[PATCH 24/25] fs,mm/fbatch: lru_cache_disable() keep off buffer_head lrus only

[Hugh Dickins]

Now that the per-cpu fbatch folio references are gone, there seems to
be no excuse for lru_cache_disable() there - other than offline_pages()
needing to lru_add_drain_all() to erase stale pointers from the fbatches.

Remove lru_cache_disabled() checks from all except bh_lru_install():
assuming that lru_cache_disable() might have value in preventing repeated
calls to invalidate_bh_lrus() when migrating folios in memory hotremoval.

So move all that from mm/folio.c to fs/buffer.c: but I can't see how any
of the paranoid synchronize_rcu_expedited() business is needed now (or
even before) - leave it out. And back at the mm end, lru_add_drain_all()
does not need that force_all_cpus either - it stopped forcing all cpus
in 5.18 commit ff042f4a9b05 ("mm: lru_cache_disable: replace work queue
synchronization with synchronize_rcu").

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 fs/buffer.c                 | 24 +++++++++++++++---
 include/linux/buffer_head.h |  4 +++
 include/linux/swap.h        |  7 ------
 mm/folio.c                  | 49 ++++---------------------------------
 mm/internal.h               |  6 -----
 mm/memory_hotplug.c         |  4 +++
 mm/mlock.c                  |  4 +--
 7 files changed, 36 insertions(+), 62 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 7d114e5b9c62..7455a11dfc4a 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1200,6 +1200,24 @@ static inline void check_irqs_on(void)
 #endif
 }
 
+static atomic_t lru_disable_count = ATOMIC_INIT(0);
+
+void lru_cache_disable(void)
+{
+	if (atomic_inc_return(&lru_disable_count) == 1)
+		invalidate_bh_lrus();
+}
+
+static inline bool lru_cache_disabled(void)
+{
+	return atomic_read(&lru_disable_count);
+}
+
+void lru_cache_enable(void)
+{
+	atomic_dec(&lru_disable_count);
+}
+
 /*
  * Install a buffer_head into this cpu's LRU.  If not already in the LRU, it is
  * inserted at the front, and the buffer_head at the back if any is evicted.
@@ -1215,9 +1233,9 @@ static void bh_lru_install(struct buffer_head *bh)
 	bh_lru_lock();
 
 	/*
-	 * the refcount of buffer_head in bh_lru prevents dropping the
-	 * attached page(i.e., try_to_free_buffers) so it could cause
-	 * failing page migration.
+	 * The refcount of buffer_head in bh_lru prevents dropping the
+	 * attached page (i.e., try_to_free_buffers), so it could cause
+	 * repeated calls to invalidate_bh_lrus() during page migration.
 	 * Skip putting upcoming bh into bh_lru until migration is done.
 	 */
 	if (lru_cache_disabled() || cpu_is_isolated(smp_processor_id())) {
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index f19f9e80be8f..3b6a41b7932f 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -517,6 +517,8 @@ void mmb_init(struct mapping_metadata_bhs *mmb, struct address_space *mapping);
 bool mmb_has_buffers(struct mapping_metadata_bhs *mmb);
 void mmb_invalidate(struct mapping_metadata_bhs *mmb);
 int mmb_sync(struct mapping_metadata_bhs *mmb);
+void lru_cache_disable(void);
+void lru_cache_enable(void);
 void invalidate_bh_lrus(void);
 extern int buffer_heads_over_limit;
 
@@ -525,6 +527,8 @@ extern int buffer_heads_over_limit;
 static inline void buffer_init(void) {}
 static inline bool try_to_free_buffers(struct folio *folio) { return true; }
 static inline int mmb_sync(struct mapping_metadata_bhs *mmb) { return 0; }
+static inline void lru_cache_disable(void) {}
+static inline void lru_cache_enable(void) {}
 static inline void invalidate_bh_lrus(void) {}
 #define buffer_heads_over_limit 0
 
diff --git a/include/linux/swap.h b/include/linux/swap.h
index f21e1dd6febc..ddfd9b7bb861 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -305,13 +305,6 @@ void lru_add_drain_all(void);
 /* linux/mm/folio-compat.c */
 void mark_page_accessed(struct page *page);
 
-extern atomic_t lru_disable_count;
-
-static inline bool lru_cache_disabled(void)
-{
-	return atomic_read(&lru_disable_count);
-}
-
 extern unsigned long shrink_all_memory(unsigned long nr_pages);
 extern int vm_swappiness;
 long remove_mapping(struct address_space *mapping, struct folio *folio);
diff --git a/mm/folio.c b/mm/folio.c
index 3212c7a58623..dac2f2d5dcc1 100644
--- a/mm/folio.c
+++ b/mm/folio.c
@@ -173,7 +173,7 @@ 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_may_be_lru_cached(folio) || lru_cache_disabled())
+			!folio_may_be_lru_cached(folio))
 		folio_batch_move_lru(this_cpu_ptr(fbatch), move_fn);
 
 	if (disable_irq)
@@ -501,7 +501,7 @@ void __folio_add_lru(struct folio *folio, bool mlockit)
 	smp_mb__before_atomic();
 	folio_set_lru(folio);
 
-	if (full || !folio_may_be_lru_cached(folio) || lru_cache_disabled())
+	if (full || !folio_may_be_lru_cached(folio))
 		folio_batch_move_lru(fbatch, lru_add);
 
 	local_unlock(&cpu_fbatches.lock);
@@ -786,7 +786,7 @@ static bool cpu_needs_drain(unsigned int cpu)
  * Calling this function with cpu hotplug locks held can actually lead
  * to obscure indirect dependencies via WQ context.
  */
-static inline void __lru_add_drain_all(bool force_all_cpus)
+void lru_add_drain_all(void)
 {
 	/*
 	 * lru_drain_gen - Global pages generation number
@@ -810,7 +810,7 @@ static inline void __lru_add_drain_all(bool force_all_cpus)
 	if (WARN_ON(!mm_percpu_wq))
 		return;
 
-	trace_mm_lru_add_drain_all_tp(force_all_cpus);
+	trace_mm_lru_add_drain_all_tp(false);
 
 	/*
 	 * Guarantee folio_batch counter stores visible by this CPU
@@ -837,7 +837,7 @@ static inline void __lru_add_drain_all(bool force_all_cpus)
 	 * (C) Exit the draining operation if a newer generation, from another
 	 * lru_add_drain_all(), was already scheduled for draining. Check (A).
 	 */
-	if (unlikely(this_gen != lru_drain_gen && !force_all_cpus))
+	if (unlikely(this_gen != lru_drain_gen))
 		goto done;
 
 	/*
@@ -883,11 +883,6 @@ static inline void __lru_add_drain_all(bool force_all_cpus)
 done:
 	mutex_unlock(&lock);
 }
-
-void lru_add_drain_all(void)
-{
-	__lru_add_drain_all(false);
-}
 #else
 void lru_add_drain_all(void)
 {
@@ -896,40 +891,6 @@ void lru_add_drain_all(void)
 }
 #endif /* CONFIG_SMP */
 
-atomic_t lru_disable_count = ATOMIC_INIT(0);
-
-/*
- * lru_cache_disable() needs to be called before we start compiling
- * a list of folios to be migrated using folio_isolate_lru().
- * It drains folios on LRU cache and then disable on all cpus until
- * lru_cache_enable is called.
- *
- * Must be paired with a call to lru_cache_enable().
- */
-void lru_cache_disable(void)
-{
-	atomic_inc(&lru_disable_count);
-	/*
-	 * Readers of lru_disable_count are protected by either disabling
-	 * preemption or rcu_read_lock:
-	 *
-	 * preempt_disable, local_irq_disable  [bh_lru_lock()]
-	 * rcu_read_lock		       [rt_spin_lock CONFIG_PREEMPT_RT]
-	 * preempt_disable		       [local_lock !CONFIG_PREEMPT_RT]
-	 *
-	 * Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on
-	 * preempt_disable() regions of code. So any CPU which sees
-	 * lru_disable_count = 0 will have exited the critical
-	 * section when synchronize_rcu() returns.
-	 */
-	synchronize_rcu_expedited();
-#ifdef CONFIG_SMP
-	__lru_add_drain_all(true);
-#else
-	lru_add_drain_all();
-#endif
-}
-
 /**
  * folios_put_refs - Reduce the reference count on a batch of folios.
  * @folios: The folios.
diff --git a/mm/internal.h b/mm/internal.h
index 9a25552cbd83..7a301c401d39 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -56,12 +56,6 @@ static inline bool folio_may_be_lru_cached(struct folio *folio)
 	return !folio_test_large(folio);
 }
 
-static inline void lru_cache_enable(void)
-{
-	atomic_dec(&lru_disable_count);
-}
-
-void lru_cache_disable(void);
 void lru_add_drain(void);
 void lru_add_drain_cpu(int cpu);
 void lru_add_drain_cpu_zone(struct zone *zone);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 226ab9cb078a..19756c45b5e7 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -24,6 +24,7 @@
 #include <linux/ioport.h>
 #include <linux/delay.h>
 #include <linux/migrate.h>
+#include <linux/buffer_head.h>
 #include <linux/page-isolation.h>
 #include <linux/pfn.h>
 #include <linux/suspend.h>
@@ -2095,6 +2096,9 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 
 	} while (ret);
 
+	/* Remove any instances of the freed pages from per-cpu fbatches. */
+	lru_add_drain_all();
+
 	/* Mark all sections offline and remove free pages from the buddy. */
 	managed_pages = __offline_isolated_pages(start_pfn, end_pfn);
 	pr_debug("Offlined Pages %ld\n", nr_pages);
diff --git a/mm/mlock.c b/mm/mlock.c
index 971430e6251e..a3cfdb274fc7 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -247,7 +247,7 @@ void mlock_folio(struct folio *folio)
 	local_lock(&mlock_fbatch.lock);
 	fbatch = this_cpu_ptr(&mlock_fbatch.fbatch);
 	if (!folio_batch_add(fbatch, mlock_flagged(folio)) ||
-	    !folio_may_be_lru_cached(folio) || lru_cache_disabled())
+	    !folio_may_be_lru_cached(folio))
 		mlock_folio_batch(fbatch);
 	local_unlock(&mlock_fbatch.lock);
 }
@@ -278,7 +278,7 @@ void munlock_folio(struct folio *folio)
 	local_lock(&mlock_fbatch.lock);
 	fbatch = this_cpu_ptr(&mlock_fbatch.fbatch);
 	if (!folio_batch_add(fbatch, folio) ||
-	    !folio_may_be_lru_cached(folio) || lru_cache_disabled())
+	    !folio_may_be_lru_cached(folio))
 		mlock_folio_batch(fbatch);
 	local_unlock(&mlock_fbatch.lock);
 }
-- 
2.51.0