mm: compaction: support non-movable compaction for pageblock requests

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.

Link: https://lore.kernel.org/20260722150006.3848560-3-hannes@cmpxchg.org
Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Gregory Price <gourry@gourry.net>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions