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    SubjectRe: [PATCH -mm -v4 01/21] mm, THP, swap: Enable PMD swap operations for CONFIG_THP_SWAP
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    Dave Hansen <dave.hansen@linux.intel.com> writes:

    > On 06/21/2018 08:51 PM, Huang, Ying wrote:
    >> From: Huang Ying <ying.huang@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.

    This looks much better than my original words. Thanks for help!

    > 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.

    Because both you and Dan thinks it's better to add new Kconfig option, I
    will do that.

    Best Regards,
    Huang, Ying

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