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Message-ID: <861c061c-51cd-b940-49df-9f55e1fee2c8@google.com>
Date: Sun, 31 Aug 2025 02:16:25 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: Will Deacon <will@...nel.org>, David Hildenbrand <david@...hat.com>,
Shivank Garg <shivankg@....com>, Matthew Wilcox <willy@...radead.org>,
Christoph Hellwig <hch@...radead.org>, Keir Fraser <keirf@...gle.com>,
Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
Frederick Mayle <fmayle@...gle.com>, Peter Xu <peterx@...hat.com>,
"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>,
Alexander Krabler <Alexander.Krabler@...a.com>,
Ge Yang <yangge1116@....com>, Li Zhe <lizhe.67@...edance.com>,
Chris Li <chrisl@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>,
Konstantin Khlebnikov <koct9i@...il.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: [PATCH 6/7] mm: folio_may_be_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_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".
Suggested-by: David Hildenbrand <david@...hat.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Cc: <stable@...r.kernel.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(-)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 2fe6ed2cc3fd..b49a61c32238 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_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 9f7c87f504a9..e70544c0f958 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2309,8 +2309,9 @@ static unsigned long collect_longterm_unpinnable_folios(
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;
}
diff --git a/mm/mlock.c b/mm/mlock.c
index a1d93ad33c6d..427339dea380 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_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);
}
diff --git a/mm/swap.c b/mm/swap.c
index 6ae2d5680574..17438fd1f51a 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_cached(folio) || lru_cache_disabled())
folio_batch_move_lru(this_cpu_ptr(fbatch), move_fn);
if (disable_irq)
--
2.51.0
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