[PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

[Mel Gorman]

struct per_cpu_pages is no longer strictly local as PCP lists can be
drained remotely using a lock for protection. While the use of local_lock
works, it goes against the intent of local_lock which is for "pure
CPU local concurrency control mechanisms and not suited for inter-CPU
concurrency control" (Documentation/locking/locktypes.rst)

local_lock protects against migration between when the percpu pointer is
accessed and the pcp->lock acquired. The lock acquisition is a preemption
point so in the worst case, a task could migrate to another NUMA node
and accidentally allocate remote memory. The main requirement is to pin
the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.

Replace local_lock with helpers that pin a task to a CPU, lookup the
per-cpu structure and acquire the embedded lock. It's similar to local_lock
without breaking the intent behind the API. It is not a complete API
as only the parts needed for PCP-alloc are implemented but in theory,
the generic helpers could be promoted to a general API if there was
demand for an embedded lock within a per-cpu struct with a guarantee
that the per-cpu structure locked matches the running CPU and cannot use
get_cpu_var due to RT concerns. PCP requires these semantics to avoid
accidentally allocating remote memory.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 mm/page_alloc.c | 226 ++++++++++++++++++++++++++----------------------
 1 file changed, 121 insertions(+), 105 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 03882ce7765f..f10782ab7cc7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -126,13 +126,6 @@ typedef int __bitwise fpi_t;
 static DEFINE_MUTEX(pcp_batch_high_lock);
 #define MIN_PERCPU_PAGELIST_HIGH_FRACTION (8)
 
-struct pagesets {
-	local_lock_t lock;
-};
-static DEFINE_PER_CPU(struct pagesets, pagesets) = {
-	.lock = INIT_LOCAL_LOCK(lock),
-};
-
 #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
 /*
  * On SMP, spin_trylock is sufficient protection.
@@ -147,6 +140,81 @@ static DEFINE_PER_CPU(struct pagesets, pagesets) = {
 #define pcp_trylock_finish(flags)	local_irq_restore(flags)
 #endif
 
+/*
+ * Locking a pcp requires a PCP lookup followed by a spinlock. To avoid
+ * a migration causing the wrong PCP to be locked and remote memory being
+ * potentially allocated, pin the task to the CPU for the lookup+lock.
+ * preempt_disable is used on !RT because it is faster than migrate_disable.
+ * migrate_disable is used on RT because otherwise RT spinlock usage is
+ * interfered with and a high priority task cannot preempt the allocator.
+ */
+#ifndef CONFIG_PREEMPT_RT
+#define pcpu_task_pin()		preempt_disable()
+#define pcpu_task_unpin()	preempt_enable()
+#else
+#define pcpu_task_pin()		migrate_disable()
+#define pcpu_task_unpin()	migrate_enable()
+#endif
+
+/*
+ * Generic helper to lookup and a per-cpu variable with an embedded spinlock.
+ * Return value should be used with equivalent unlock helper.
+ */
+#define pcpu_spin_lock(type, member, ptr)				\
+({									\
+	type *_ret;							\
+	pcpu_task_pin();						\
+	_ret = this_cpu_ptr(ptr);					\
+	spin_lock(&_ret->member);					\
+	_ret;								\
+})
+
+#define pcpu_spin_lock_irqsave(type, member, ptr, flags)		\
+({									\
+	type *_ret;							\
+	pcpu_task_pin();						\
+	_ret = this_cpu_ptr(ptr);					\
+	spin_lock_irqsave(&_ret->member, flags);			\
+	_ret;								\
+})
+
+#define pcpu_spin_trylock_irqsave(type, member, ptr, flags)		\
+({									\
+	type *_ret;							\
+	pcpu_task_pin();						\
+	_ret = this_cpu_ptr(ptr);					\
+	if (!spin_trylock_irqsave(&_ret->member, flags))		\
+		_ret = NULL;						\
+	_ret;								\
+})
+
+#define pcpu_spin_unlock(member, ptr)					\
+({									\
+	spin_unlock(&ptr->member);					\
+	pcpu_task_unpin();						\
+})
+
+#define pcpu_spin_unlock_irqrestore(member, ptr, flags)			\
+({									\
+	spin_unlock_irqrestore(&ptr->member, flags);			\
+	pcpu_task_unpin();						\
+})
+
+/* struct per_cpu_pages specific helpers. */
+#define pcp_spin_lock(ptr)						\
+	pcpu_spin_lock(struct per_cpu_pages, lock, ptr)
+
+#define pcp_spin_lock_irqsave(ptr, flags)				\
+	pcpu_spin_lock_irqsave(struct per_cpu_pages, lock, ptr, flags)
+
+#define pcp_spin_trylock_irqsave(ptr, flags)				\
+	pcpu_spin_trylock_irqsave(struct per_cpu_pages, lock, ptr, flags)
+
+#define pcp_spin_unlock(ptr)						\
+	pcpu_spin_unlock(lock, ptr)
+
+#define pcp_spin_unlock_irqrestore(ptr, flags)				\
+	pcpu_spin_unlock_irqrestore(lock, ptr, flags)
 #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
 DEFINE_PER_CPU(int, numa_node);
 EXPORT_PER_CPU_SYMBOL(numa_node);
@@ -1481,10 +1549,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 	/* Ensure requested pindex is drained first. */
 	pindex = pindex - 1;
 
-	/*
-	 * local_lock_irq held so equivalent to spin_lock_irqsave for
-	 * both PREEMPT_RT and non-PREEMPT_RT configurations.
-	 */
+	/* Caller must hold IRQ-safe pcp->lock so IRQs are disabled. */
 	spin_lock(&zone->lock);
 	isolated_pageblocks = has_isolate_pageblock(zone);
 
@@ -3052,10 +3117,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 {
 	int i, allocated = 0;
 
-	/*
-	 * local_lock_irq held so equivalent to spin_lock_irqsave for
-	 * both PREEMPT_RT and non-PREEMPT_RT configurations.
-	 */
+	/* Caller must hold IRQ-safe pcp->lock so IRQs are disabled. */
 	spin_lock(&zone->lock);
 	for (i = 0; i < count; ++i) {
 		struct page *page = __rmqueue(zone, order, migratetype,
@@ -3367,30 +3429,17 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
 	return min(READ_ONCE(pcp->batch) << 2, high);
 }
 
-/* Returns true if the page was committed to the per-cpu list. */
-static bool free_unref_page_commit(struct page *page, int migratetype,
-				   unsigned int order, bool locked)
+static void free_unref_page_commit(struct per_cpu_pages *pcp, struct zone *zone,
+				   struct page *page, int migratetype,
+				   unsigned int order)
 {
-	struct zone *zone = page_zone(page);
-	struct per_cpu_pages *pcp;
 	int high;
 	int pindex;
 	bool free_high;
-	unsigned long __maybe_unused UP_flags;
 
 	__count_vm_event(PGFREE);
-	pcp = this_cpu_ptr(zone->per_cpu_pageset);
 	pindex = order_to_pindex(migratetype, order);
 
-	if (!locked) {
-		/* Protect against a parallel drain. */
-		pcp_trylock_prepare(UP_flags);
-		if (!spin_trylock(&pcp->lock)) {
-			pcp_trylock_finish(UP_flags);
-			return false;
-		}
-	}
-
 	list_add(&page->pcp_list, &pcp->lists[pindex]);
 	pcp->count += 1 << order;
 
@@ -3408,13 +3457,6 @@ static bool free_unref_page_commit(struct page *page, int migratetype,
 
 		free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch, free_high), pcp, pindex);
 	}
-
-	if (!locked) {
-		spin_unlock(&pcp->lock);
-		pcp_trylock_finish(UP_flags);
-	}
-
-	return true;
 }
 
 /*
@@ -3422,10 +3464,12 @@ static bool free_unref_page_commit(struct page *page, int migratetype,
  */
 void free_unref_page(struct page *page, unsigned int order)
 {
-	unsigned long flags;
+	struct per_cpu_pages *pcp;
+	struct zone *zone;
 	unsigned long pfn = page_to_pfn(page);
 	int migratetype;
-	bool freed_pcp = false;
+	unsigned long flags;
+	unsigned long __maybe_unused UP_flags;
 
 	if (!free_unref_page_prepare(page, pfn, order))
 		return;
@@ -3446,12 +3490,16 @@ void free_unref_page(struct page *page, unsigned int order)
 		migratetype = MIGRATE_MOVABLE;
 	}
 
-	local_lock_irqsave(&pagesets.lock, flags);
-	freed_pcp = free_unref_page_commit(page, migratetype, order, false);
-	local_unlock_irqrestore(&pagesets.lock, flags);
-
-	if (unlikely(!freed_pcp))
+	zone = page_zone(page);
+	pcp_trylock_prepare(UP_flags);
+	pcp = pcpu_spin_trylock_irqsave(struct per_cpu_pages, lock, zone->per_cpu_pageset, flags);
+	if (pcp) {
+		free_unref_page_commit(pcp, zone, page, migratetype, order);
+		pcp_spin_unlock_irqrestore(pcp, flags);
+	} else {
 		free_one_page(page_zone(page), page, pfn, order, migratetype, FPI_NONE);
+	}
+	pcp_trylock_finish(UP_flags);
 }
 
 /*
@@ -3500,20 +3548,20 @@ void free_unref_page_list(struct list_head *list)
 	if (list_empty(list))
 		return;
 
-	local_lock_irqsave(&pagesets.lock, flags);
-
 	page = lru_to_page(list);
 	locked_zone = page_zone(page);
-	pcp = this_cpu_ptr(locked_zone->per_cpu_pageset);
-	spin_lock(&pcp->lock);
+	pcp = pcp_spin_lock_irqsave(locked_zone->per_cpu_pageset, flags);
 
 	list_for_each_entry_safe(page, next, list, lru) {
 		struct zone *zone = page_zone(page);
 
 		/* Different zone, different pcp lock. */
 		if (zone != locked_zone) {
+			/* Leave IRQs enabled as a new lock is acquired. */
 			spin_unlock(&pcp->lock);
 			locked_zone = zone;
+
+			/* Preemption disabled by pcp_spin_lock_irqsave. */
 			pcp = this_cpu_ptr(zone->per_cpu_pageset);
 			spin_lock(&pcp->lock);
 		}
@@ -3528,33 +3576,19 @@ void free_unref_page_list(struct list_head *list)
 
 		trace_mm_page_free_batched(page);
 
-		/*
-		 * If there is a parallel drain in progress, free to the buddy
-		 * allocator directly. This is expensive as the zone lock will
-		 * be acquired multiple times but if a drain is in progress
-		 * then an expensive operation is already taking place.
-		 *
-		 * TODO: Always false at the moment due to local_lock_irqsave
-		 *       and is preparation for converting to local_lock.
-		 */
-		if (unlikely(!free_unref_page_commit(page, migratetype, 0, true)))
-			free_one_page(page_zone(page), page, page_to_pfn(page), 0, migratetype, FPI_NONE);
+		free_unref_page_commit(pcp, zone, page, migratetype, 0);
 
 		/*
 		 * Guard against excessive IRQ disabled times when we get
 		 * a large list of pages to free.
 		 */
 		if (++batch_count == SWAP_CLUSTER_MAX) {
-			spin_unlock(&pcp->lock);
-			local_unlock_irqrestore(&pagesets.lock, flags);
+			pcp_spin_unlock_irqrestore(pcp, flags);
 			batch_count = 0;
-			local_lock_irqsave(&pagesets.lock, flags);
-			pcp = this_cpu_ptr(locked_zone->per_cpu_pageset);
-			spin_lock(&pcp->lock);
+			pcp = pcp_spin_lock_irqsave(locked_zone->per_cpu_pageset, flags);
 		}
 	}
-	spin_unlock(&pcp->lock);
-	local_unlock_irqrestore(&pagesets.lock, flags);
+	pcp_spin_unlock_irqrestore(pcp, flags);
 }
 
 /*
@@ -3722,28 +3756,9 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
 			int migratetype,
 			unsigned int alloc_flags,
 			struct per_cpu_pages *pcp,
-			struct list_head *list,
-			bool locked)
+			struct list_head *list)
 {
 	struct page *page;
-	unsigned long __maybe_unused UP_flags;
-
-	/*
-	 * spin_trylock is not necessary right now due to due to
-	 * local_lock_irqsave and is a preparation step for
-	 * a conversion to local_lock using the trylock to prevent
-	 * IRQ re-entrancy. If pcp->lock cannot be acquired, the caller
-	 * uses rmqueue_buddy.
-	 *
-	 * TODO: Convert local_lock_irqsave to local_lock.
-	 */
-	if (unlikely(!locked)) {
-		pcp_trylock_prepare(UP_flags);
-		if (!spin_trylock(&pcp->lock)) {
-			pcp_trylock_finish(UP_flags);
-			return NULL;
-		}
-	}
 
 	do {
 		if (list_empty(list)) {
@@ -3776,10 +3791,6 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
 	} while (check_new_pcp(page, order));
 
 out:
-	if (!locked) {
-		spin_unlock(&pcp->lock);
-		pcp_trylock_finish(UP_flags);
-	}
 
 	return page;
 }
@@ -3794,19 +3805,29 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
 	struct list_head *list;
 	struct page *page;
 	unsigned long flags;
+	unsigned long __maybe_unused UP_flags;
 
-	local_lock_irqsave(&pagesets.lock, flags);
+	/*
+	 * spin_trylock_irqsave is not necessary right now as it'll only be
+	 * true when contending with a remote drain. It's in place as a
+	 * preparation step before converting pcp locking to spin_trylock
+	 * to protect against IRQ reentry.
+	 */
+	pcp_trylock_prepare(UP_flags);
+	pcp = pcp_spin_trylock_irqsave(zone->per_cpu_pageset, flags);
+	if (!pcp)
+		return NULL;
 
 	/*
 	 * On allocation, reduce the number of pages that are batch freed.
 	 * See nr_pcp_free() where free_factor is increased for subsequent
 	 * frees.
 	 */
-	pcp = this_cpu_ptr(zone->per_cpu_pageset);
 	pcp->free_factor >>= 1;
 	list = &pcp->lists[order_to_pindex(migratetype, order)];
-	page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list, false);
-	local_unlock_irqrestore(&pagesets.lock, flags);
+	page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
+	pcp_spin_unlock_irqrestore(pcp, flags);
+	pcp_trylock_finish(UP_flags);
 	if (page) {
 		__count_zid_vm_events(PGALLOC, page_zonenum(page), 1);
 		zone_statistics(preferred_zone, zone, 1);
@@ -5410,10 +5431,8 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 		goto failed;
 
 	/* Attempt the batch allocation */
-	local_lock_irqsave(&pagesets.lock, flags);
-	pcp = this_cpu_ptr(zone->per_cpu_pageset);
+	pcp = pcp_spin_lock_irqsave(zone->per_cpu_pageset, flags);
 	pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
-	spin_lock(&pcp->lock);
 
 	while (nr_populated < nr_pages) {
 
@@ -5424,13 +5443,11 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 		}
 
 		page = __rmqueue_pcplist(zone, 0, ac.migratetype, alloc_flags,
-							pcp, pcp_list, true);
+							pcp, pcp_list);
 		if (unlikely(!page)) {
 			/* Try and allocate at least one page */
-			if (!nr_account) {
-				spin_unlock(&pcp->lock);
+			if (!nr_account)
 				goto failed_irq;
-			}
 			break;
 		}
 		nr_account++;
@@ -5443,8 +5460,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 		nr_populated++;
 	}
 
-	spin_unlock(&pcp->lock);
-	local_unlock_irqrestore(&pagesets.lock, flags);
+	pcp_spin_unlock_irqrestore(pcp, flags);
 
 	__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
 	zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
@@ -5453,7 +5469,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
 	return nr_populated;
 
 failed_irq:
-	local_unlock_irqrestore(&pagesets.lock, flags);
+	pcp_spin_unlock_irqrestore(pcp, flags);
 
 failed:
 	page = __alloc_pages(gfp, 0, preferred_nid, nodemask);
-- 
2.35.3