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@xxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> 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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