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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jul 01, 2026 at 04:19:29PM +0200, Vlastimil Babka (SUSE) wrote:
> On 6/26/26 20:21, Johannes Weiner wrote:
> > While trying to fix a reclaim storm in defrag_mode, I noticed that
> > non-movable direct compaction is extremely inefficient.
> > 
> > When searching for space to evacuate, compaction only allows blocks of
> > the same type as the incoming request. This is to prevent migratetype
> > pollution, where a small non-movable request frees space in a movable
> > block and provokes the allocator to fall back and pollute it.
> > 
> > This protection is reasonable on one hand, but the downside is that it
> > makes non-movable direct compaction nearly useless: if we get the type
> > annotations right, by definition there aren't any movable pages inside
> > the non-movable blocks it is allowed to scan.
> > 
> > With defrag_mode, the goal is the production of whole blocks, which
> > are essentially type neutral: __rmqueue_claim() will convert them
> > wholesale on alloc. This makes type mixing and pollution a non-issue.
> > 
> > Fix the pollution gates to take the requested order into account, and
> > allow whole-block requests to scan blocks of other types.
> > 
> > The only exception is CMA blocks. That type is sticky and these blocks
> > cannot be claimed to other types. Continue to be strict with them, and
> > allow only explicit ALLOC_CMA requests and kcompactd to evacuate them.
> > 
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Thanks!

> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index f08765ade014..7df3a85d43af 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1381,12 +1381,33 @@ static bool suitable_migration_source(struct compact_control *cc,
> >  	if (pageblock_skip_persistent(page))
> >  		return false;
> >  
> > -	if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
> > +	/*
> > +	 * Background compaction produces blocks for the zone at
> > +	 * large, with no particular allocation context. Allow all
> > +	 * block types, including CMA.
> > +	 */
> > +	if (!cc->direct_compaction)
> >  		return true;
> >  
> >  	block_mt = get_pageblock_migratetype(page);
> >  
> > -	if (cc->migratetype == MIGRATE_MOVABLE)
> > +	/*
> > +	 * CMA pages can only be taken by ALLOC_CMA requests. For anybody
> > +	 * else, vacating a CMA block consumes free pages the caller
> > +	 * could have used, and produces free pages it cannot.
> > +	 */
> > +	if (is_migrate_cma(block_mt) && !(cc->alloc_flags & ALLOC_CMA))
> > +		return false;
> > +
> > +	if (cc->mode != MIGRATE_ASYNC)
> > +		return true;
> 
> This now stands out as uncommented. Can we come up with a rationale? :)

Let's see. Originally it came from here:

commit 9927af740b1b9b1e769310bd0b91425e8047b803
Author: Mel Gorman <mel@csn.ul.ie>
Date:   Thu Jan 13 15:45:59 2011 -0800

    mm: compaction: perform a faster migration scan when migrating asynchronously

This limited async scanners to movable blocks. By keeping them to the
most productive space, it keeps their latencies down.

But then there was a follow up here:

commit 282722b0d258ec23fc79d80165418fee83f01736
Author: Vlastimil Babka <vbabka@kernel.org>
Date:   Mon May 8 15:54:49 2017 -0700

    mm, compaction: restrict async compaction to pageblocks of same migratetype

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?

Another way of looking at it would be this:

	/*
	 * Allocation fallbacks can spread migratable pages
	 * into non-movable blocks. This is high-effort,
	 * low-result work. Restrict it to sync scans.
	 */