Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver_tag

Shakeel Butt
2020-09-25T20:38:49Z

On Fri, Sep 25, 2020 at 10:58 AM Shakeel Butt <shakeelb@google.com>
wrote:
>
[snip]
>
> I don't think you can ignore the flushing. The __free_once() in
> ___cache_free() assumes there is a space available.
>
> BTW do_drain() also have the same issue.
>
> Why not move slabs_destroy() after we update ac->avail and memmove()?

Ming, can you please try the following patch?

From: Shakeel Butt <shakeelb@google.com>

[PATCH] mm: slab: fix potential infinite recursion in ___cache_free

With the commit 10befea91b61 ("mm: memcg/slab: use a single set of
kmem_caches for all allocations"), it becomes possible to call kfree()
from the slabs_destroy(). However if slabs_destroy() is being called for
the array_cache of the local CPU then this opens the potential scenario
of infinite recursion because kfree() called from slabs_destroy() can
call slabs_destroy() with the same array_cache of the local CPU. Since
the array_cache of the local CPU is not updated before calling
slabs_destroy(), it will try to free the same pages.

To fix the issue, simply update the cache before calling
slabs_destroy().

Signed-off-by: Shakeel Butt <shakeelb@google.com>
---
mm/slab.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 3160dff6fd76..f658e86ec8ce 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1632,6 +1632,10 @@ static void slab_destroy(struct kmem_cache *cachep, struct page *page)
kmem_cache_free(cachep->freelist_cache, freelist);
}

+/*
+ * Update the size of the caches before calling slabs_destroy as it may
+ * recursively call kfree.
+ */
static void slabs_destroy(struct kmem_cache *cachep, struct list_head *list)
{
struct page *page, *n;
@@ -2153,8 +2157,8 @@ static void do_drain(void *arg)
spin_lock(&n->list_lock);
free_block(cachep, ac->entry, ac->avail, node, &list);
spin_unlock(&n->list_lock);
- slabs_destroy(cachep, &list);
ac->avail = 0;
+ slabs_destroy(cachep, &list);
}

static void drain_cpu_caches(struct kmem_cache *cachep)
@@ -3402,9 +3406,9 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
}
#endif
spin_unlock(&n->list_lock);
- slabs_destroy(cachep, &list);
ac->avail -= batchcount;
memmove(ac->entry, &(ac->entry[batchcount]), sizeof(void *)*ac->avail);
+ slabs_destroy(cachep, &list);
}

/*
--
2.28.0.681.g6f77f65b4e-goog