[PATCH v2 2/5] slub: Use slab_list instead of lru

[Tobin C. Harding]

Currently we use the page->lru list for maintaining lists of slabs.  We
have a list_head in the page structure (slab_list) that can be used for
this purpose.  Doing so makes the code cleaner since we are not
overloading the lru list.

The slab_list is part of a union within the page struct (included here
stripped down):

	union {
		struct {	/* Page cache and anonymous pages */
			struct list_head lru;
			...
		};
		struct {
			dma_addr_t dma_addr;
		};
		struct {	/* slab, slob and slub */
			union {
				struct list_head slab_list;
				struct {	/* Partial pages */
					struct page *next;
					int pages;	/* Nr of pages left */
					int pobjects;	/* Approximate count */
				};
			};
		...

Here we see that slab_list and lru are the same bits.  We can verify
that this change is safe to do by examining the object file produced from
slub.c before and after this patch is applied.

Steps taken to verify:

 1. checkout current tip of Linus' tree

    commit a667cb7a94d4 ("Merge branch 'akpm' (patches from Andrew)")

 2. configure and build (defaults to SLUB allocator)

    CONFIG_SLUB=y
    CONFIG_SLUB_DEBUG=y
    CONFIG_SLUB_DEBUG_ON=y
    CONFIG_SLUB_STATS=y
    CONFIG_HAVE_DEBUG_KMEMLEAK=y
    CONFIG_SLAB_FREELIST_RANDOM=y
    CONFIG_SLAB_FREELIST_HARDENED=y

 3. dissasemble object file `objdump -dr mm/slub.o > before.s
 4. apply patch
 5. build
 6. dissasemble object file `objdump -dr mm/slub.o > after.s
 7. diff before.s after.s

Use slab_list list_head instead of the lru list_head for maintaining
lists of slabs.

Reviewed-by: Roman Gushchin <guro@xxxxxx>
Signed-off-by: Tobin C. Harding <tobin@xxxxxxxxxx>
---
 mm/slub.c | 40 ++++++++++++++++++++--------------------
 1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index b282e22885cd..d692b5e0163d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1023,7 +1023,7 @@ static void add_full(struct kmem_cache *s,
 		return;
 
 	lockdep_assert_held(&n->list_lock);
-	list_add(&page->lru, &n->full);
+	list_add(&page->slab_list, &n->full);
 }
 
 static void remove_full(struct kmem_cache *s, struct kmem_cache_node *n, struct page *page)
@@ -1032,7 +1032,7 @@ static void remove_full(struct kmem_cache *s, struct kmem_cache_node *n, struct
 		return;
 
 	lockdep_assert_held(&n->list_lock);
-	list_del(&page->lru);
+	list_del(&page->slab_list);
 }
 
 /* Tracking of the number of slabs for debugging purposes */
@@ -1773,9 +1773,9 @@ __add_partial(struct kmem_cache_node *n, struct page *page, int tail)
 {
 	n->nr_partial++;
 	if (tail == DEACTIVATE_TO_TAIL)
-		list_add_tail(&page->lru, &n->partial);
+		list_add_tail(&page->slab_list, &n->partial);
 	else
-		list_add(&page->lru, &n->partial);
+		list_add(&page->slab_list, &n->partial);
 }
 
 static inline void add_partial(struct kmem_cache_node *n,
@@ -1789,7 +1789,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
 					struct page *page)
 {
 	lockdep_assert_held(&n->list_lock);
-	list_del(&page->lru);
+	list_del(&page->slab_list);
 	n->nr_partial--;
 }
 
@@ -1863,7 +1863,7 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
 		return NULL;
 
 	spin_lock(&n->list_lock);
-	list_for_each_entry_safe(page, page2, &n->partial, lru) {
+	list_for_each_entry_safe(page, page2, &n->partial, slab_list) {
 		void *t;
 
 		if (!pfmemalloc_match(page, flags))
@@ -2407,7 +2407,7 @@ static unsigned long count_partial(struct kmem_cache_node *n,
 	struct page *page;
 
 	spin_lock_irqsave(&n->list_lock, flags);
-	list_for_each_entry(page, &n->partial, lru)
+	list_for_each_entry(page, &n->partial, slab_list)
 		x += get_count(page);
 	spin_unlock_irqrestore(&n->list_lock, flags);
 	return x;
@@ -3702,10 +3702,10 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
 
 	BUG_ON(irqs_disabled());
 	spin_lock_irq(&n->list_lock);
-	list_for_each_entry_safe(page, h, &n->partial, lru) {
+	list_for_each_entry_safe(page, h, &n->partial, slab_list) {
 		if (!page->inuse) {
 			remove_partial(n, page);
-			list_add(&page->lru, &discard);
+			list_add(&page->slab_list, &discard);
 		} else {
 			list_slab_objects(s, page,
 			"Objects remaining in %s on __kmem_cache_shutdown()");
@@ -3713,7 +3713,7 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
 	}
 	spin_unlock_irq(&n->list_lock);
 
-	list_for_each_entry_safe(page, h, &discard, lru)
+	list_for_each_entry_safe(page, h, &discard, slab_list)
 		discard_slab(s, page);
 }
 
@@ -3993,7 +3993,7 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 		 * Note that concurrent frees may occur while we hold the
 		 * list_lock. page->inuse here is the upper limit.
 		 */
-		list_for_each_entry_safe(page, t, &n->partial, lru) {
+		list_for_each_entry_safe(page, t, &n->partial, slab_list) {
 			int free = page->objects - page->inuse;
 
 			/* Do not reread page->inuse */
@@ -4003,10 +4003,10 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 			BUG_ON(free <= 0);
 
 			if (free == page->objects) {
-				list_move(&page->lru, &discard);
+				list_move(&page->slab_list, &discard);
 				n->nr_partial--;
 			} else if (free <= SHRINK_PROMOTE_MAX)
-				list_move(&page->lru, promote + free - 1);
+				list_move(&page->slab_list, promote + free - 1);
 		}
 
 		/*
@@ -4019,7 +4019,7 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 		spin_unlock_irqrestore(&n->list_lock, flags);
 
 		/* Release empty slabs */
-		list_for_each_entry_safe(page, t, &discard, lru)
+		list_for_each_entry_safe(page, t, &discard, slab_list)
 			discard_slab(s, page);
 
 		if (slabs_node(s, node))
@@ -4211,11 +4211,11 @@ static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
 	for_each_kmem_cache_node(s, node, n) {
 		struct page *p;
 
-		list_for_each_entry(p, &n->partial, lru)
+		list_for_each_entry(p, &n->partial, slab_list)
 			p->slab_cache = s;
 
 #ifdef CONFIG_SLUB_DEBUG
-		list_for_each_entry(p, &n->full, lru)
+		list_for_each_entry(p, &n->full, slab_list)
 			p->slab_cache = s;
 #endif
 	}
@@ -4432,7 +4432,7 @@ static int validate_slab_node(struct kmem_cache *s,
 
 	spin_lock_irqsave(&n->list_lock, flags);
 
-	list_for_each_entry(page, &n->partial, lru) {
+	list_for_each_entry(page, &n->partial, slab_list) {
 		validate_slab_slab(s, page, map);
 		count++;
 	}
@@ -4443,7 +4443,7 @@ static int validate_slab_node(struct kmem_cache *s,
 	if (!(s->flags & SLAB_STORE_USER))
 		goto out;
 
-	list_for_each_entry(page, &n->full, lru) {
+	list_for_each_entry(page, &n->full, slab_list) {
 		validate_slab_slab(s, page, map);
 		count++;
 	}
@@ -4639,9 +4639,9 @@ static int list_locations(struct kmem_cache *s, char *buf,
 			continue;
 
 		spin_lock_irqsave(&n->list_lock, flags);
-		list_for_each_entry(page, &n->partial, lru)
+		list_for_each_entry(page, &n->partial, slab_list)
 			process_slab(&t, s, page, alloc, map);
-		list_for_each_entry(page, &n->full, lru)
+		list_for_each_entry(page, &n->full, slab_list)
 			process_slab(&t, s, page, alloc, map);
 		spin_unlock_irqrestore(&n->list_lock, flags);
 	}
-- 
2.21.0