Subject: [PATCH] mm/zsmalloc: Replace bit spinlock and get_cpu_var() usage.
From: Mike Galbraith <[email-masked]>
Date: Tue, 28 Sep 2021 09:38:47 +0200

For efficiency reasons, zsmalloc is using a slim `handle'. The value is
the address of a memory allocation of 4 or 8 bytes depending on the size
of the long data type. The lowest bit in that allocated memory is used
as a bit spin lock.
The usage of the bit spin lock is problematic because with the bit spin
lock held zsmalloc acquires a rwlock_t and spinlock_t which are both
sleeping locks on PREEMPT_RT and therefore must not be acquired with
disabled preemption.

Extend the handle to struct zsmalloc_handle which holds the old handle as
addr and a spinlock_t which replaces the bit spinlock. Replace all the
wrapper functions accordingly.

The usage of get_cpu_var() in zs_map_object() is problematic because
it disables preemption and makes it impossible to acquire any sleeping
lock on PREEMPT_RT such as a spinlock_t.
Replace the get_cpu_var() usage with a local_lock_t which is embedded
struct mapping_area. It ensures that the access the struct is
synchronized against all users on the same CPU.

This survived LTP testing.

Signed-off-by: Mike Galbraith <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
[bigeasy: replace the bitspin_lock() with a mutex, get_locked_var() and
 patch description. Mike then fixed the size magic and made handle lock
 spinlock_t.]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 mm/Kconfig    |    3 --
 mm/zsmalloc.c |   84 +++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 79 insertions(+), 8 deletions(-)

--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -640,7 +640,6 @@ config ZSWAP_ZPOOL_DEFAULT_Z3FOLD
 
 config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC
 	bool "zsmalloc"
-	depends on !PREEMPT_RT
 	select ZSMALLOC
 	help
 	  Use the zsmalloc allocator as the default allocator.
@@ -691,7 +690,7 @@ config Z3FOLD
 
 config ZSMALLOC
 	tristate "Memory allocator for compressed pages"
-	depends on MMU && !PREEMPT_RT
+	depends on MMU
 	help
 	  zsmalloc is a slab-based memory allocator designed to store
 	  compressed RAM pages.  zsmalloc uses virtual memory mapping
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -57,6 +57,7 @@
 #include <linux/wait.h>
 #include <linux/pagemap.h>
 #include <linux/fs.h>
+#include <linux/local_lock.h>
 
 #define ZSPAGE_MAGIC	0x58
 
@@ -77,6 +78,20 @@
 
 #define ZS_HANDLE_SIZE (sizeof(unsigned long))
 
+#ifdef CONFIG_PREEMPT_RT
+
+struct zsmalloc_handle {
+	unsigned long addr;
+	spinlock_t lock;
+};
+
+#define ZS_HANDLE_ALLOC_SIZE (sizeof(struct zsmalloc_handle))
+
+#else
+
+#define ZS_HANDLE_ALLOC_SIZE (sizeof(unsigned long))
+#endif
+
 /*
  * Object location (<PFN>, <obj_idx>) is encoded as
  * a single (unsigned long) handle value.
@@ -293,6 +308,7 @@ struct zspage {
 };
 
 struct mapping_area {
+	local_lock_t lock;
 	char *vm_buf; /* copy buffer for objects that span pages */
 	char *vm_addr; /* address of kmap_atomic()'ed pages */
 	enum zs_mapmode vm_mm; /* mapping mode */
@@ -322,7 +338,7 @@ static void SetZsPageMovable(struct zs_p
 
 static int create_cache(struct zs_pool *pool)
 {
-	pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
+	pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_ALLOC_SIZE,
 					0, 0, NULL);
 	if (!pool->handle_cachep)
 		return 1;
@@ -346,10 +362,27 @@ static void destroy_cache(struct zs_pool
 
 static unsigned long cache_alloc_handle(struct zs_pool *pool, gfp_t gfp)
 {
-	return (unsigned long)kmem_cache_alloc(pool->handle_cachep,
-			gfp & ~(__GFP_HIGHMEM|__GFP_MOVABLE));
+	void *p;
+
+	p = kmem_cache_alloc(pool->handle_cachep,
+			     gfp & ~(__GFP_HIGHMEM|__GFP_MOVABLE));
+#ifdef CONFIG_PREEMPT_RT
+	if (p) {
+		struct zsmalloc_handle *zh = p;
+
+		spin_lock_init(&zh->lock);
+	}
+#endif
+	return (unsigned long)p;
 }
 
+#ifdef CONFIG_PREEMPT_RT
+static struct zsmalloc_handle *zs_get_pure_handle(unsigned long handle)
+{
+	return (void *)(handle & ~((1 << OBJ_TAG_BITS) - 1));
+}
+#endif
+
 static void cache_free_handle(struct zs_pool *pool, unsigned long handle)
 {
 	kmem_cache_free(pool->handle_cachep, (void *)handle);
@@ -368,12 +401,18 @@ static void cache_free_zspage(struct zs_
 
 static void record_obj(unsigned long handle, unsigned long obj)
 {
+#ifdef CONFIG_PREEMPT_RT
+	struct zsmalloc_handle *zh = zs_get_pure_handle(handle);
+
+	WRITE_ONCE(zh->addr, obj);
+#else
 	/*
 	 * lsb of @obj represents handle lock while other bits
 	 * represent object value the handle is pointing so
 	 * updating shouldn't do store tearing.
 	 */
 	WRITE_ONCE(*(unsigned long *)handle, obj);
+#endif
 }
 
 /* zpool driver */
@@ -455,7 +494,9 @@ MODULE_ALIAS("zpool-zsmalloc");
 #endif /* CONFIG_ZPOOL */
 
 /* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
-static DEFINE_PER_CPU(struct mapping_area, zs_map_area);
+static DEFINE_PER_CPU(struct mapping_area, zs_map_area) = {
+	.lock	= INIT_LOCAL_LOCK(lock),
+};
 
 static bool is_zspage_isolated(struct zspage *zspage)
 {
@@ -862,7 +903,13 @@ static unsigned long location_to_obj(str
 
 static unsigned long handle_to_obj(unsigned long handle)
 {
+#ifdef CONFIG_PREEMPT_RT
+	struct zsmalloc_handle *zh = zs_get_pure_handle(handle);
+
+	return zh->addr;
+#else
 	return *(unsigned long *)handle;
+#endif
 }
 
 static unsigned long obj_to_head(struct page *page, void *obj)
@@ -876,22 +923,46 @@ static unsigned long obj_to_head(struct
 
 static inline int testpin_tag(unsigned long handle)
 {
+#ifdef CONFIG_PREEMPT_RT
+	struct zsmalloc_handle *zh = zs_get_pure_handle(handle);
+
+	return spin_is_locked(&zh->lock);
+#else
 	return bit_spin_is_locked(HANDLE_PIN_BIT, (unsigned long *)handle);
+#endif
 }
 
 static inline int trypin_tag(unsigned long handle)
 {
+#ifdef CONFIG_PREEMPT_RT
+	struct zsmalloc_handle *zh = zs_get_pure_handle(handle);
+
+	return spin_trylock(&zh->lock);
+#else
 	return bit_spin_trylock(HANDLE_PIN_BIT, (unsigned long *)handle);
+#endif
 }
 
 static void pin_tag(unsigned long handle) __acquires(bitlock)
 {
+#ifdef CONFIG_PREEMPT_RT
+	struct zsmalloc_handle *zh = zs_get_pure_handle(handle);
+
+	return spin_lock(&zh->lock);
+#else
 	bit_spin_lock(HANDLE_PIN_BIT, (unsigned long *)handle);
+#endif
 }
 
 static void unpin_tag(unsigned long handle) __releases(bitlock)
 {
+#ifdef CONFIG_PREEMPT_RT
+	struct zsmalloc_handle *zh = zs_get_pure_handle(handle);
+
+	return spin_unlock(&zh->lock);
+#else
 	bit_spin_unlock(HANDLE_PIN_BIT, (unsigned long *)handle);
+#endif
 }
 
 static void reset_page(struct page *page)
@@ -1274,7 +1345,8 @@ void *zs_map_object(struct zs_pool *pool
 	class = pool->size_class[class_idx];
 	off = (class->size * obj_idx) & ~PAGE_MASK;
 
-	area = &get_cpu_var(zs_map_area);
+	local_lock(&zs_map_area.lock);
+	area = this_cpu_ptr(&zs_map_area);
 	area->vm_mm = mm;
 	if (off + class->size <= PAGE_SIZE) {
 		/* this object is contained entirely within a page */
@@ -1328,7 +1400,7 @@ void zs_unmap_object(struct zs_pool *poo
 
 		__zs_unmap_object(area, pages, off, class->size);
 	}
-	put_cpu_var(zs_map_area);
+	local_unlock(&zs_map_area.lock);
 
 	migrate_read_unlock(zspage);
 	unpin_tag(handle);