Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to helpers

[Andrew Morton]

On Tue,  5 Apr 2016 13:25:31 +0200 Michal Hocko <mho...@kernel.org> wrote:

> From: Michal Hocko <mho...@suse.com>
> 
> Compaction can provide a wild variation of feedback to the caller. Many
> of them are implementation specific and the caller of the compaction
> (especially the page allocator) shouldn't be bound to specifics of the
> current implementation.
> 
> This patch abstracts the feedback into three basic types:
>       - compaction_made_progress - compaction was active and made some
>         progress.
>       - compaction_failed - compaction failed and further attempts to
>         invoke it would most probably fail and therefore it is not
>         worth retrying
>       - compaction_withdrawn - compaction wasn't invoked for an
>           implementation specific reasons. In the current implementation
>           it means that the compaction was deferred, contended or the
>           page scanners met too early without any progress. Retrying is
>           still worthwhile.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3362,25 +3362,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int 
> order,
>       if (page)
>               goto got_pg;
>  
> -     /* Checks for THP-specific high-order allocations */
> -     if (is_thp_gfp_mask(gfp_mask)) {
> -             /*
> -              * If compaction is deferred for high-order allocations, it is
> -              * because sync compaction recently failed. If this is the case
> -              * and the caller requested a THP allocation, we do not want
> -              * to heavily disrupt the system, so we fail the allocation
> -              * instead of entering direct reclaim.
> -              */
> -             if (compact_result == COMPACT_DEFERRED)
> -                     goto nopage;
> -
> -             /*
> -              * Compaction is contended so rather back off than cause
> -              * excessive stalls.
> -              */
> -             if(compact_result == COMPACT_CONTENDED)
> -                     goto nopage;
> -     }
> +     /*
> +      * Checks for THP-specific high-order allocations and back off
> +      * if the the compaction backed off
> +      */
> +     if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(compact_result))
> +             goto nopage;

This change smashed into Hugh's "huge tmpfs: shmem_huge_gfpmask and
shmem_recovery_gfpmask".

I ended up doing this:

        /* Checks for THP-specific high-order allocations */
        if (!is_thp_allocation(gfp_mask, order))
                migration_mode = MIGRATE_SYNC_LIGHT;

        /*
         * Checks for THP-specific high-order allocations and back off
         * if the the compaction backed off
         */
        if (is_thp_allocation(gfp_mask) && compaction_withdrawn(compact_result))
                goto nopage;