Message-ID: <20250212063153.179231-13-senozhatsky@chromium.org>
Date: Wed, 12 Feb 2025 15:27:10 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Kairui Song <ryncsn@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH v5 12/18] zsmalloc: make zspage lock preemptible

Switch over from rwlock_t to a atomic_t variable that takes negative
value when the page is under migration, or positive values when the
page is used by zsmalloc users (object map, etc.)   Using a rwsem
per-zspage is a little too memory heavy, a simple atomic_t should
suffice.

zspage lock is a leaf lock for zs_map_object(), where it's read-acquired.
Since this lock now permits preemption extra care needs to be taken when
it is write-acquired - all writers grab it in atomic context, so they
cannot spin and wait for (potentially preempted) reader to unlock zspage.
There are only two writers at this moment - migration and compaction.  In
both cases we use write-try-lock and bail out if zspage is read locked.
Writers, on the other hand, never get preempted, so readers can spin
waiting for the writer to unlock zspage.

With this we can implement a preemptible object mapping.

Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 mm/zsmalloc.c | 183 +++++++++++++++++++++++++++++++++++---------------
 1 file changed, 128 insertions(+), 55 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index c82c24b8e6a4..80261bb78cf8 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -226,6 +226,9 @@ struct zs_pool {
 	/* protect page/zspage migration */
 	rwlock_t lock;
 	atomic_t compaction_in_progress;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lock_class_key lockdep_key;
+#endif
 };
 
 static void pool_write_unlock(struct zs_pool *pool)
@@ -292,6 +295,9 @@ static inline void free_zpdesc(struct zpdesc *zpdesc)
 	__free_page(page);
 }
 
+#define ZS_PAGE_UNLOCKED	0
+#define ZS_PAGE_WRLOCKED	-1
+
 struct zspage {
 	struct {
 		unsigned int huge:HUGE_BITS;
@@ -304,7 +310,11 @@ struct zspage {
 	struct zpdesc *first_zpdesc;
 	struct list_head list; /* fullness list */
 	struct zs_pool *pool;
-	rwlock_t lock;
+	atomic_t lock;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map lockdep_map;
+#endif
 };
 
 struct mapping_area {
@@ -314,6 +324,88 @@ struct mapping_area {
 	enum zs_mapmode vm_mm; /* mapping mode */
 };
 
+static void zspage_lock_init(struct zspage *zspage)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	lockdep_init_map(&zspage->lockdep_map, "zsmalloc-page",
+			 &zspage->pool->lockdep_key, 0);
+#endif
+
+	atomic_set(&zspage->lock, ZS_PAGE_UNLOCKED);
+}
+
+/*
+ * zspage locking rules:
+ *
+ * 1) writer-lock is exclusive
+ *
+ * 2) writer-lock owner cannot sleep
+ *
+ * 3) writer-lock owner cannot spin waiting for the lock
+ *   - caller (e.g. compaction and migration) must check return value and
+ *     handle locking failures
+ *   - there is only TRY variant of writer-lock function
+ *
+ * 4) reader-lock owners (multiple) can sleep
+ *
+ * 5) reader-lock owners can spin waiting for the lock, in any context
+ *   - existing readers (even preempted ones) don't block new readers
+ *   - writer-lock owners never sleep, always unlock at some point
+ */
+static void zspage_read_lock(struct zspage *zspage)
+{
+	atomic_t *lock = &zspage->lock;
+	int old = atomic_read_acquire(lock);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	rwsem_acquire_read(&zspage->lockdep_map, 0, 0, _RET_IP_);
+#endif
+
+	do {
+		if (old == ZS_PAGE_WRLOCKED) {
+			cpu_relax();
+			old = atomic_read_acquire(lock);
+			continue;
+		}
+	} while (!atomic_try_cmpxchg_acquire(lock, &old, old + 1));
+}
+
+static void zspage_read_unlock(struct zspage *zspage)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	rwsem_release(&zspage->lockdep_map, _RET_IP_);
+#endif
+	atomic_dec_return_release(&zspage->lock);
+}
+
+static __must_check bool zspage_try_write_lock(struct zspage *zspage)
+{
+	atomic_t *lock = &zspage->lock;
+	int old = ZS_PAGE_UNLOCKED;
+
+	WARN_ON_ONCE(preemptible());
+
+	preempt_disable();
+	if (atomic_try_cmpxchg_acquire(lock, &old, ZS_PAGE_WRLOCKED)) {
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+		rwsem_acquire(&zspage->lockdep_map, 0, 1, _RET_IP_);
+#endif
+		return true;
+	}
+
+	preempt_enable();
+	return false;
+}
+
+static void zspage_write_unlock(struct zspage *zspage)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	rwsem_release(&zspage->lockdep_map, _RET_IP_);
+#endif
+	atomic_set_release(&zspage->lock, ZS_PAGE_UNLOCKED);
+	preempt_enable();
+}
+
 /* huge object: pages_per_zspage == 1 && maxobj_per_zspage == 1 */
 static void SetZsHugePage(struct zspage *zspage)
 {
@@ -325,12 +417,6 @@ static bool ZsHugePage(struct zspage *zspage)
 	return zspage->huge;
 }
 
-static void migrate_lock_init(struct zspage *zspage);
-static void migrate_read_lock(struct zspage *zspage);
-static void migrate_read_unlock(struct zspage *zspage);
-static void migrate_write_lock(struct zspage *zspage);
-static void migrate_write_unlock(struct zspage *zspage);
-
 #ifdef CONFIG_COMPACTION
 static void kick_deferred_free(struct zs_pool *pool);
 static void init_deferred_free(struct zs_pool *pool);
@@ -1026,7 +1112,9 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
 		return NULL;
 
 	zspage->magic = ZSPAGE_MAGIC;
-	migrate_lock_init(zspage);
+	zspage->pool = pool;
+	zspage->class = class->index;
+	zspage_lock_init(zspage);
 
 	for (i = 0; i < class->pages_per_zspage; i++) {
 		struct zpdesc *zpdesc;
@@ -1049,8 +1137,6 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
 
 	create_page_chain(class, zspage, zpdescs);
 	init_zspage(class, zspage);
-	zspage->pool = pool;
-	zspage->class = class->index;
 
 	return zspage;
 }
@@ -1251,7 +1337,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
 	 * zs_unmap_object API so delegate the locking from class to zspage
 	 * which is smaller granularity.
 	 */
-	migrate_read_lock(zspage);
+	zspage_read_lock(zspage);
 	pool_read_unlock(pool);
 
 	class = zspage_class(pool, zspage);
@@ -1311,7 +1397,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
 	}
 	local_unlock(&zs_map_area.lock);
 
-	migrate_read_unlock(zspage);
+	zspage_read_unlock(zspage);
 }
 EXPORT_SYMBOL_GPL(zs_unmap_object);
 
@@ -1705,18 +1791,18 @@ static void lock_zspage(struct zspage *zspage)
 	/*
 	 * Pages we haven't locked yet can be migrated off the list while we're
 	 * trying to lock them, so we need to be careful and only attempt to
-	 * lock each page under migrate_read_lock(). Otherwise, the page we lock
+	 * lock each page under zspage_read_lock(). Otherwise, the page we lock
 	 * may no longer belong to the zspage. This means that we may wait for
 	 * the wrong page to unlock, so we must take a reference to the page
-	 * prior to waiting for it to unlock outside migrate_read_lock().
+	 * prior to waiting for it to unlock outside zspage_read_lock().
 	 */
 	while (1) {
-		migrate_read_lock(zspage);
+		zspage_read_lock(zspage);
 		zpdesc = get_first_zpdesc(zspage);
 		if (zpdesc_trylock(zpdesc))
 			break;
 		zpdesc_get(zpdesc);
-		migrate_read_unlock(zspage);
+		zspage_read_unlock(zspage);
 		zpdesc_wait_locked(zpdesc);
 		zpdesc_put(zpdesc);
 	}
@@ -1727,41 +1813,16 @@ static void lock_zspage(struct zspage *zspage)
 			curr_zpdesc = zpdesc;
 		} else {
 			zpdesc_get(zpdesc);
-			migrate_read_unlock(zspage);
+			zspage_read_unlock(zspage);
 			zpdesc_wait_locked(zpdesc);
 			zpdesc_put(zpdesc);
-			migrate_read_lock(zspage);
+			zspage_read_lock(zspage);
 		}
 	}
-	migrate_read_unlock(zspage);
+	zspage_read_unlock(zspage);
 }
 #endif /* CONFIG_COMPACTION */
 
-static void migrate_lock_init(struct zspage *zspage)
-{
-	rwlock_init(&zspage->lock);
-}
-
-static void migrate_read_lock(struct zspage *zspage) __acquires(&zspage->lock)
-{
-	read_lock(&zspage->lock);
-}
-
-static void migrate_read_unlock(struct zspage *zspage) __releases(&zspage->lock)
-{
-	read_unlock(&zspage->lock);
-}
-
-static void migrate_write_lock(struct zspage *zspage)
-{
-	write_lock(&zspage->lock);
-}
-
-static void migrate_write_unlock(struct zspage *zspage)
-{
-	write_unlock(&zspage->lock);
-}
-
 #ifdef CONFIG_COMPACTION
 
 static const struct movable_operations zsmalloc_mops;
@@ -1803,7 +1864,7 @@ static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
 }
 
 static int zs_page_migrate(struct page *newpage, struct page *page,
-		enum migrate_mode mode)
+			   enum migrate_mode mode)
 {
 	struct zs_pool *pool;
 	struct size_class *class;
@@ -1819,15 +1880,12 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 
 	VM_BUG_ON_PAGE(!zpdesc_is_isolated(zpdesc), zpdesc_page(zpdesc));
 
-	/* We're committed, tell the world that this is a Zsmalloc page. */
-	__zpdesc_set_zsmalloc(newzpdesc);
-
 	/* The page is locked, so this pointer must remain valid */
 	zspage = get_zspage(zpdesc);
 	pool = zspage->pool;
 
 	/*
-	 * The pool lock protects the race between zpage migration
+	 * The pool->lock protects the race between zpage migration
 	 * and zs_free.
 	 */
 	pool_write_lock(pool);
@@ -1837,8 +1895,15 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 	 * the class lock protects zpage alloc/free in the zspage.
 	 */
 	size_class_lock(class);
-	/* the migrate_write_lock protects zpage access via zs_map_object */
-	migrate_write_lock(zspage);
+	/* the zspage write_lock protects zpage access via zs_map_object */
+	if (!zspage_try_write_lock(zspage)) {
+		size_class_unlock(class);
+		pool_write_unlock(pool);
+		return -EINVAL;
+	}
+
+	/* We're committed, tell the world that this is a Zsmalloc page. */
+	__zpdesc_set_zsmalloc(newzpdesc);
 
 	offset = get_first_obj_offset(zpdesc);
 	s_addr = kmap_local_zpdesc(zpdesc);
@@ -1869,7 +1934,7 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 	 */
 	pool_write_unlock(pool);
 	size_class_unlock(class);
-	migrate_write_unlock(zspage);
+	zspage_write_unlock(zspage);
 
 	zpdesc_get(newzpdesc);
 	if (zpdesc_zone(newzpdesc) != zpdesc_zone(zpdesc)) {
@@ -2005,9 +2070,11 @@ static unsigned long __zs_compact(struct zs_pool *pool,
 		if (!src_zspage)
 			break;
 
-		migrate_write_lock(src_zspage);
+		if (!zspage_try_write_lock(src_zspage))
+			break;
+
 		migrate_zspage(pool, src_zspage, dst_zspage);
-		migrate_write_unlock(src_zspage);
+		zspage_write_unlock(src_zspage);
 
 		fg = putback_zspage(class, src_zspage);
 		if (fg == ZS_INUSE_RATIO_0) {
@@ -2267,7 +2334,9 @@ struct zs_pool *zs_create_pool(const char *name)
 	 * trigger compaction manually. Thus, ignore return code.
 	 */
 	zs_register_shrinker(pool);
-
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	lockdep_register_key(&pool->lockdep_key);
+#endif
 	return pool;
 
 err:
@@ -2304,6 +2373,10 @@ void zs_destroy_pool(struct zs_pool *pool)
 		kfree(class);
 	}
 
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	lockdep_unregister_key(&pool->lockdep_key);
+#endif
+
 	destroy_cache(pool);
 	kfree(pool->name);
 	kfree(pool);
-- 
2.48.1.502.g6dc24dfdaf-goog

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