxxxxxxxxx>
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