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

[Roman Gushchin <guro@fb.com>]

On Fri, Sep 25, 2020 at 12:19:02PM -0700, Shakeel Butt wrote:
> 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>

I like the patch and I think it should fix the problem.

However the description above should be likely asjusted a bit.
It seems that the problem is not necessary caused by an infinite recursion,
it can be even simpler.

In cache_flusharray() we rely on the state of ac, which is described
by ac->avail. In particular we rely on batchcount < ac->avail,
as we shift the batchcount number of pointers by memmove.
But if slabs_destroy() is called before and leaded to a change of the
ac state, it can lead to a memory corruption.

Also, unconditionally resetting ac->avail to 0 in do_drain() after calling
to slab_destroy() seems to be wrong.
It explains double free BUGs we've seen in stacktraces.

Thanks!

> ---
>  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
>