drm/ttm: Fix possible stack overflow by recursive shrinker calls.

While ttm_dma_pool_shrink_scan() tries to take mutex before doing GFP_KERNEL
allocation, ttm_pool_shrink_scan() does not do it. This can result in stack
overflow if kmalloc() in ttm_page_pool_free() triggered recursion due to
memory pressure.

  shrink_slab()
  => ttm_pool_shrink_scan()
     => ttm_page_pool_free()
        => kmalloc(GFP_KERNEL)
           => shrink_slab()
              => ttm_pool_shrink_scan()
                 => ttm_page_pool_free()
                    => kmalloc(GFP_KERNEL)

Change ttm_pool_shrink_scan() to do like ttm_dma_pool_shrink_scan() does.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable <stable@kernel.org> [2.6.35+]
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/drivers/gpu/drm/ttm/ttm_page_alloc.c b/drivers/gpu/drm/ttm/ttm_page_alloc.c
index beb8e75a3f00..edb83151041f 100644
--- a/drivers/gpu/drm/ttm/ttm_page_alloc.c
+++ b/drivers/gpu/drm/ttm/ttm_page_alloc.c
@@ -391,14 +391,17 @@ out:
 static unsigned long
 ttm_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 {
-        static atomic_t start_pool = ATOMIC_INIT(0);
+        static DEFINE_MUTEX(lock);
+        static unsigned start_pool;
         unsigned i;
-        unsigned pool_offset = atomic_add_return(1, &start_pool);
+        unsigned pool_offset;
         struct ttm_page_pool *pool;
         int shrink_pages = sc->nr_to_scan;
         unsigned long freed = 0;
 
-        pool_offset = pool_offset % NUM_POOLS;
+        if (!mutex_trylock(&lock))
+                return SHRINK_STOP;
+        pool_offset = ++start_pool % NUM_POOLS;
         /* select start pool in round robin fashion */
         for (i = 0; i < NUM_POOLS; ++i) {
                 unsigned nr_free = shrink_pages;
@@ -408,6 +411,7 @@ ttm_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
                 shrink_pages = ttm_page_pool_free(pool, nr_free);
                 freed += nr_free - shrink_pages;
         }
+        mutex_unlock(&lock);
         return freed;
 }