linux-kernel - [PATCH RFC v2 1/3] mm/page_alloc: ignore the exact initial compaction result

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Message-Id: <20251219-thp-thisnode-tweak-v2-1-0c01f231fd1c@suse.cz>
Date: Fri, 19 Dec 2025 18:38:51 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>, 
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>, 
 Zi Yan <ziy@...dia.com>, David Rientjes <rientjes@...gle.com>, 
 David Hildenbrand <david@...nel.org>, 
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
 Mike Rapoport <rppt@...nel.org>, Joshua Hahn <joshua.hahnjy@...il.com>, 
 Pedro Falcato <pfalcato@...e.de>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
 Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH RFC v2 1/3] mm/page_alloc: ignore the exact initial
 compaction result

For allocations that are of costly order and __GFP_NORETRY (and can
perform compaction) we attempt direct compaction first. If that fails,
we continue with a single round of direct reclaim+compaction (as for
other __GFP_NORETRY allocations, except the compaction is of lower
priority), with two exceptions that fail immediately:

- __GFP_THISNODE is specified, to prevent zone_reclaim_mode-like
  behavior for e.g. THP page faults

- compaction failed because it was deferred (i.e. has been failing
  recently so further attempts are not done for a while) or skipped,
  which means there are insufficient free base pages to defragment to
  begin with

Upon closer inspection, the second condition has a somewhat flawed
reasoning. If there are not enough base pages and reclaim could create
them, we instead fail. When there are enough base pages and compaction
has already ran and failed, we proceed and hope that reclaim and the
subsequent compaction attempt will succeed. But it's unclear why they
should and whether it will be as inexpensive as intended.

It might make therefore more sense to just fail unconditionally after
the initial compaction attempt. However that would change the semantics
of __GFP_NORETRY to attempt reclaim at least once.

Alternatively we can remove the compaction result checks and proceed
with the single reclaim and (lower priority) compaction attempt, leaving
only the __GFP_THISNODE exception for failing immediately.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/page_alloc.c | 34 ++++++----------------------------
 1 file changed, 6 insertions(+), 28 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6f5e1b902999..9e7b0967f1b5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4767,44 +4767,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * includes some THP page fault allocations
 		 */
 		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
-			/*
-			 * If allocating entire pageblock(s) and compaction
-			 * failed because all zones are below low watermarks
-			 * or is prohibited because it recently failed at this
-			 * order, fail immediately unless the allocator has
-			 * requested compaction and reclaim retry.
-			 *
-			 * Reclaim is
-			 *  - potentially very expensive because zones are far
-			 *    below their low watermarks or this is part of very
-			 *    bursty high order allocations,
-			 *  - not guaranteed to help because isolate_freepages()
-			 *    may not iterate over freed pages as part of its
-			 *    linear scan, and
-			 *  - unlikely to make entire pageblocks free on its
-			 *    own.
-			 */
-			if (compact_result == COMPACT_SKIPPED ||
-			    compact_result == COMPACT_DEFERRED)
-				goto nopage;
-
 			/*
 			 * THP page faults may attempt local node only first,
 			 * but are then allowed to only compact, not reclaim,
 			 * see alloc_pages_mpol().
 			 *
-			 * Compaction can fail for other reasons than those
-			 * checked above and we don't want such THP allocations
-			 * to put reclaim pressure on a single node in a
-			 * situation where other nodes might have plenty of
-			 * available memory.
+			 * Compaction has failed above and we don't want such
+			 * THP allocations to put reclaim pressure on a single
+			 * node in a situation where other nodes might have
+			 * plenty of available memory.
 			 */
 			if (gfp_mask & __GFP_THISNODE)
 				goto nopage;

 			/*
-			 * Looks like reclaim/compaction is worth trying, but
-			 * sync compaction could be very expensive, so keep
+			 * Proceed with single round of reclaim/compaction, but
+			 * since sync compaction could be very expensive, keep
 			 * using async compaction.
 			 */
 			compact_priority = INIT_COMPACT_PRIORITY;

-- 
2.52.0