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

[Johannes Weiner <hannes@cmpxchg.org>]

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?

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);