From: Hugh Dickins <hughd@google.com>
Date: Mon, 8 Sep 2025 22:23:15 +0000 (-0700)
Subject: mm: folio_may_be_lru_cached() unless folio_test_large()
X-Git-Tag: v6.1.155~53
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=85fc33541cc56d364ec0c6b9cf9088674d9eb4c5;p=thirdparty%2Fkernel%2Fstable.git

mm: folio_may_be_lru_cached() unless folio_test_large()

[ Upstream commit 2da6de30e60dd9bb14600eff1cc99df2fa2ddae3 ]

mm/swap.c and mm/mlock.c agree to drain any per-CPU batch as soon as a
large folio is added: so collect_longterm_unpinnable_folios() just wastes
effort when calling lru_add_drain[_all]() on a large folio.

But although there is good reason not to batch up PMD-sized folios, we
might well benefit from batching a small number of low-order mTHPs (though
unclear how that "small number" limitation will be implemented).

So ask if folio_may_be_lru_cached() rather than !folio_test_large(), to
insulate those particular checks from future change.  Name preferred to
"folio_is_batchable" because large folios can well be put on a batch: it's
just the per-CPU LRU caches, drained much later, which need care.

Marked for stable, to counter the increase in lru_add_drain_all()s from
"mm/gup: check ref_count instead of lru before migration".

Link: https://lkml.kernel.org/r/57d2eaf8-3607-f318-e0c5-be02dce61ad0@google.com
Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region")
Signed-off-by: Hugh Dickins <hughd@google.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Keir Fraser <keirf@google.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Li Zhe <lizhe.67@bytedance.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shivank Garg <shivankg@amd.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Xu <weixugc@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: yangge <yangge1116@126.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ Resolved conflicts in mm/swap.c; left "page" parts of mm/mlock.c as is ]
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

diff --git a/include/linux/swap.h b/include/linux/swap.h
index add47f43e568e..3eecf97dfbb8d 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -392,6 +392,16 @@ void lru_cache_add(struct page *);
 void mark_page_accessed(struct page *);
 void folio_mark_accessed(struct folio *);
 
+static inline bool folio_may_be_lru_cached(struct folio *folio)
+{
+	/*
+	 * Holding PMD-sized folios in per-CPU LRU cache unbalances accounting.
+	 * Holding small numbers of low-order mTHP folios in per-CPU LRU cache
+	 * will be sensible, but nobody has implemented and tested that yet.
+	 */
+	return !folio_test_large(folio);
+}
+
 extern atomic_t lru_disable_count;
 
 static inline bool lru_cache_disabled(void)
diff --git a/mm/gup.c b/mm/gup.c
index e1f125af9c844..b02993c9a8cdf 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1990,13 +1990,13 @@ static unsigned long collect_longterm_unpinnable_pages(
 			continue;
 		}
 
-		if (drained == 0 &&
+		if (drained == 0 && folio_may_be_lru_cached(folio) &&
 				folio_ref_count(folio) !=
 				folio_expected_ref_count(folio) + 1) {
 			lru_add_drain();
 			drained = 1;
 		}
-		if (drained == 1 &&
+		if (drained == 1 && folio_may_be_lru_cached(folio) &&
 				folio_ref_count(folio) !=
 				folio_expected_ref_count(folio) + 1) {
 			lru_add_drain_all();
diff --git a/mm/mlock.c b/mm/mlock.c
index 7032f6dd0ce19..3bf9e1d263da4 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -256,7 +256,7 @@ void mlock_folio(struct folio *folio)
 
 	folio_get(folio);
 	if (!pagevec_add(pvec, mlock_lru(&folio->page)) ||
-	    folio_test_large(folio) || lru_cache_disabled())
+	    !folio_may_be_lru_cached(folio) || lru_cache_disabled())
 		mlock_pagevec(pvec);
 	local_unlock(&mlock_pvec.lock);
 }
diff --git a/mm/swap.c b/mm/swap.c
index 85aa04fc48a67..e0fdf25350002 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -249,8 +249,8 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
 static void folio_batch_add_and_move(struct folio_batch *fbatch,
 		struct folio *folio, move_fn_t move_fn)
 {
-	if (folio_batch_add(fbatch, folio) && !folio_test_large(folio) &&
-	    !lru_cache_disabled())
+	if (folio_batch_add(fbatch, folio) &&
+	    folio_may_be_lru_cached(folio) && !lru_cache_disabled())
 		return;
 	folio_batch_move_lru(fbatch, move_fn);
 }