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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id 00721157ae682-7227d634d22sm7255777b3.26.2025.08.31.02.16.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 31 Aug 2025 02:16:28 -0700 (PDT) Date: Sun, 31 Aug 2025 02:16:25 -0700 (PDT) From: Hugh Dickins To: Andrew Morton cc: Will Deacon , David Hildenbrand , Shivank Garg , Matthew Wilcox , Christoph Hellwig , Keir Fraser , Jason Gunthorpe , John Hubbard , Frederick Mayle , Peter Xu , "Aneesh Kumar K.V" , Johannes Weiner , Vlastimil Babka , Alexander Krabler , Ge Yang , Li Zhe , Chris Li , Yu Zhao , Axel Rasmussen , Yuanchu Xie , Wei Xu , Konstantin Khlebnikov , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH 6/7] mm: folio_may_be_cached() unless folio_test_large() In-Reply-To: Message-ID: <861c061c-51cd-b940-49df-9f55e1fee2c8@google.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 Signed-off-by: Hugh Dickins Cc: Suggested-by that's making you uncomfortable, we can delete it. --- 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 *); =20 +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; =20 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_foli= os( continue; } =20 - if (drain_allow && folio_ref_count(folio) !=3D - folio_expected_ref_count(folio) + 1) { + if (drain_allow && folio_may_be_cached(folio) && + folio_ref_count(folio) !=3D + folio_expected_ref_count(folio) + 1) { lru_add_drain_all(); drain_allow =3D 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) =20 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) =20 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_bat= ch __percpu *fbatch, local_lock(&cpu_fbatches.lock); =20 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); =20 if (disable_irq) --=20 2.51.0