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    SubjectRe: [PATCH v6 08/11] mm, compaction: create compact_gap wrapper
    On Wed, Aug 10, 2016 at 11:12:23AM +0200, Vlastimil Babka wrote:
    > Compaction uses a watermark gap of (2UL << order) pages at various places and
    > it's not immediately obvious why. Abstract it through a compact_gap() wrapper
    > to create a single place with a thorough explanation.
    >
    > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
    > Acked-by: Michal Hocko <mhocko@suse.com>
    > ---
    > include/linux/compaction.h | 16 ++++++++++++++++
    > mm/compaction.c | 7 +++----
    > mm/vmscan.c | 6 +++---
    > 3 files changed, 22 insertions(+), 7 deletions(-)
    >
    > diff --git a/include/linux/compaction.h b/include/linux/compaction.h
    > index a1fba9994728..e7f0d34a90fe 100644
    > --- a/include/linux/compaction.h
    > +++ b/include/linux/compaction.h
    > @@ -58,6 +58,22 @@ enum compact_result {
    >
    > struct alloc_context; /* in mm/internal.h */
    >
    > +/*
    > + * Number of free order-0 pages that should be available above given watermark
    > + * to make sure compaction has reasonable chance of not running out of free
    > + * pages that it needs to isolate as migration target during its work.
    > + */
    > +static inline unsigned long compact_gap(unsigned int order)
    > +{
    > + /*
    > + * Although all the isolations for migration are temporary, compaction
    > + * may have up to 1 << order pages on its list and then try to split
    > + * an (order - 1) free page. At that point, a gap of 1 << order might
    > + * not be enough, so it's safer to require twice that amount.
    > + */
    > + return 2UL << order;
    > +}

    I agree with this wrapper function but there is a question.

    Could you elaborate more on this code comment? Freescanner could keep
    COMPACT_CLUSTER_MAX freepages on the list. It's not associated with
    requested order at least for now. Why compact_gap is 2UL << order in
    this case?

    Thanks.

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