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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jul 01, 2026 at 09:47:21PM -0400, Zi Yan wrote:
> On Fri Jun 26, 2026 at 2:21 PM EDT, 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>
> > ---
> >  mm/compaction.c | 35 ++++++++++++++++++++++++++++-------
> >  1 file changed, 28 insertions(+), 7 deletions(-)
> >
> 
> <snip>
> 
> > +
> > +	/*
> > +	 * Prevent small unmovable/reclaimable requests from polluting
> 
> "small" here means smaller than pageblock_order requests? If yes, it is
> better to spell it out to prevent confusion.

Ah, I thought the order < pageblock_order in the code would make it
clear. But I don't mind spelling it out.

I will change this to "<pageblock_order". Shout if you prefer
something else :)

> 
> > +	 * movable blocks through fallbacks. Whole-block production is
> > +	 * exempt as the allocator claims and converts these.
> > +	 */
> > +	if (cc->migratetype == MIGRATE_MOVABLE || cc->order >= pageblock_order)
> >  		return is_migrate_movable(block_mt);
> >  	else
> >  		return block_mt == cc->migratetype;
> > @@ -1974,12 +1995,12 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
> >  		return pfn;
> >  
> >  	/*
> > -	 * Only allow kcompactd and direct requests for movable pages to
> > -	 * quickly clear out a MOVABLE pageblock for allocation. This
> > -	 * reduces the risk that a large movable pageblock is freed for
> > -	 * an unmovable/reclaimable small allocation.
> > +	 * Prevent small unmovable/reclaimable requests from polluting
> 
> This "small" as well.

Yep, will keep these in sync.