Re: [PATCH] slab: fix an infinite loop in leaks_show()

[Vlastimil Babka <vbabka@suse.cz>]

On 4/11/19 5:26 AM, Qian Cai wrote:
> "cat /proc/slab_allocators" could hang forever on SMP machines with
> kmemleak or object debugging enabled due to other CPUs running do_drain()
> will keep making kmemleak_object or debug_objects_cache dirty and unable
> to escape the first loop in leaks_show(),

So what if we don't remove SLAB (yet?) but start removing the debugging
functionality that has been broken for years and nobody noticed. I think
Linus already mentioned that we remove at least the
/proc/slab_allocators file...

> do {
> 	set_store_user_clean(cachep);
> 	drain_cpu_caches(cachep);
> 	...
> 
> } while (!is_store_user_clean(cachep));
> 
> For example,
> 
> do_drain
>   slabs_destroy
>     slab_destroy
>       kmem_cache_free
>         __cache_free
>           ___cache_free
>             kmemleak_free_recursive
>               delete_object_full
>                 __delete_object
>                   put_object
>                     free_object_rcu
>                       kmem_cache_free
>                         cache_free_debugcheck --> dirty kmemleak_object
> 
> One approach is to check cachep->name and skip both kmemleak_object and
> debug_objects_cache in leaks_show(). The other is to set
> store_user_clean after drain_cpu_caches() which leaves a small window
> between drain_cpu_caches() and set_store_user_clean() where per-CPU
> caches could be dirty again lead to slightly wrong information has been
> stored but could also speed up things significantly which sounds like a
> good compromise. For example,
> 
>  # cat /proc/slab_allocators
>  0m42.778s # 1st approach
>  0m0.737s  # 2nd approach
> 
> Fixes: d31676dfde25 ("mm/slab: alternative implementation for DEBUG_SLAB_LEAK")
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  mm/slab.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index 9142ee992493..3e1b7ff0360c 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4328,8 +4328,12 @@ static int leaks_show(struct seq_file *m, void *p)
>  	 * whole processing.
>  	 */
>  	do {
> -		set_store_user_clean(cachep);
>  		drain_cpu_caches(cachep);
> +		/*
> +		 * drain_cpu_caches() could always make kmemleak_object and
> +		 * debug_objects_cache dirty, so reset afterwards.
> +		 */
> +		set_store_user_clean(cachep);
>  
>  		x[1] = 0;
>  
>