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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jul 01, 2026 at 08:14:05PM +0200, Vlastimil Babka (SUSE) wrote:
> On 7/1/26 17:28, Johannes Weiner wrote:
> > 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
> 
> Aha :)
> 
> > 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?

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.

I'll try to work that second part into the comment as well.