Re: [PATCH 9/9] mm, slab/slub: move and improve cache_from_obj()

[Kees Cook <keescook@chromium.org>]

On Thu, Jun 18, 2020 at 01:05:53PM -0700, Roman Gushchin wrote:
> On Thu, Jun 18, 2020 at 12:10:38PM +0200, Vlastimil Babka wrote:
> > To prvent the churn of your patch moving the cache_from_obj() back to slab.h, I
> > think it's best if we modify my patch. The patch below should be squashed into
> > the current version in mmots, with the commit log used for the whole result.
> > 
> > This will cause conflicts while reapplying Roman's
> > mm-memcg-slab-use-a-single-set-of-kmem_caches-for-all-allocations.patch which
> > can be fixed by
> > a) throwing away the conflicting hunks for cache_from_obj() in slab.c and slub.c
> > b) applying this hunk instead:
> > 
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -455,12 +455,11 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> >  	struct kmem_cache *cachep;
> >  
> >  	if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
> > -	    !memcg_kmem_enabled() &&
> >  	    !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
> >  		return s;
> >  
> >  	cachep = virt_to_cache(x);
> > -	if (WARN(cachep && !slab_equal_or_root(cachep, s),
> > +	if (WARN(cachep && cachep != s,
> >  		  "%s: Wrong slab cache. %s but object is from %s\n",
> >  		  __func__, s->name, cachep->name))
> >  		print_tracking(cachep, x);
> > 
> > The fixup patch itself:
> > ----8<----
> > From b8df607d92b37e5329ce7bda62b2b364cc249893 Mon Sep 17 00:00:00 2001
> > From: Vlastimil Babka <vbabka@suse.cz>
> > Date: Thu, 18 Jun 2020 11:52:03 +0200
> > Subject: [PATCH] mm, slab/slub: improve error reporting and overhead of
> >  cache_from_obj()

Andrew, do you need this separately, or can you extract this fixup from
this thread?

-Kees

> > 
> > The function cache_from_obj() was added by commit b9ce5ef49f00 ("sl[au]b:
> > always get the cache from its page in kmem_cache_free()") to support
> > kmemcg, where per-memcg cache can be different from the root one, so we
> > can't use the kmem_cache pointer given to kmem_cache_free().
> > 
> > Prior to that commit, SLUB already had debugging check+warning that could
> > be enabled to compare the given kmem_cache pointer to one referenced by
> > the slab page where the object-to-be-freed resides.  This check was moved
> > to cache_from_obj().  Later the check was also enabled for
> > SLAB_FREELIST_HARDENED configs by commit 598a0717a816 ("mm/slab: validate
> > cache membership under freelist hardening").
> > 
> > These checks and warnings can be useful especially for the debugging,
> > which can be improved.  Commit 598a0717a816 changed the pr_err() with
> > WARN_ON_ONCE() to WARN_ONCE() so only the first hit is now reported,
> > others are silent.  This patch changes it to WARN() so that all errors are
> > reported.
> > 
> > It's also useful to print SLUB allocation/free tracking info for the offending
> > object, if tracking is enabled. Thus, export the SLUB print_tracking() function
> > and provide an empty one for SLAB.
> > 
> > For SLUB we can also benefit from the static key check in
> > kmem_cache_debug_flags(), but we need to move this function to slab.h and
> > declare the static key there.
> > 
> > [1] https://lore.kernel.org/r/20200608230654.828134-18-guro@fb.com
> > 
> > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Acked-by: Roman Gushchin <guro@fb.com>
> 
> Thanks!
> 
> > ---
> >  mm/slab.c |  8 --------
> >  mm/slab.h | 45 +++++++++++++++++++++++++++++++++++++++++++++
> >  mm/slub.c | 38 +-------------------------------------
> >  3 files changed, 46 insertions(+), 45 deletions(-)
> > 
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 6134c4c36d4c..9350062ffc1a 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3672,14 +3672,6 @@ void *__kmalloc_track_caller(size_t size, gfp_t flags, unsigned long caller)
> >  }
> >  EXPORT_SYMBOL(__kmalloc_track_caller);
> >  
> > -static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> > -{
> > -	if (memcg_kmem_enabled())
> > -		return virt_to_cache(x);
> > -	else
> > -		return s;
> > -}
> > -
> >  /**
> >   * kmem_cache_free - Deallocate an object
> >   * @cachep: The cache the allocation was from.
> > diff --git a/mm/slab.h b/mm/slab.h
> > index a2696d306b62..a9f5ba9ce9a7 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -275,6 +275,34 @@ static inline int cache_vmstat_idx(struct kmem_cache *s)
> >  		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE;
> >  }
> >  
> > +#ifdef CONFIG_SLUB_DEBUG
> > +#ifdef CONFIG_SLUB_DEBUG_ON
> > +DECLARE_STATIC_KEY_TRUE(slub_debug_enabled);
> > +#else
> > +DECLARE_STATIC_KEY_FALSE(slub_debug_enabled);
> > +#endif
> > +extern void print_tracking(struct kmem_cache *s, void *object);
> > +#else
> > +static inline void print_tracking(struct kmem_cache *s, void *object)
> > +{
> > +}
> > +#endif
> > +
> > +/*
> > + * Returns true if any of the specified slub_debug flags is enabled for the
> > + * cache. Use only for flags parsed by setup_slub_debug() as it also enables
> > + * the static key.
> > + */
> > +static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
> > +{
> > +	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
> > +#ifdef CONFIG_SLUB_DEBUG
> > +	if (static_branch_unlikely(&slub_debug_enabled))
> > +		return s->flags & flags;
> > +#endif
> > +	return false;
> > +}
> > +
> >  #ifdef CONFIG_MEMCG_KMEM
> >  
> >  /* List of all root caches. */
> > @@ -503,6 +531,23 @@ static __always_inline void uncharge_slab_page(struct page *page, int order,
> >  	memcg_uncharge_slab(page, order, s);
> >  }
> >  
> > +static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> > +{
> > +	struct kmem_cache *cachep;
> > +
> > +	if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
> > +	    !memcg_kmem_enabled() &&
> > +	    !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
> > +		return s;
> > +
> > +	cachep = virt_to_cache(x);
> > +	if (WARN(cachep && !slab_equal_or_root(cachep, s),
> > +		  "%s: Wrong slab cache. %s but object is from %s\n",
> > +		  __func__, s->name, cachep->name))
> > +		print_tracking(cachep, x);
> > +	return cachep;
> > +}
> > +
> >  static inline size_t slab_ksize(const struct kmem_cache *s)
> >  {
> >  #ifndef CONFIG_SLUB
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 202fb423d195..0e635a8aa340 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -122,21 +122,6 @@ DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
> >  #endif
> >  #endif
> >  
> > -/*
> > - * Returns true if any of the specified slub_debug flags is enabled for the
> > - * cache. Use only for flags parsed by setup_slub_debug() as it also enables
> > - * the static key.
> > - */
> > -static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
> > -{
> > -	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
> > -#ifdef CONFIG_SLUB_DEBUG
> > -	if (static_branch_unlikely(&slub_debug_enabled))
> > -		return s->flags & flags;
> > -#endif
> > -	return false;
> > -}
> > -
> >  static inline bool kmem_cache_debug(struct kmem_cache *s)
> >  {
> >  	return kmem_cache_debug_flags(s, SLAB_DEBUG_FLAGS);
> > @@ -653,7 +638,7 @@ static void print_track(const char *s, struct track *t, unsigned long pr_time)
> >  #endif
> >  }
> >  
> > -static void print_tracking(struct kmem_cache *s, void *object)
> > +void print_tracking(struct kmem_cache *s, void *object)
> >  {
> >  	unsigned long pr_time = jiffies;
> >  	if (!(s->flags & SLAB_STORE_USER))
> > @@ -1525,10 +1510,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
> >  {
> >  	return false;
> >  }
> > -
> > -static void print_tracking(struct kmem_cache *s, void *object)
> > -{
> > -}
> >  #endif /* CONFIG_SLUB_DEBUG */
> >  
> >  /*
> > @@ -3180,23 +3161,6 @@ void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
> >  }
> >  #endif
> >  
> > -static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> > -{
> > -	struct kmem_cache *cachep;
> > -
> > -	if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
> > -	    !memcg_kmem_enabled() &&
> > -	    !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
> > -		return s;
> > -
> > -	cachep = virt_to_cache(x);
> > -	if (WARN(cachep && !slab_equal_or_root(cachep, s),
> > -		  "%s: Wrong slab cache. %s but object is from %s\n",
> > -		  __func__, s->name, cachep->name))
> > -		print_tracking(cachep, x);
> > -	return cachep;
> > -}
> > -
> >  void kmem_cache_free(struct kmem_cache *s, void *x)
> >  {
> >  	s = cache_from_obj(s, x);
> > -- 
> > 2.27.0
> > 
> > 

-- 
Kees Cook