Re: [RFC PATCH v2 4/4] mm: compaction: skip unmovable blocks for non-costly proactive mTHP compaction

[Dev Jain]

On 01/10/26 5:44 am, Bo Zhang wrote:
> Proactive compaction scans all pageblocks including unmovable/reclaimable
> ones. For non-costly mTHP orders (<= PAGE_ALLOC_COSTLY_ORDER), those blocks
> are dominated by non-migratable kernel allocations (slab, page tables) and
> scanning them wastes cycles: a small order is easily satisfied from movable
> blocks alone, so there is no need to scavenge the few migratable fallback
> pages inside unmovable blocks.
> 
> Costly orders keep scanning all blocks: they are hard to form and benefit
> from any migratable page available, including allocator fallbacks inside
> unmovable blocks.

I think this is a reasonable compromise for now.

> 
> Restrict non-costly proactive compaction to movable/CMA blocks.
> 
> Signed-off-by: Bo Zhang <zhangbo56@xxxxxxxxxx>
> ---
>  mm/compaction.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 0ff454ee5604..deddf931d0f2 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1404,10 +1404,19 @@ static bool suitable_migration_source(struct compact_control *cc,
>  							struct page *page)
>  {
>  	int block_mt;
> +	unsigned int order = compact_hpage_order();
> +	bool costly = order > PAGE_ALLOC_COSTLY_ORDER;
>  
>  	if (pageblock_skip_persistent(page))
>  		return false;
>  
> +	if (is_via_compact_memory(cc->order) && !costly) {
> +		block_mt = get_pageblock_migratetype(page);
> +		if (block_mt == MIGRATE_MOVABLE || is_migrate_cma(block_mt))
> +			return true;
> +		return false;
> +	}
> +
>  	/*
>  	 * Background compaction produces blocks for the zone at
>  	 * large, with no particular allocation context. Allow al