Date:   Thu, 10 Dec 2020 19:33:59 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        andrii@...nel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 sl-b 1/6] mm: Add mem_dump_obj() to print source of
 memory block

On Fri, Dec 11, 2020 at 11:22:10AM +0900, Joonsoo Kim wrote:
> On Thu, Dec 10, 2020 at 05:19:58PM -0800, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> > 
> > There are kernel facilities such as per-CPU reference counts that give
> > error messages in generic handlers or callbacks, whose messages are
> > unenlightening.  In the case of per-CPU reference-count underflow, this
> > is not a problem when creating a new use of this facility because in that
> > case the bug is almost certainly in the code implementing that new use.
> > However, trouble arises when deploying across many systems, which might
> > exercise corner cases that were not seen during development and testing.
> > Here, it would be really nice to get some kind of hint as to which of
> > several uses the underflow was caused by.
> > 
> > This commit therefore exposes a mem_dump_obj() function that takes
> > a pointer to memory (which must still be allocated if it has been
> > dynamically allocated) and prints available information on where that
> > memory came from.  This pointer can reference the middle of the block as
> > well as the beginning of the block, as needed by things like RCU callback
> > functions and timer handlers that might not know where the beginning of
> > the memory block is.  These functions and handlers can use mem_dump_obj()
> > to print out better hints as to where the problem might lie.
> > 
> > The information printed can depend on kernel configuration.  For example,
> > the allocation return address can be printed only for slab and slub,
> > and even then only when the necessary debug has been enabled.  For slab,
> > build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
> > to the next power of two or use the SLAB_STORE_USER when creating the
> > kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
> > boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
> > if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
> > to enable printing of the allocation-time stack trace.
> > 
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Pekka Enberg <penberg@...nel.org>
> > Cc: David Rientjes <rientjes@...gle.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: <linux-mm@...ck.org>
> > Reported-by: Andrii Nakryiko <andrii@...nel.org>
> > [ paulmck: Convert to printing and change names per Joonsoo Kim. ]
> > [ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
> > [ paulmck: Handle CONFIG_MMU=n case where vmalloc() is kmalloc(). ]
> > [ paulmck: Apply Vlastimil Babka feedback on slab.c kmem_provenance(). ]
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  include/linux/mm.h   |  2 ++
> >  include/linux/slab.h |  2 ++
> >  mm/slab.c            | 20 ++++++++++++++
> >  mm/slab.h            | 12 +++++++++
> >  mm/slab_common.c     | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  mm/slob.c            |  6 +++++
> >  mm/slub.c            | 36 +++++++++++++++++++++++++
> >  mm/util.c            | 24 +++++++++++++++++
> >  8 files changed, 176 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index ef360fe..1eea266 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3153,5 +3153,7 @@ unsigned long wp_shared_mapping_range(struct address_space *mapping,
> >  
> >  extern int sysctl_nr_trim_pages;
> >  
> > +void mem_dump_obj(void *object);
> > +
> >  #endif /* __KERNEL__ */
> >  #endif /* _LINUX_MM_H */
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index dd6897f..169b511 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -186,6 +186,8 @@ void kfree(const void *);
> >  void kfree_sensitive(const void *);
> >  size_t __ksize(const void *);
> >  size_t ksize(const void *);
> > +bool kmem_valid_obj(void *object);
> > +void kmem_dump_obj(void *object);
> >  
> >  #ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
> >  void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
> > diff --git a/mm/slab.c b/mm/slab.c
> > index b111356..66f00ad 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3633,6 +3633,26 @@ void *__kmalloc_node_track_caller(size_t size, gfp_t flags,
> >  EXPORT_SYMBOL(__kmalloc_node_track_caller);
> >  #endif /* CONFIG_NUMA */
> >  
> > +void kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct page *page)
> > +{
> > +	struct kmem_cache *cachep;
> > +	unsigned int objnr;
> > +	void *objp;
> > +
> > +	kpp->kp_ptr = object;
> > +	kpp->kp_page = page;
> > +	cachep = page->slab_cache;
> > +	kpp->kp_slab_cache = cachep;
> > +	objp = object - obj_offset(cachep);
> > +	kpp->kp_data_offset = obj_offset(cachep);
> > +	page = virt_to_head_page(objp);
> > +	objnr = obj_to_index(cachep, page, objp);
> > +	objp = index_to_obj(cachep, page, objnr);
> > +	kpp->kp_objp = objp;
> > +	if (DEBUG && cachep->flags & SLAB_STORE_USER)
> > +		kpp->kp_ret = *dbg_userword(cachep, objp);
> > +}
> > +
> >  /**
> >   * __do_kmalloc - allocate memory
> >   * @size: how many bytes of memory are required.
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 6d7c6a5..0dc705b 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -630,4 +630,16 @@ static inline bool slab_want_init_on_free(struct kmem_cache *c)
> >  	return false;
> >  }
> >  
> > +#define KS_ADDRS_COUNT 16
> > +struct kmem_obj_info {
> > +	void *kp_ptr;
> > +	struct page *kp_page;
> > +	void *kp_objp;
> > +	unsigned long kp_data_offset;
> > +	struct kmem_cache *kp_slab_cache;
> > +	void *kp_ret;
> > +	void *kp_stack[KS_ADDRS_COUNT];
> > +};
> > +void kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct page *page);
> > +
> >  #endif /* MM_SLAB_H */
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index f9ccd5d..df2e203 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -536,6 +536,80 @@ bool slab_is_available(void)
> >  	return slab_state >= UP;
> >  }
> >  
> > +/**
> > + * kmem_valid_obj - does the pointer reference a valid slab object?
> > + * @object: pointer to query.
> > + *
> > + * Return: %true if the pointer is to a not-yet-freed object from
> > + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> > + * is to an already-freed object, and %false otherwise.
> > + */
> > +bool kmem_valid_obj(void *object)
> > +{
> > +	struct page *page;
> > +
> > +	if (!virt_addr_valid(object))
> > +		return false;
> > +	page = virt_to_head_page(object);
> > +	return PageSlab(page);
> > +}
> > +
> > +/**
> > + * kmem_dump_obj - Print available slab provenance information
> > + * @object: slab object for which to find provenance information.
> > + *
> > + * This function uses pr_cont(), so that the caller is expected to have
> > + * printed out whatever preamble is appropriate.  The provenance information
> > + * depends on the type of object and on how much debugging is enabled.
> > + * For a slab-cache object, the fact that it is a slab object is printed,
> > + * and, if available, the slab name, return address, and stack trace from
> > + * the allocation of that object.
> > + *
> > + * This function will splat if passed a pointer to a non-slab object.
> > + * If you are not sure what type of object you have, you should instead
> > + * use mem_dump_obj().
> > + */
> > +void kmem_dump_obj(void *object)
> > +{
> > +	char *cp = IS_ENABLED(CONFIG_MMU) ? "" : "/vmalloc";
> > +	int i;
> > +	struct page *page;
> > +	unsigned long ptroffset;
> > +	struct kmem_obj_info kp = { };
> > +
> > +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> > +		return;
> > +	page = virt_to_head_page(object);
> > +	if (WARN_ON_ONCE(!PageSlab(page))) {
> > +		pr_cont(" non-slab memory.\n");
> > +		return;
> > +	}
> > +	kmem_obj_info(&kp, object, page);
> > +	if (kp.kp_slab_cache)
> > +		pr_cont(" slab%s %s", cp, kp.kp_slab_cache->name);
> > +	else
> > +		pr_cont(" slab%s", cp);
> > +	if (kp.kp_objp)
> > +		pr_cont(" start %px", kp.kp_objp);
> > +	if (kp.kp_data_offset)
> > +		pr_cont(" data offset %lu", kp.kp_data_offset);
> 
> I don't check the code deeply but kp_data_offset could be 0 in normal
> situation. Is it intentional not to print a message in this case?

Yes, so that it tells you of the offset only if it is non-zero, which as
you say happens only if certain debugging options are enabled.  Easy to
print it unconditionally if that is preferred!

> > +	if (kp.kp_objp) {
> > +		ptroffset = ((char *)object - (char *)kp.kp_objp) - kp.kp_data_offset;
> > +		pr_cont(" pointer offset %lu", ptroffset);
> > +	}
> > +	if (kp.kp_slab_cache && kp.kp_slab_cache->usersize)
> > +		pr_cont(" size %u", kp.kp_slab_cache->usersize);
> > +	if (kp.kp_ret)
> > +		pr_cont(" allocated at %pS\n", kp.kp_ret);
> > +	else
> > +		pr_cont("\n");
> > +	for (i = 0; i < ARRAY_SIZE(kp.kp_stack); i++) {
> > +		if (!kp.kp_stack[i])
> > +			break;
> > +		pr_info("    %pS\n", kp.kp_stack[i]);
> > +	}
> > +}
> > +
> >  #ifndef CONFIG_SLOB
> >  /* Create a cache during boot when no slab services are available yet */
> >  void __init create_boot_cache(struct kmem_cache *s, const char *name,
> > diff --git a/mm/slob.c b/mm/slob.c
> > index 7cc9805..2ed1de2 100644
> > --- a/mm/slob.c
> > +++ b/mm/slob.c
> > @@ -461,6 +461,12 @@ static void slob_free(void *block, int size)
> >  	spin_unlock_irqrestore(&slob_lock, flags);
> >  }
> >  
> > +void kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct page *page)
> > +{
> > +	kpp->kp_ptr = object;
> > +	kpp->kp_page = page;
> > +}
> > +
> >  /*
> >   * End of slob allocator proper. Begin kmem_cache_alloc and kmalloc frontend.
> >   */
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b30be23..0459d2a 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3918,6 +3918,42 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
> >  	return 0;
> >  }
> >  
> > +void kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct page *page)
> > +{
> > +#ifdef CONFIG_SLUB_DEBUG
> 
> We can get some infos even if CONFIG_SLUB_DEBUG isn't defined.
> Please move them out.

I guess since I worry about CONFIG_MMU=n it only makes sense to also
worry about CONFIG_SLUB_DEBUG=n.  Fix update.

							Thanx, Paul

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