On Tue, 8 Apr 2008, Pekka Enberg wrote:
quoted text
> Actually, a trivial way to implement that is to have a few "emergency kmalloc"
> caches say for sizes 64, 128, 256, and 512 that have some pre-allocated pages
> into which these GFP_TEMPORARY allocations are allowed to dip into on OOM and
> OOM only.

So something like the following (totally untested) patch modulo the 
pre-allocation bits.

		Pekka

diff --git a/mm/slub.c b/mm/slub.c
index acc975f..e8f7cf3 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -264,6 +264,67 @@ static inline void stat(struct kmem_cache_cpu *c, enum stat_item si)
 #endif
 }
 
+/*
+ * Emergency caches are for GFP_TEMPORARY allocations to dip into when we're
+ * OOM to make sure we can make some progress for writeback.
+ */
+
+#define MIN_EMERGENCY_SIZE 32
+#define NR_EMERGENCY_CACHES 4
+
+struct kmem_cache *emergency_caches[NR_EMERGENCY_CACHES];
+
+static void init_emergency_caches(void)
+{
+	unsigned long size = MIN_EMERGENCY_SIZE;
+	int i;
+
+	for (i = 0; i < NR_EMERGENCY_CACHES; i++) {
+		struct kmem_cache *cache;
+		char *name;
+
+		name = kasprintf(GFP_KERNEL, "kmalloc-%d", 1 << i);
+		BUG_ON(!name);
+
+		cache = kmem_cache_create(name, size, 0, 0, NULL);
+		BUG_ON(!cache);
+		kfree(name);
+
+		emergency_caches[i] = cache;
+
+		size *= 2;
+	}
+}
+
+static struct kmem_cache *lookup_emergency_cache(unsigned long size)
+{
+	unsigned long cache_size = MIN_EMERGENCY_SIZE;
+	int i;
+
+	for (i = 0; i < NR_EMERGENCY_CACHES; i++) {
+		if (size < cache_size)
+			return emergency_caches[i];
+
+		cache_size *= 2;
+	}
+	return NULL;
+}
+
+static void *emergency_alloc(struct kmem_cache *cache, gfp_t gfp)
+{
+	struct kmem_cache *emergency_cache;
+	void *p;
+
+	emergency_cache = lookup_emergency_cache(cache->objsize);
+	if (!emergency_cache)
+		return NULL;
+
+	p = kmem_cache_alloc(emergency_cache, gfp);
+	if (p && cache->ctor)
+		cache->ctor(cache, p);
+	return p;
+}
+
 /********************************************************************
  * 			Core slab cache functions
  *******************************************************************/
@@ -1528,6 +1589,9 @@ new_slab:
 		goto load_freelist;
 	}
 
+	if ((gfpflags & GFP_TEMPORARY) == GFP_TEMPORARY)
+		return emergency_alloc(s, gfpflags);
+
 	/*
 	 * No memory available.
 	 *
@@ -2970,6 +3034,7 @@ void __init kmem_cache_init(void)
 #else
 	kmem_size = sizeof(struct kmem_cache);
 #endif
+	init_emergency_caches();
 
 	printk(KERN_INFO
 		"SLUB: Genslabs=%d, HWalign=%d, Order=%d-%d, MinObjects=%d,"
--
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