Re: [RFC PATCH 3/5] mm, slub: remove runtime allocation order changes

[Roman Gushchin <guro@fb.com>]

On Tue, Jun 02, 2020 at 04:15:17PM +0200, Vlastimil Babka wrote:
> SLUB allows runtime changing of page allocation order by writing into the
> /sys/kernel/slab/<cache>/order file. Jann has reported [1] that this interface
> allows the order to be set too small, leading to crashes.
> 
> While it's possible to fix the immediate issue, closer inspection reveals
> potential races. Storing the new order calls calculate_sizes() which
> non-atomically updates a lot of kmem_cache fields while the cache is still in
> use. Unexpected behavior might occur even if the fields are set to the same
> value as they were.
> 
> This could be fixed by splitting out the part of calculate_sizes() that depends
> on forced_order, so that we only update kmem_cache.oo field. This could still
> race with init_cache_random_seq(), shuffle_freelist(), allocate_slab(). Perhaps
> it's possible to audit and e.g. add some READ_ONCE/WRITE_ONCE accesses, it
> might be easier just to remove the runtime order changes, which is what this
> patch does. If there are valid usecases for per-cache order setting, we could
> e.g. extend the boot parameters to do that.
> 
> [1] https://lore.kernel.org/r/CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@mail.gmail.com
> 
> Reported-by: Jann Horn <jannh@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Roman Gushchin <guro@fb.com>

Thanks!

> ---
>  mm/slub.c | 19 +------------------
>  1 file changed, 1 insertion(+), 18 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index ac198202dbb0..58c1e9e7b3b3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5111,28 +5111,11 @@ static ssize_t objs_per_slab_show(struct kmem_cache *s, char *buf)
>  }
>  SLAB_ATTR_RO(objs_per_slab);
>  
> -static ssize_t order_store(struct kmem_cache *s,
> -				const char *buf, size_t length)
> -{
> -	unsigned int order;
> -	int err;
> -
> -	err = kstrtouint(buf, 10, &order);
> -	if (err)
> -		return err;
> -
> -	if (order > slub_max_order || order < slub_min_order)
> -		return -EINVAL;
> -
> -	calculate_sizes(s, order);
> -	return length;
> -}
> -
>  static ssize_t order_show(struct kmem_cache *s, char *buf)
>  {
>  	return sprintf(buf, "%u\n", oo_order(s->oo));
>  }
> -SLAB_ATTR(order);
> +SLAB_ATTR_RO(order);
>  
>  static ssize_t min_partial_show(struct kmem_cache *s, char *buf)
>  {
> -- 
> 2.26.2
> 
>