Re: [PATCH] slab: introduce the flag SLAB_MINIMIZE_WASTE

[Christopher Lameter <cl@linux.com>]

On Wed, 21 Mar 2018, Mikulas Patocka wrote:

> For example, if someone creates a slab cache with the flag SLAB_CACHE_DMA,
> and he allocates an object from this cache and this allocation races with
> the user writing to /sys/kernel/slab/cache/order - then the allocator can
> for a small period of time see "s->allocflags == 0" and allocate a non-DMA
> page. That is a bug.

True we need to fix that:

Subject: Avoid potentially visible allocflags without all flags set

During slab size recalculation s->allocflags may be temporarily set
to 0 and thus the flags may not be set which may result in the wrong
flags being passed. Slab size calculation happens in two cases:

1. When a slab is created (which is safe since we cannot have
   concurrent allocations)

2. When the slab order is changed via /sysfs.

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


Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c
+++ linux/mm/slub.c
@@ -3457,6 +3457,7 @@ static void set_cpu_partial(struct kmem_
 static int calculate_sizes(struct kmem_cache *s, int forced_order)
 {
 	slab_flags_t flags = s->flags;
+	gfp_t allocflags;
 	size_t size = s->object_size;
 	int order;

@@ -3551,16 +3552,17 @@ static int calculate_sizes(struct kmem_c
 	if (order < 0)
 		return 0;

-	s->allocflags = 0;
+	allocflags = 0;
 	if (order)
-		s->allocflags |= __GFP_COMP;
+		allocflags |= __GFP_COMP;

 	if (s->flags & SLAB_CACHE_DMA)
-		s->allocflags |= GFP_DMA;
+		allocflags |= GFP_DMA;

 	if (s->flags & SLAB_RECLAIM_ACCOUNT)
-		s->allocflags |= __GFP_RECLAIMABLE;
+		allocflags |= __GFP_RECLAIMABLE;

+	s->allocflags = allocflags;
 	/*
 	 * Determine the number of objects per slab
 	 */