On 06/21/2018 08:51 PM, Huang, Ying wrote:
> From: Huang Ying <ying.hu...@intel.com>
> 
> Previously, the PMD swap operations are only enabled for
> CONFIG_ARCH_ENABLE_THP_MIGRATION.  Because they are only used by the
> THP migration support.  We will support PMD swap mapping to the huge
> swap cluster and swapin the THP as a whole.  That will be enabled via
> CONFIG_THP_SWAP and needs these PMD swap operations.  So enable the
> PMD swap operations for CONFIG_THP_SWAP too.

This commit message kinda skirts around the real reasons for this patch.
 Shouldn't we just say something like:

        Currently, "swap entries" in the page tables are used for a
        number of things outside of actual swap, like page migration.
        We support THP/PMD "swap entries" for page migration currently
        and the functions behind this are tied to page migration's
        config option (CONFIG_ARCH_ENABLE_THP_MIGRATION).

        But, we also need them for THP swap.
        ...

It would also be nice to explain a bit why you are moving code around.

Would this look any better if we made a Kconfig option:

        config HAVE_THP_SWAP_ENTRIES
                def_bool n
                # "Swap entries" in the page tables are used
                # both for migration and actual swap.
                depends on THP_SWAP || ARCH_ENABLE_THP_MIGRATION

You logically talked about this need for PMD swap operations in your
commit message, so I think it makes sense to codify that in a single
place where it can be coherently explained.

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