New subject: [PATCH 6.12.y] mm: folio_may_be_lru_cached() unless folio_test_large()

21 Sep 2025

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to stable@vger.kernel.org.
To reproduce the conflict and resubmit, you may use the following commands:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 2da6de30e60dd9bb14600eff1cc99df2fa2ddae3
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025092142-easiness-blatancy-23af@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 2da6de30e60dd9bb14600eff1cc99df2fa2ddae3 Mon Sep 17 00:00:00 2001
From: Hugh Dickins hughd@google.com
Date: Mon, 8 Sep 2025 15:23:15 -0700
Subject: [PATCH] mm: folio_may_be_lru_cached() unless folio_test_large()
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

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 2fe6ed2cc3fd..7012a0f758d8 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -385,6 +385,16 @@ void folio_add_lru_vma(struct folio *, struct vm_area_struct *);
 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 b47066a54f52..0bc4d140fc07 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2307,13 +2307,13 @@ static unsigned long collect_longterm_unpinnable_folios(
    		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 a1d93ad33c6d..bb0776f5ef7c 100644
--- a/mm/mlock.c
+++ b/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_lru_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_lru_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_lru_cached(folio) || lru_cache_disabled())
    	mlock_folio_batch(fbatch);
    local_unlock(&mlock_fbatch.lock);
 }
diff --git a/mm/swap.c b/mm/swap.c
index 6ae2d5680574..b74ebe865dd9 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -192,7 +192,7 @@ static void __folio_batch_add_and_move(struct folio_batch __percpu *fbatch,
    	local_lock(&cpu_fbatches.lock);
if (!folio_batch_add(this_cpu_ptr(fbatch), folio) ||
-			folio_test_large(folio) || lru_cache_disabled())
+			!folio_may_be_lru_cached(folio) || lru_cache_disabled())
    	folio_batch_move_lru(this_cpu_ptr(fbatch), move_fn);
if (disable_irq)

    

FAILED: patch "[PATCH] mm: folio_may_be_lru_cached() unless folio_test_large()" failed to apply to 6.12-stable tree