On Wed, Jun 10, 2020 at 06:31:35PM +0200, Vlastimil Babka wrote: > 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. We could export the SLUB print_tracking() > function and provide an empty one for SLAB, or realize that both the debugging > and hardening cases in cache_from_obj() are only supported by SLUB anyway. So > this patch moves cache_from_obj() from slab.h to separate instances in slab.c > and slub.c, where the SLAB version only does the kmemcg lookup and even could > be completely removed once the kmemcg rework [1] is merged. The SLUB version > can thus easily use the print_tracking() function. It can also use the > kmem_cache_debug_flags() static key check for improved performance in kernels > without the hardening and with debugging not enabled on boot. > > [1] https://lore.kernel.org/r/20200608230654.828134-18-g...@fb.com > > Signed-off-by: Vlastimil Babka <vba...@suse.cz> > --- > mm/slab.c | 8 ++++++++ > mm/slab.h | 23 ----------------------- > mm/slub.c | 21 +++++++++++++++++++++ > 3 files changed, 29 insertions(+), 23 deletions(-) > > diff --git a/mm/slab.c b/mm/slab.c > index 9350062ffc1a..6134c4c36d4c 100644 > --- a/mm/slab.c > +++ b/mm/slab.c > @@ -3672,6 +3672,14 @@ 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; > +}
Hm, it looks like all the SLAB version doesn't perform any sanity checks anymore. Is it intended? Also, Is it ever possible that s != virt_to_cache(x) if there are no bugs? kmem_cache_free_bulk() in slab.c does contain the following: if (!orig_s) /* called via kfree_bulk */ s = virt_to_cache(objp); else s = cache_from_obj(orig_s, objp); which looks a bit strange with the version above. > + > /** > * kmem_cache_free - Deallocate an object > * @cachep: The cache the allocation was from. > diff --git a/mm/slab.h b/mm/slab.h > index 815e4e9a94cd..c0c4244f75da 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -503,29 +503,6 @@ 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; > - > - /* > - * When kmemcg is not being used, both assignments should return the > - * same value. but we don't want to pay the assignment price in that > - * case. If it is not compiled in, the compiler should be smart enough > - * to not do even the assignment. In that case, slab_equal_or_root > - * will also be a constant. > - */ > - if (!memcg_kmem_enabled() && > - !IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) && > - !unlikely(s->flags & SLAB_CONSISTENCY_CHECKS)) > - return s; > - > - cachep = virt_to_cache(x); > - WARN_ONCE(cachep && !slab_equal_or_root(cachep, s), > - "%s: Wrong slab cache. %s but object is from %s\n", > - __func__, s->name, cachep->name); > - 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 efb08f2e9c66..f7a1d8537674 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -1524,6 +1524,10 @@ 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 */ > > /* > @@ -3175,6 +3179,23 @@ 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; > +} Maybe we can define a trivial SLAB version of kmem_cache_debug_flags() and keep a single version of cache_from_obj()? Thank you!