[patch 036/163] mm, slab/slub: move and improve cache_from_obj()

Andrew Morton
2020-08-07T06:19:05Z

From: Vlastimil Babka <vbabka@suse.cz>
Subject: mm, slab/slub: move and improve cache_from_obj()

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-guro@fb.com

Link: http://lkml.kernel.org/r/20200610163135.17364-10-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/slab.c | 8 ++++++++
mm/slab.h | 23 -----------------------
mm/slub.c | 21 +++++++++++++++++++++
3 files changed, 29 insertions(+), 23 deletions(-)

--- a/mm/slab.c~mm-slab-slub-move-and-improve-cache_from_obj
+++ a/mm/slab.c
@@ -3678,6 +3678,14 @@ void *__kmalloc_track_caller(size_t size
}
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.
--- a/mm/slab.h~mm-slab-slub-move-and-improve-cache_from_obj
+++ a/mm/slab.h
@@ -504,29 +504,6 @@ static __always_inline void uncharge_sla
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
--- a/mm/slub.c~mm-slab-slub-move-and-improve-cache_from_obj
+++ a/mm/slub.c
@@ -1525,6 +1525,10 @@ static bool freelist_corrupted(struct km
{
return false;
}
+
+static void print_tracking(struct kmem_cache *s, void *object)
+{
+}
#endif /* CONFIG_SLUB_DEBUG */

/*
@@ -3171,6 +3175,23 @@ void ___cache_free(struct kmem_cache *ca
}
#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);
_