[PATCH 0/4] rcu: don't assume the size of struct rcu_head

[PATCH 0/4] rcu: don't assume the size of struct rcu_head

[Lai Jiangshan]

We always keep the struct rcu_head very small, but we may change it in future
or under some CONFIGs.

There are some other systems may assume the size of struct rcu_head as 2 * sizeof(long).
These assumptions obstruct us to add debug information or priority information
to struct rcu_head. It is time to fix them.

It is glad that I just find 3 places which need to be fixed. These 4 patches
are just cleanup patches when the size of struct rcu_head == 2 * sizeof(long).
NO overhead added and NO behavior changed.
Even when the size of struct rcu_head becomes larger, only slub is changed a little.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---

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[PATCH 1/3 V2] slub: automatically reserve bytes at the end of slab

[Lai Jiangshan]

There is no "struct" for slub's slab, it shares with struct page.
But struct page is very small, it is insufficient when we need
to add some metadata for slab.

So we add a field "reserved" to struct kmem_cache, when a slab
is allocated, kmem_cache->reserved bytes are automatically reserved
at the end of the slab for slab's metadata.

Changed from v1:
	Export the reserved field via sysfs

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 include/linux/slub_def.h |    1 +
 mm/slub.c                |   47 +++++++++++++++++++++++++++++----------------
 2 files changed, 31 insertions(+), 17 deletions(-)

diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index 8b6e8ae..ae0093c 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -83,6 +83,7 @@ struct kmem_cache {
 	void (*ctor)(void *);
 	int inuse;		/* Offset to metadata */
 	int align;		/* Alignment */
+	int reserved;		/* Reserved bytes at the end of slabs */
 	unsigned long min_partial;
 	const char *name;	/* Name (only for display!) */
 	struct list_head list;	/* List of slab caches */
diff --git a/mm/slub.c b/mm/slub.c
index e15aa7f..d3d1767 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -281,11 +281,16 @@ static inline int slab_index(void *p, struct kmem_cache *s, void *addr)
 	return (p - addr) / s->size;
 }
 
+static inline int order_objects(int order, unsigned long size, int reserved)
+{
+	return ((PAGE_SIZE << order) - reserved) / size;
+}
+
 static inline struct kmem_cache_order_objects oo_make(int order,
-						unsigned long size)
+		unsigned long size, int reserved)
 {
 	struct kmem_cache_order_objects x = {
-		(order << OO_SHIFT) + (PAGE_SIZE << order) / size
+		(order << OO_SHIFT) + order_objects(order, size, reserved)
 	};
 
 	return x;
@@ -617,7 +622,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
 		return 1;
 
 	start = page_address(page);
-	length = (PAGE_SIZE << compound_order(page));
+	length = (PAGE_SIZE << compound_order(page)) - s->reserved;
 	end = start + length;
 	remainder = length % s->size;
 	if (!remainder)
@@ -698,7 +703,7 @@ static int check_slab(struct kmem_cache *s, struct page *page)
 		return 0;
 	}
 
-	maxobj = (PAGE_SIZE << compound_order(page)) / s->size;
+	maxobj = order_objects(compound_order(page), s->size, s->reserved);
 	if (page->objects > maxobj) {
 		slab_err(s, page, "objects %u > max %u",
 			s->name, page->objects, maxobj);
@@ -748,7 +753,7 @@ static int on_freelist(struct kmem_cache *s, struct page *page, void *search)
 		nr++;
 	}
 
-	max_objects = (PAGE_SIZE << compound_order(page)) / s->size;
+	max_objects = order_objects(compound_order(page), s->size, s->reserved);
 	if (max_objects > MAX_OBJS_PER_PAGE)
 		max_objects = MAX_OBJS_PER_PAGE;
 
@@ -1988,13 +1993,13 @@ static int slub_nomerge;
  * the smallest order which will fit the object.
  */
 static inline int slab_order(int size, int min_objects,
-				int max_order, int fract_leftover)
+				int max_order, int fract_leftover, int reserved)
 {
 	int order;
 	int rem;
 	int min_order = slub_min_order;
 
-	if ((PAGE_SIZE << min_order) / size > MAX_OBJS_PER_PAGE)
+	if (order_objects(min_order, size, reserved) > MAX_OBJS_PER_PAGE)
 		return get_order(size * MAX_OBJS_PER_PAGE) - 1;
 
 	for (order = max(min_order,
@@ -2003,10 +2008,10 @@ static inline int slab_order(int size, int min_objects,
 
 		unsigned long slab_size = PAGE_SIZE << order;
 
-		if (slab_size < min_objects * size)
+		if (slab_size < min_objects * size + reserved)
 			continue;
 
-		rem = slab_size % size;
+		rem = (slab_size - reserved) % size;
 
 		if (rem <= slab_size / fract_leftover)
 			break;
@@ -2016,7 +2021,7 @@ static inline int slab_order(int size, int min_objects,
 	return order;
 }
 
-static inline int calculate_order(int size)
+static inline int calculate_order(int size, int reserved)
 {
 	int order;
 	int min_objects;
@@ -2034,14 +2039,14 @@ static inline int calculate_order(int size)
 	min_objects = slub_min_objects;
 	if (!min_objects)
 		min_objects = 4 * (fls(nr_cpu_ids) + 1);
-	max_objects = (PAGE_SIZE << slub_max_order)/size;
+	max_objects = order_objects(slub_max_order, size, reserved);
 	min_objects = min(min_objects, max_objects);
 
 	while (min_objects > 1) {
 		fraction = 16;
 		while (fraction >= 4) {
 			order = slab_order(size, min_objects,
-						slub_max_order, fraction);
+					slub_max_order, fraction, reserved);
 			if (order <= slub_max_order)
 				return order;
 			fraction /= 2;
@@ -2053,14 +2058,14 @@ static inline int calculate_order(int size)
 	 * We were unable to place multiple objects in a slab. Now
 	 * lets see if we can place a single object there.
 	 */
-	order = slab_order(size, 1, slub_max_order, 1);
+	order = slab_order(size, 1, slub_max_order, 1, reserved);
 	if (order <= slub_max_order)
 		return order;
 
 	/*
 	 * Doh this slab cannot be placed using slub_max_order.
 	 */
-	order = slab_order(size, 1, MAX_ORDER, 1);
+	order = slab_order(size, 1, MAX_ORDER, 1, reserved);
 	if (order < MAX_ORDER)
 		return order;
 	return -ENOSYS;
@@ -2311,7 +2316,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 	if (forced_order >= 0)
 		order = forced_order;
 	else
-		order = calculate_order(size);
+		order = calculate_order(size, s->reserved);
 
 	if (order < 0)
 		return 0;
@@ -2329,8 +2334,8 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 	/*
 	 * Determine the number of objects per slab
 	 */
-	s->oo = oo_make(order, size);
-	s->min = oo_make(get_order(size), size);
+	s->oo = oo_make(order, size, s->reserved);
+	s->min = oo_make(get_order(size), size, s->reserved);
 	if (oo_objects(s->oo) > oo_objects(s->max))
 		s->max = s->oo;
 
@@ -2349,6 +2354,7 @@ static int kmem_cache_open(struct kmem_cache *s,
 	s->objsize = size;
 	s->align = align;
 	s->flags = kmem_cache_flags(size, flags, name, ctor);
+	s->reserved = 0;
 
 	if (!calculate_sizes(s, -1))
 		goto error;
@@ -4017,6 +4023,12 @@ static ssize_t destroy_by_rcu_show(struct kmem_cache *s, char *buf)
 }
 SLAB_ATTR_RO(destroy_by_rcu);
 
+static ssize_t reserved_show(struct kmem_cache *s, char *buf)
+{
+	return sprintf(buf, "%d\n", s->reserved);
+}
+SLAB_ATTR_RO(reserved);
+
 #ifdef CONFIG_SLUB_DEBUG
 static ssize_t slabs_show(struct kmem_cache *s, char *buf)
 {
@@ -4303,6 +4315,7 @@ static struct attribute *slab_attrs[] = {
 	&reclaim_account_attr.attr,
 	&destroy_by_rcu_attr.attr,
 	&shrink_attr.attr,
+	&reserved_attr.attr,
 #ifdef CONFIG_SLUB_DEBUG
 	&total_objects_attr.attr,
 	&slabs_attr.attr,

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[PATCH 2/3 V2] slub,rcu: don't assume the size of struct rcu_head

[Lai Jiangshan]

The size of struct rcu_head may be changed. When it becomes larger,
it will pollute the page array.

We reserve some some bytes for struct rcu_head when a slab
is allocated in this situation.

Changed from V1:
	use VM_BUG_ON instead BUG_ON

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Christoph Lameter <cl@linux.com>
---
 mm/slub.c |   30 +++++++++++++++++++++++++-----
 1 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index d3d1767..ebba3eb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1254,21 +1254,38 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 	__free_pages(page, order);
 }
 
+#define need_reserve_slab_rcu						\
+	(sizeof(((struct page *)NULL)->lru) < sizeof(struct rcu_head))
+
 static void rcu_free_slab(struct rcu_head *h)
 {
 	struct page *page;
 
-	page = container_of((struct list_head *)h, struct page, lru);
+	if (need_reserve_slab_rcu)
+		page = virt_to_head_page(h);
+	else
+		page = container_of((struct list_head *)h, struct page, lru);
+
 	__free_slab(page->slab, page);
 }
 
 static void free_slab(struct kmem_cache *s, struct page *page)
 {
 	if (unlikely(s->flags & SLAB_DESTROY_BY_RCU)) {
-		/*
-		 * RCU free overloads the RCU head over the LRU
-		 */
-		struct rcu_head *head = (void *)&page->lru;
+		struct rcu_head *head;
+
+		if (need_reserve_slab_rcu) {
+			int order = compound_order(page);
+			int offset = (PAGE_SIZE << order) - s->reserved;
+
+			VM_BUG_ON(s->reserved != sizeof(*head));
+			head = page_address(page) + offset;
+		} else {
+			/*
+			 * RCU free overloads the RCU head over the LRU
+			 */
+			head = (void *)&page->lru;
+		}
 
 		call_rcu(head, rcu_free_slab);
 	} else
@@ -2356,6 +2373,9 @@ static int kmem_cache_open(struct kmem_cache *s,
 	s->flags = kmem_cache_flags(size, flags, name, ctor);
 	s->reserved = 0;
 
+	if (need_reserve_slab_rcu && (s->flags & SLAB_DESTROY_BY_RCU))
+		s->reserved = sizeof(struct rcu_head);
+
 	if (!calculate_sizes(s, -1))
 		goto error;
 	if (disable_higher_order_debug) {

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[PATCH 3/3 V2] slab,rcu: don't assume the size of struct rcu_head

[Lai Jiangshan]

The size of struct rcu_head may be changed. When it becomes larger,
it may pollute the data after struct slab.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/mm/slab.c b/mm/slab.c
index 37961d1..52cf0b4 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -191,22 +191,6 @@ typedef unsigned int kmem_bufctl_t;
 #define	SLAB_LIMIT	(((kmem_bufctl_t)(~0U))-3)
 
 /*
- * struct slab
- *
- * Manages the objs in a slab. Placed either at the beginning of mem allocated
- * for a slab, or allocated from an general cache.
- * Slabs are chained into three list: fully used, partial, fully free slabs.
- */
-struct slab {
-	struct list_head list;
-	unsigned long colouroff;
-	void *s_mem;		/* including colour offset */
-	unsigned int inuse;	/* num of objs active in slab */
-	kmem_bufctl_t free;
-	unsigned short nodeid;
-};
-
-/*
  * struct slab_rcu
  *
  * slab_destroy on a SLAB_DESTROY_BY_RCU cache uses this structure to
@@ -219,8 +203,6 @@ struct slab {
  *
  * rcu_read_lock before reading the address, then rcu_read_unlock after
  * taking the spinlock within the structure expected at that address.
- *
- * We assume struct slab_rcu can overlay struct slab when destroying.
  */
 struct slab_rcu {
 	struct rcu_head head;
@@ -229,6 +211,27 @@ struct slab_rcu {
 };
 
 /*
+ * struct slab
+ *
+ * Manages the objs in a slab. Placed either at the beginning of mem allocated
+ * for a slab, or allocated from an general cache.
+ * Slabs are chained into three list: fully used, partial, fully free slabs.
+ */
+struct slab {
+	union {
+		struct {
+			struct list_head list;
+			unsigned long colouroff;
+			void *s_mem;		/* including colour offset */
+			unsigned int inuse;	/* num of objs active in slab */
+			kmem_bufctl_t free;
+			unsigned short nodeid;
+		};
+		struct slab_rcu __slab_cover_slab_rcu;
+	};
+};
+
+/*
  * struct array_cache
  *
  * Purpose:
--

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Re: [PATCH 1/3 V2] slub: automatically reserve bytes at the end of slab

[Christoph Lameter]

Acked-by: Christoph Lameter <cl@linux.com>

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Re: [PATCH 2/3 V2] slub,rcu: don't assume the size of struct rcu_head

[Christoph Lameter]

Acked-by: Christoph Lameter <cl@linux.com>

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Re: [PATCH 3/3 V2] slab,rcu: don't assume the size of struct rcu_head

[Christoph Lameter]

Acked-by: Christoph Lameter <cl@linux.com>

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