Re: [PATCH v17 03/21] mm/compaction: correct the comments of compact_defer_shift

[Alexander Duyck <alexander.duyck@gmail.com>]

On Sat, Jul 25, 2020 at 6:00 AM Alex Shi <alex.shi@linux.alibaba.com> wrote:
>
> There is no compact_defer_limit. It should be compact_defer_shift in
> use. and add compact_order_failed explanation.
>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  include/linux/mmzone.h | 1 +
>  mm/compaction.c        | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index f6f884970511..14c668b7e793 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -512,6 +512,7 @@ struct zone {
>          * On compaction failure, 1<<compact_defer_shift compactions
>          * are skipped before trying again. The number attempted since
>          * last failure is tracked with compact_considered.
> +        * compact_order_failed is the minimum compaction failed order.
>          */
>         unsigned int            compact_considered;
>         unsigned int            compact_defer_shift;
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 86375605faa9..cd1ef9e5e638 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -136,7 +136,7 @@ void __ClearPageMovable(struct page *page)
>
>  /*
>   * Compaction is deferred when compaction fails to result in a page
> - * allocation success. 1 << compact_defer_limit compactions are skipped up
> + * allocation success. compact_defer_shift++, compactions are skipped up
>   * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
>   */
>  void defer_compaction(struct zone *zone, int order)

So this doesn't read right. I wouldn't keep the "++," in the
explanation, and if we are going to refer to a limit of "1 <<
COMPACT_MAX_DEFER_SHIFT" then maybe this should be left as "1 <<
compact_defer_shift".