Re: [PATCH v2 2/4] mm: compaction: support non-movable compaction for pageblock requests

[Gregory Price <gourry@gourry.net>]

On Wed, Jul 22, 2026 at 10:56:45AM -0400, 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.
> 
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

The CMA interaction took a second to grok but this makes sense.

Reviewed-by: Gregory Price <gourry@gourry.net>