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McKenney" > > 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 > Cc: Pekka Enberg > Cc: David Rientjes > Cc: Joonsoo Kim > Cc: Andrew Morton > Cc: > Reported-by: Andrii Nakryiko > [ 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 > --- > 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? > + 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. Thanks.