Re: [PATCH 2/4] mm: compaction: support non-movable compaction for pageblock requests

[Vlastimil Babka (SUSE)]

On 7/9/26 19:39, Johannes Weiner wrote:
> On Thu, Jul 02, 2026 at 11:28:47AM +0200, Vlastimil Babka (SUSE) wrote:
>> On 7/1/26 23:11, Johannes Weiner wrote:
>> > On Wed, Jul 01, 2026 at 08:14:05PM +0200, Vlastimil Babka (SUSE) wrote:
>> >> 
>> >> > This made the migratetype filtering about preventing block
>> >> > pollution. The patch quotes reduced extfrag numbers.
>> >> > 
>> >> > So now we have a block pollution guard that we apply only if... the
>> >> > scanner is latency sensitive? :) Is this actually desired behavior?
>> >> 
>> >> Yeah indeed I was wondering in this direction.
>> >> 
>> >> > Another way of looking at it would be this:
>> >> > 
>> >> > 	/*
>> >> > 	 * Allocation fallbacks can spread migratable pages
>> >> > 	 * into non-movable blocks.
>> >> 
>> >> But also vice versa, non-movable pages into movable blocks? (without
>> >> defrag_mode?).
>> > 
>> > Uhm but those aren't compactable anymore then, right?
>> 
>> Yeah.
>> 
>> > There is a flipside, but it isn't quite symmetrical. Movable requests
>> > are allowed to look for movables in unmovable blocks once they become
>> > sync (high-effort, low-result). Non-movable requests are finally
>> > allowed to empty movable blocks once they become sync; they *start*
>> > with the high-effort, low-result mode to avoid block contamination but
>> > are allowed to escalate when that doesn't produce results.
>> 
>> Hm, true!
>> 
>> I was also thinking, if defrag mode doesn't allow that escalation ever
>> (IIRC?), should it be reflected here as well even with sync compaction?
> 
> Ok I made a clarification regarding defrag_mode in the other comment.
> 
> How about this?

Looks good.

> diff --git a/mm/compaction.c b/mm/compaction.c
> index 7df3a85d43af..a12a49eefe85 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1399,13 +1399,24 @@ static bool suitable_migration_source(struct compact_control *cc,
>  	if (is_migrate_cma(block_mt) && !(cc->alloc_flags & ALLOC_CMA))
>  		return false;
>  
> +	/*
> +	 * Per default, scans are restricted to blocks compatible with
> +	 * the request, to prevent cross-contamination. Once
> +	 * compaction priority escalates to synchronous scans, though,
> +	 * scan all blocks to try to make forward progress. For
> +	 * movable request, this likely helps little: there shouldn't
> +	 * be many migratable pages inside non-movable blocks besides
> +	 * allocator fallbacks. For non-movable requests, this helps a
> +	 * lot, as they can finally scan movable blocks.
> +	 */
>  	if (cc->mode != MIGRATE_ASYNC)
>  		return true;
>  
>  	/*
>  	 * Prevent small unmovable/reclaimable requests from polluting
> -	 * movable blocks through fallbacks. Whole-block production is
> -	 * exempt as the allocator claims and converts these.
> +	 * movable blocks through fallbacks. Whole-block production
> +	 * (directly requested, or defrag_mode) is exempt as the
> +	 * allocator claims and converts these.
>  	 */
>  	if (cc->migratetype == MIGRATE_MOVABLE || cc->order >= pageblock_order)
>  		return is_migrate_movable(block_mt);