Re: [RFC PATCH 2/5] mm, slub: make some slub_debug related attributes read-only

[Roman Gushchin <guro@fb.com>]

On Tue, Jun 02, 2020 at 04:15:16PM +0200, Vlastimil Babka wrote:
> SLUB_DEBUG creates several files under /sys/kernel/slab/<cache>/ that can be
> read to check if the respective debugging options are enabled for given cache.
> The options can be also toggled at runtime by writing into the files. Some of
> those, namely red_zone, poison, and store_user can be toggled only when no
> objects yet exist in the cache.
> 
> Vijayanand reports [1] that there is a problem with freelist randomization if
> changing the debugging option's state results in different number of objects
> per page, and the random sequence cache needs thus needs to be recomputed.
> 
> However, another problem is that the check for "no objects yet exist in the
> cache" is racy, as noted by Jann [2] and fixing that would add overhead or
> otherwise complicate the allocation/freeing paths. Thus it would be much
> simpler just to remove the runtime toggling support. The documentation
> describes it's "In case you forgot to enable debugging on the kernel command
> line", but the neccessity of having no objects limits its usefulness anyway for
> many caches.
> 
> Vijayanand describes an use case [3] where debugging is enabled for all but
> zram caches for memory overhead reasons, and using the runtime toggles was the
> only way to achieve such configuration. After the previous patch it's now
> possible to do that directly from the kernel boot option, so we can remove the
> dangerous runtime toggles by making the /sys attribute files read-only.
> 
> While updating it, also improve the documentation of the debugging /sys files.
> 
> [1] https://lkml.kernel.org/r/1580379523-32272-1-git-send-email-vjitta@codeaurora.org
> [2] https://lore.kernel.org/r/CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@mail.gmail.com
> [3] https://lore.kernel.org/r/1383cd32-1ddc-4dac-b5f8-9c42282fa81c@codeaurora.org
> 
> Reported-by: Vijayanand Jitta <vjitta@codeaurora.org>
> Reported-by: Jann Horn <jannh@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

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

Thanks!

> ---
>  Documentation/vm/slub.rst | 28 ++++++++++++++----------
>  mm/slub.c                 | 46 +++------------------------------------
>  2 files changed, 20 insertions(+), 54 deletions(-)
> 
> diff --git a/Documentation/vm/slub.rst b/Documentation/vm/slub.rst
> index f1154f707e06..61805e984a0d 100644
> --- a/Documentation/vm/slub.rst
> +++ b/Documentation/vm/slub.rst
> @@ -100,20 +100,26 @@ except some that are deemed too performance critical and don't need to be
>  
>  	slub_debug=FZ;-,zs_handle,zspage
>  
> -In case you forgot to enable debugging on the kernel command line: It is
> -possible to enable debugging manually when the kernel is up. Look at the
> -contents of::
> +The state of each debug option for a slab can be found in the respective files
> +under::
>  
>  	/sys/kernel/slab/<slab name>/
>  
> -Look at the writable files. Writing 1 to them will enable the
> -corresponding debug option. All options can be set on a slab that does
> -not contain objects. If the slab already contains objects then sanity checks
> -and tracing may only be enabled. The other options may cause the realignment
> -of objects.
> -
> -Careful with tracing: It may spew out lots of information and never stop if
> -used on the wrong slab.
> +If the file contains 1, the option is enabled, 0 means disabled. The debug
> +options from the ``slub_debug`` parameter translate to the following files::
> +
> +	F	sanity_checks
> +	Z	red_zone
> +	P	poison
> +	U	store_user
> +	T	trace
> +	A	failslab
> +
> +The sanity_checks, trace and failslab files are writable, so writing 1 or 0
> +will enable or disable the option at runtime. The writes to trace and failslab
> +may return -EINVAL if the cache is subject to slab merging. Careful with
> +tracing: It may spew out lots of information and never stop if used on the
> +wrong slab.
>  
>  Slab merging
>  ============
> diff --git a/mm/slub.c b/mm/slub.c
> index 47430aad9a65..ac198202dbb0 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5351,61 +5351,21 @@ static ssize_t red_zone_show(struct kmem_cache *s, char *buf)
>  	return sprintf(buf, "%d\n", !!(s->flags & SLAB_RED_ZONE));
>  }
>  
> -static ssize_t red_zone_store(struct kmem_cache *s,
> -				const char *buf, size_t length)
> -{
> -	if (any_slab_objects(s))
> -		return -EBUSY;
> -
> -	s->flags &= ~SLAB_RED_ZONE;
> -	if (buf[0] == '1') {
> -		s->flags |= SLAB_RED_ZONE;
> -	}
> -	calculate_sizes(s, -1);
> -	return length;
> -}
> -SLAB_ATTR(red_zone);
> +SLAB_ATTR_RO(red_zone);
>  
>  static ssize_t poison_show(struct kmem_cache *s, char *buf)
>  {
>  	return sprintf(buf, "%d\n", !!(s->flags & SLAB_POISON));
>  }
>  
> -static ssize_t poison_store(struct kmem_cache *s,
> -				const char *buf, size_t length)
> -{
> -	if (any_slab_objects(s))
> -		return -EBUSY;
> -
> -	s->flags &= ~SLAB_POISON;
> -	if (buf[0] == '1') {
> -		s->flags |= SLAB_POISON;
> -	}
> -	calculate_sizes(s, -1);
> -	return length;
> -}
> -SLAB_ATTR(poison);
> +SLAB_ATTR_RO(poison);
>  
>  static ssize_t store_user_show(struct kmem_cache *s, char *buf)
>  {
>  	return sprintf(buf, "%d\n", !!(s->flags & SLAB_STORE_USER));
>  }
>  
> -static ssize_t store_user_store(struct kmem_cache *s,
> -				const char *buf, size_t length)
> -{
> -	if (any_slab_objects(s))
> -		return -EBUSY;
> -
> -	s->flags &= ~SLAB_STORE_USER;
> -	if (buf[0] == '1') {
> -		s->flags &= ~__CMPXCHG_DOUBLE;
> -		s->flags |= SLAB_STORE_USER;
> -	}
> -	calculate_sizes(s, -1);
> -	return length;
> -}
> -SLAB_ATTR(store_user);
> +SLAB_ATTR_RO(store_user);
>  
>  static ssize_t validate_show(struct kmem_cache *s, char *buf)
>  {
> -- 
> 2.26.2
> 
>