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Subject[PATCH] mm: avoid slub allocation while holding list_lock
From
If we are already under list_lock, don't call kmalloc(). Otherwise we
will run into deadlock because kmalloc() also tries to grab the same
lock.

Instead, allocate pages directly. Given currently page->objects has
15 bits, we only need 1 page. We may waste some memory but we only do
so when slub debug is on.

WARNING: possible recursive locking detected
--------------------------------------------
mount-encrypted/4921 is trying to acquire lock:
(&(&n->list_lock)->rlock){-.-.}, at: ___slab_alloc+0x104/0x437

but task is already holding lock:
(&(&n->list_lock)->rlock){-.-.}, at: __kmem_cache_shutdown+0x81/0x3cb

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&(&n->list_lock)->rlock);
lock(&(&n->list_lock)->rlock);

*** DEADLOCK ***

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
mm/slub.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 8834563cdb4b..574a53ee31e1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3683,7 +3683,11 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
#ifdef CONFIG_SLUB_DEBUG
void *addr = page_address(page);
void *p;
- unsigned long *map = bitmap_zalloc(page->objects, GFP_ATOMIC);
+ int order;
+ unsigned long *map;
+
+ order = get_order(DIV_ROUND_UP(page->objects, BITS_PER_BYTE));
+ map = (void *)__get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
if (!map)
return;
slab_err(s, page, text, s->name);
@@ -3698,7 +3702,7 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
}
}
slab_unlock(page);
- bitmap_free(map);
+ free_pages((unsigned long)map, order);
#endif
}

--
2.23.0.187.g17f5b7556c-goog
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