+ mm-folio_may_be_cached-unless-folio_test_large.patch added to mm-new branch

+ mm-folio_may_be_cached-unless-folio_test_large.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: folio_may_be_cached() unless folio_test_large()
has been added to the -mm mm-new branch.  Its filename is
     mm-folio_may_be_cached-unless-folio_test_large.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-folio_may_be_cached-unless-folio_test_large.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
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fixup patches in mm-new.

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------------------------------------------------------
From: Hugh Dickins <hughd@google.com>
Subject: mm: folio_may_be_cached() unless folio_test_large()
Date: Sun, 31 Aug 2025 02:16:25 -0700 (PDT)

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_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/861c061c-51cd-b940-49df-9f55e1fee2c8@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: <stable@vger.kernel.org>
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: 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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/swap.h |   10 ++++++++++
 mm/gup.c             |    5 +++--
 mm/mlock.c           |    6 +++---
 mm/swap.c            |    2 +-
 4 files changed, 17 insertions(+), 6 deletions(-)

--- a/include/linux/swap.h~mm-folio_may_be_cached-unless-folio_test_large
+++ a/include/linux/swap.h
@@ -381,6 +381,16 @@ void folio_add_lru_vma(struct folio *, s
 void mark_page_accessed(struct page *);
 void folio_mark_accessed(struct folio *);
 
+static inline bool folio_may_be_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)
--- a/mm/gup.c~mm-folio_may_be_cached-unless-folio_test_large
+++ a/mm/gup.c
@@ -2309,8 +2309,9 @@ static unsigned long collect_longterm_un
 			continue;
 		}
 
-		if (drain_allow && folio_ref_count(folio) !=
-				   folio_expected_ref_count(folio) + 1) {
+		if (drain_allow && folio_may_be_cached(folio) &&
+				folio_ref_count(folio) !=
+				folio_expected_ref_count(folio) + 1) {
 			lru_add_drain_all();
 			drain_allow = false;
 		}
--- a/mm/mlock.c~mm-folio_may_be_cached-unless-folio_test_large
+++ a/mm/mlock.c
@@ -255,7 +255,7 @@ void mlock_folio(struct folio *folio)
 
 	folio_get(folio);
 	if (!folio_batch_add(fbatch, mlock_lru(folio)) ||
-	    folio_test_large(folio) || lru_cache_disabled())
+	    !folio_may_be_cached(folio) || lru_cache_disabled())
 		mlock_folio_batch(fbatch);
 	local_unlock(&mlock_fbatch.lock);
 }
@@ -278,7 +278,7 @@ void mlock_new_folio(struct folio *folio
 
 	folio_get(folio);
 	if (!folio_batch_add(fbatch, mlock_new(folio)) ||
-	    folio_test_large(folio) || lru_cache_disabled())
+	    !folio_may_be_cached(folio) || lru_cache_disabled())
 		mlock_folio_batch(fbatch);
 	local_unlock(&mlock_fbatch.lock);
 }
@@ -299,7 +299,7 @@ void munlock_folio(struct folio *folio)
 	 */
 	folio_get(folio);
 	if (!folio_batch_add(fbatch, folio) ||
-	    folio_test_large(folio) || lru_cache_disabled())
+	    !folio_may_be_cached(folio) || lru_cache_disabled())
 		mlock_folio_batch(fbatch);
 	local_unlock(&mlock_fbatch.lock);
 }
--- a/mm/swap.c~mm-folio_may_be_cached-unless-folio_test_large
+++ a/mm/swap.c
@@ -192,7 +192,7 @@ static void __folio_batch_add_and_move(s
 		local_lock(&cpu_fbatches.lock);
 
 	if (!folio_batch_add(this_cpu_ptr(fbatch), folio) ||
-			folio_test_large(folio) || lru_cache_disabled())
+			!folio_may_be_cached(folio) || lru_cache_disabled())
 		folio_batch_move_lru(this_cpu_ptr(fbatch), move_fn);
 
 	if (disable_irq)
_

Patches currently in -mm which might be from hughd@google.com are

mm-fix-folio_expected_ref_count-when-pg_private_2.patch
mm-gup-check-ref_count-instead-of-lru-before-migration.patch
mm-gup-local-lru_add_drain-to-avoid-lru_add_drain_all.patch
mm-revert-mm-gup-clear-the-lru-flag-of-a-page-before-adding-to-lru-batch.patch
mm-revert-mm-vmscanc-fix-oom-on-swap-stress-test.patch
mm-folio_may_be_cached-unless-folio_test_large.patch
mm-lru_add_drain_all-do-local-lru_add_drain-first.patch