Re: [PATCH 30/31] Fix use of uninitialized variable in cache_grow()

On Fri, 2 Oct 2009, Neil Brown wrote:

quoted

quoted

Index: mmotm/mm/slab.c
===================================================================

--- mmotm.orig/mm/slab.c
+++ mmotm/mm/slab.c

@@ -2760,7 +2760,7 @@ static int cache_grow(struct kmem_cache
 	size_t offset;
 	gfp_t local_flags;
 	struct kmem_list3 *l3;
-	int reserve;
+	int reserve = -1;
 
 	/*
 	 * Be lazy and only check for valid flags here,  keeping it out of the

@@ -2816,7 +2816,8 @@ static int cache_grow(struct kmem_cache
 	if (local_flags & __GFP_WAIT)
 		local_irq_disable();
 	check_irq_off();
-	slab_set_reserve(cachep, reserve);
+	if (reserve != -1)
+		slab_set_reserve(cachep, reserve);
 	spin_lock(&l3->list_lock);
 
 	/* Make slab active. */

Given the patch description, shouldn't this be a test for objp != NULL 
instead, then?

In between those to patch hunks, cache_grow contains the code:
	if (!objp)
		objp = kmem_getpages(cachep, local_flags, nodeid, &reserve);
	if (!objp)
		goto failed;

We can no longer test if objp was NULL on entry to the function.
We could take a copy of objp on entry to the function, and test it
here.  But initialising 'reserve' to an invalid value is easier.

Seems like you could do all this in kmem_getpages(), then, by calling 
slab_set_reserve(cachep, page->reserve) before returning the new page?

 [ I'd also drop the branch in slab_set_reserve(), it's faster to just 
   assign it unconditionally. ]