[PATCH 6.12.y 0/2] mm/slab: fix unbounded recursion in free path with memalloc profiling

[PATCH 6.12.y 0/2] mm/slab: fix unbounded recursion in free path with memalloc profiling

[Harry Yoo (Oracle)]

This is backport of "mm/slab: fix unbounded recursion in free path with
memalloc profiling" series [1] for 6.12.y.

The patch 1 was adjusted for 6.12 as mem_profiling_support is marked
__init in 6.12. I simply resolved this by marking
mem_alloc_profiling_permanently_disabled() __init since in 6.12 we need
it only when creating kmalloc-no-objext-* caches during the boot
process.

Patch 2 has slight functional differences compared to mainline
due to lack of SLAB_ALLOC_* flags, sheaves, and kmalloc_nolock().
I described those subtlties in the backporting notes in detail.

The first two patches from the original patchset [1] are not backported
to 6.12.y as they are no-op in 6.12.y:

 - mm/slab: fix a memory leak due to bootstrapping sheaves twice
 - mm/slab: decouple SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT

[1] https://lore.kernel.org/all/20260713-kmalloc-no-objext-v3-0-47c7bd138de7@kernel.org

To: Suren Baghdasaryan <surenb@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Vlastimil Babka <vbabka@kernel.org>
To: Christoph Lameter <cl@gentwo.org>
To: David Rientjes <rientjes@google.com>
To: Roman Gushchin <roman.gushchin@linux.dev>
To: Harry Yoo <harry@kernel.org>
To: Kees Cook <kees@kernel.org>
To: Danielle Costantino <dcostantino@meta.com>
To: Shakeel Butt <shakeel.butt@linux.dev>
To: Hao Ge <hao.ge@linux.dev>
To: Hao Li <hao.li@linux.dev>
To: Pedro Falcato <pfalcato@suse.de>,
To: "Liam R . Howlett" <liam@infradead.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org

Signed-off-by: Harry Yoo <harry@kernel.org>
---
Harry Yoo (Oracle) (2):
      lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()
      mm/slab: prevent unbounded recursion in free path with new kmalloc type

 include/linux/alloc_tag.h |  3 +++
 include/linux/slab.h      |  8 +++++++-
 lib/alloc_tag.c           |  9 +++++++++
 mm/slab.h                 | 28 ++++++++++++++++++++++++++--
 mm/slab_common.c          | 13 +++++++++++++
 mm/slub.c                 | 34 +++++++++++++++++++++++-----------
 6 files changed, 81 insertions(+), 14 deletions(-)
---
base-commit: 833291ee77846538926c990bcb468f6f54e9af50
change-id: 20260804-kmalloc-no-objext-for-6-12-y-bedd8e852aeb

Best regards,
-- 
Cheers,
Harry / Hyeonggon

[PATCH 6.12.y 1/2] lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()

[Harry Yoo (Oracle)]

commit a37b0066a10aabf3c968b4566706fb866eaf9a85 upstream.

mem_alloc_profiling_enabled() tells whether memalloc profiling is
currently enabled. However, even when this function returns false,
it can be enabled later.

However, this is not enough. Some optimizations can be applied only when
memalloc profiling is permanently disabled. For example, to skip the
creation of KMALLOC_NO_OBJ_EXT caches at boot time, mem_profiling must
be set to "never", "0" w/ debugging on, or have been shutdown so that
it can no longer be enabled.

Introduce mem_alloc_profiling_permanently_disabled() for this purpose.

Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Suren Baghdasaryan <surenb@google.com>
Link: https://patch.msgid.link/20260713-kmalloc-no-objext-v3-3-47c7bd138de7@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
[ harry@kernel.org: Move the definition of
  mem_alloc_profiling_permanently_disabled() after
  mem_profiling_support.

  Unlike 6.18 and later kernels, mem_profiling_support is marked
  __init and gets freed after boot. Since the function is needed
  only when creating kmalloc caches, mark it __init as well. ]
Signed-off-by: Harry Yoo <harry@kernel.org>
---
 include/linux/alloc_tag.h | 3 +++
 lib/alloc_tag.c           | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
index 6073a8f13c41..70025f8e2499 100644
--- a/include/linux/alloc_tag.h
+++ b/include/linux/alloc_tag.h
@@ -105,6 +105,8 @@ static inline bool mem_alloc_profiling_enabled(void)
 				   &mem_alloc_profiling_key);
 }
 
+bool __init mem_alloc_profiling_permanently_disabled(void);
+
 static inline struct alloc_tag_counters alloc_tag_read(struct alloc_tag *tag)
 {
 	struct alloc_tag_counters v = { 0, 0 };
@@ -198,6 +200,7 @@ static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes)
 
 #define DEFINE_ALLOC_TAG(_alloc_tag)
 static inline bool mem_alloc_profiling_enabled(void) { return false; }
+static inline bool mem_alloc_profiling_permanently_disabled(void) { return true; }
 static inline void alloc_tag_add(union codetag_ref *ref, struct alloc_tag *tag,
 				 size_t bytes) {}
 static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) {}
diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index e76c40bf29d0..a0d11582029e 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -183,6 +183,15 @@ static bool mem_profiling_support __meminitdata = true;
 static bool mem_profiling_support __meminitdata;
 #endif
 
+/*
+ * Memory allocation profiling is permanently disabled and cannot be enabled.
+ * Must be called after setup_early_mem_profiling().
+ */
+bool __init mem_alloc_profiling_permanently_disabled(void)
+{
+	return !mem_profiling_support;
+}
+
 static int __init setup_early_mem_profiling(char *str)
 {
 	bool enable;

-- 
2.53.0

[PATCH 6.12.y 2/2] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Harry Yoo (Oracle)]

commit d9e6a7623938968e3752b67e37eaff097e559a54 upstream.

Commit 280ea9c3154b ("mm/slab: avoid allocating slabobj_ext array from
its own slab") avoided recursive allocation of obj_exts from kmalloc
caches of the same size, by bumping the obj_exts array's allocation
size whenever the array size equals the size of the object being
allocated.

However, as reported by Danielle Costantino and Shakeel Butt,
even slabs from kmalloc caches of different sizes can form a cycle
by allocating obj_exts arrays from each other [1]:

  What happened: a KMALLOC_NORMAL slab's obj_exts array (used by
  allocation profiling / memcg accounting) is itself kmalloc()'d from a
  KMALLOC_NORMAL cache, so the "slab holds another slab's obj_exts array"
  relation can form cycles. With sizeof(struct slabobj_ext) == 16 and
  the host's geometry:

  - kmalloc-512 has 64 objects/slab -> array is 64*16 == 1024 bytes,
    served from kmalloc-1k;
  - kmalloc-1k  has 32 objects/slab -> array is 32*16 ==  512 bytes,
    served from kmalloc-512.

  A kmalloc-512 slab and a kmalloc-1k slab therefore hold each other's
  obj_exts array.  Discarding one frees the other's array, which empties
  and discards that slab, which frees the first's array, and so on:
  __free_slab() -> free_slab_obj_exts() -> kfree() -> discard_slab() ->
  __free_slab() recurses along the cycle until the stack is exhausted.

With memory allocation profiling, this allows unbounded recursion
in the free path and led to a stack overflow on a production host in
the Meta fleet [1]:

  BUG: TASK stack guard page was hit
  Oops: stack guard page
  RIP: 0010:kfree+0x8/0x5d0
  Call Trace:
   __free_slab+0x66/0xc0
   kfree+0x3f0/0x5d0
   ... ( ~125x __free_slab <-> kfree ) ...
   <kernel driver freeing a resource>
   do_syscall_64

It is proposed [1] to resolve this issue by always serving the obj_exts
array allocation from kmalloc caches (or large kmalloc) of sizes larger
than the object size. However, as pointed out by Vlastimil Babka [2],
this can waste an excessive amount of memory as slabs from large
kmalloc sizes (e.g. kmalloc-8k) generally need obj_exts arrays much
smaller than the object size.

Therefore, rather than bumping the size, let us take a different
approach; disallow formation of cycles between kmalloc types when
allocating obj_exts arrays. Currently, all obj_exts arrays are served
from normal kmalloc caches. Cycles cannot be created if obj_exts arrays
of normal kmalloc caches are served from a special kmalloc type that can
never have obj_exts arrays.

To achieve this, create a new kmalloc type called KMALLOC_NO_OBJ_EXT.
KMALLOC_NO_OBJ_EXT caches are created with SLAB_NO_OBJ_EXT flag when
either 1) memory allocation profiling is not permanently disabled,
or 2) kmalloc types with a priority higher than KMALLOC_CGROUP are
aliased with KMALLOC_NORMAL.

Sheaf bootstrapping for KMALLOC_NO_OBJ_EXT caches now must be deferred
because allocation of a barn can trigger obj_exts array allocation of
normal kmalloc caches when the KMALLOC_NO_OBJ_EXT cache for that size
is not ready yet. For simplicity, perform bootstrapping of sheaves for
all kmalloc caches later.

Introduce a new slab alloc flag, SLAB_ALLOC_NO_OBJ_EXT, to prevent
allocation of obj_exts arrays, and let kmalloc_slab() override the type
to KMALLOC_NO_OBJ_EXT when specified. Note that kmalloc_type() remains
unchanged because kmalloc_flags() bypasses the kmalloc fastpath.

Do not pass SLAB_ALLOC_NO_RECURSE to kmalloc_flags() in
alloc_slab_obj_exts() and instead use SLAB_ALLOC_NO_OBJ_EXT only when
the objects are allocated from normal kmalloc caches. While this
prevents unbounded recursive allocation of obj_exts, it allows
KMALLOC_NO_OBJ_EXT caches to have sheaves.

Since sheaf allocations specify SLAB_ALLOC_NO_RECURSE that prevents
allocation of both sheaves and obj_exts arrays, the recursion depth
is bounded.

obj_exts arrays for non-kmalloc-normal caches can now have a valid tag.
Do not call mark_obj_codetag_empty() when freeing an obj_exts array to
avoid false warnings. KMALLOC_NO_OBJ_EXT don't need this as they never
allocate those arrays.

Reported-by: Danielle Costantino <dcostantino@meta.com>
Reported-by: Shakeel Butt <shakeel.butt@linux.dev>
Closes: https://lore.kernel.org/linux-mm/20260625230029.703750-1-shakeel.butt@linux.dev [1]
Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/linux-mm/c5c4208d-a6f0-413e-bad9-49be12f12d55@kernel.org [2]
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Link: https://patch.msgid.link/20260713-kmalloc-no-objext-v3-4-47c7bd138de7@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
[harry@kernel.org: Backport notes:
 - Fix a minor conflict due to missing partitioned
   kmalloc caches in 6.12.

 - Use __GFP_NO_OBJ_EXT instead of SLAB_ALLOC_NO_OBJ_EXT
   since slab's internal alloc_flags do not exist in 6.12.

 - Deferring sheaf bootstrapping for kmalloc caches is not applied
   as 6.12 doesn't have sheaves.

 - Adjust the comment for SLAB_NO_OBJ_EXT, like in the commit
   982e31382d9a ("mm/slab: decouple SLAB_NO_SHEAVES from
   SLAB_NO_OBJ_EXT"). The rest of that commit is a no-op in 6.12 as
   sheaves are not supported. Thus only adjust the comment.

 - Resolve conflicts due to missing kmalloc_nolock() support in 6.12.

 - Mark need_kmalloc_no_objext() __always_inline to make sure
   the compiler does not generate a out-of-line function that could
   access mem_profiling_support (which is marked __init) ]
Signed-off-by: Harry Yoo <harry@kernel.org>
---
 include/linux/slab.h |  8 +++++++-
 mm/slab.h            | 28 ++++++++++++++++++++++++++--
 mm/slab_common.c     | 13 +++++++++++++
 mm/slub.c            | 34 +++++++++++++++++++++++-----------
 4 files changed, 69 insertions(+), 14 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 773843a71960..28eba6cbb767 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -206,7 +206,7 @@ enum _slab_flag_bits {
 #endif
 #define SLAB_TEMPORARY		SLAB_RECLAIM_ACCOUNT	/* Objects are short-lived */
 
-/* Slab created using create_boot_cache */
+/* Slab caches without obj_exts array */
 #ifdef CONFIG_SLAB_OBJ_EXT
 #define SLAB_NO_OBJ_EXT		__SLAB_FLAG_BIT(_SLAB_NO_OBJ_EXT)
 #else
@@ -578,6 +578,9 @@ enum kmalloc_cache_type {
 #endif
 #ifndef CONFIG_MEMCG
 	KMALLOC_CGROUP = KMALLOC_NORMAL,
+#endif
+#ifndef CONFIG_SLAB_OBJ_EXT
+	KMALLOC_NO_OBJ_EXT = KMALLOC_NORMAL,
 #endif
 	KMALLOC_RANDOM_START = KMALLOC_NORMAL,
 	KMALLOC_RANDOM_END = KMALLOC_RANDOM_START + RANDOM_KMALLOC_CACHES_NR,
@@ -591,6 +594,9 @@ enum kmalloc_cache_type {
 #endif
 #ifdef CONFIG_MEMCG
 	KMALLOC_CGROUP,
+#endif
+#ifdef CONFIG_SLAB_OBJ_EXT
+	KMALLOC_NO_OBJ_EXT,
 #endif
 	NR_KMALLOC_TYPES
 };
diff --git a/mm/slab.h b/mm/slab.h
index b65d2462b3fd..34a3d65b4ef5 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -413,9 +413,13 @@ static inline struct kmem_cache *
 kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, unsigned long caller)
 {
 	unsigned int index;
+	enum kmalloc_cache_type type = kmalloc_type(flags, caller);
+
+	if (flags & __GFP_NO_OBJ_EXT)
+		type = KMALLOC_NO_OBJ_EXT;
 
 	if (!b)
-		b = &kmalloc_caches[kmalloc_type(flags, caller)];
+		b = &kmalloc_caches[type];
 	if (size <= 192)
 		index = kmalloc_size_index[size_index_elem(size)];
 	else
@@ -454,7 +458,8 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
 {
 	if (!is_kmalloc_cache(s))
 		return false;
-	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
+
+	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT|SLAB_NO_OBJ_EXT));
 }
 
 /* Legal flag mask for kmem_cache_create(), for various configurations */
@@ -557,6 +562,25 @@ bool slab_in_kunit_test(void);
 static inline bool slab_in_kunit_test(void) { return false; }
 #endif
 
+/*
+ * Return true if KMALLOC_NORMAL caches may need obj_exts arrays.
+ *
+ * Memory allocation profiling requires obj_exts for all caches.
+ * Memcg usually doesn't need them for normal kmalloc caches, but kmalloc types
+ * with a priority higher than KMALLOC_CGROUP can be aliased with KMALLOC_NORMAL.
+ */
+static __always_inline inline bool need_kmalloc_no_objext(void)
+{
+	if (!mem_alloc_profiling_permanently_disabled())
+		return true;
+
+	if (!mem_cgroup_kmem_disabled() &&
+			(KMALLOC_NORMAL == KMALLOC_RECLAIM))
+		return true;
+
+	return false;
+}
+
 #ifdef CONFIG_SLAB_OBJ_EXT
 
 /*
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 477fa471da18..4cc96c3ea18e 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -793,6 +793,12 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
 #define KMALLOC_RANDOM_NAME(N, sz)
 #endif
 
+#ifdef CONFIG_SLAB_OBJ_EXT
+#define KMALLOC_NO_OBJ_EXT_NAME(sz) .name[KMALLOC_NO_OBJ_EXT] = "kmalloc-no-objext-" #sz,
+#else
+#define KMALLOC_NO_OBJ_EXT_NAME(sz)
+#endif
+
 #define INIT_KMALLOC_INFO(__size, __short_size)			\
 {								\
 	.name[KMALLOC_NORMAL]  = "kmalloc-" #__short_size,	\
@@ -800,6 +806,7 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
 	KMALLOC_CGROUP_NAME(__short_size)			\
 	KMALLOC_DMA_NAME(__short_size)				\
 	KMALLOC_RANDOM_NAME(RANDOM_KMALLOC_CACHES_NR, __short_size)	\
+	KMALLOC_NO_OBJ_EXT_NAME(__short_size)			\
 	.size = __size,						\
 }
 
@@ -907,6 +914,12 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type)
 			return;
 		}
 		flags |= SLAB_ACCOUNT;
+	} else if (IS_ENABLED(CONFIG_SLAB_OBJ_EXT) && type == KMALLOC_NO_OBJ_EXT) {
+		if (!need_kmalloc_no_objext()) {
+			kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
+			return;
+		}
+		flags |= SLAB_NO_OBJ_EXT | SLAB_NO_MERGE;
 	} else if (IS_ENABLED(CONFIG_ZONE_DMA) && (type == KMALLOC_DMA)) {
 		flags |= SLAB_CACHE_DMA;
 	}
diff --git a/mm/slub.c b/mm/slub.c
index cf678f461725..b61225c3503b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1997,8 +1997,13 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 	struct slabobj_ext *vec;
 
 	gfp &= ~OBJCGS_CLEAR_MASK;
-	/* Prevent recursive extension vector allocation */
-	gfp |= __GFP_NO_OBJ_EXT;
+	/*
+	 * In most cases, obj_exts arrays are allocated from normal kmalloc.
+	 * However, normal kmalloc caches must allocate them from
+	 * KMALLOC_NO_OBJ_EXT caches to prevent recursion.
+	 */
+	if (is_kmalloc_normal(s))
+		gfp |= __GFP_NO_OBJ_EXT;
 	vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
 			   slab_nid(slab));
 	if (!vec) {
@@ -2014,6 +2019,22 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		return -ENOMEM;
 	}
 
+	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
+		struct kmem_cache *exts_cache;
+		struct slab *exts_slab;
+
+		exts_slab = virt_to_slab(vec);
+		if (exts_slab) {
+			/*
+			 * The vector must be allocated from either normal or
+			 * KMALLOC_NO_OBJ_EXT kmalloc caches to avoid cycles.
+			 */
+			exts_cache = exts_slab->slab_cache;
+			WARN_ON_ONCE(!is_kmalloc_normal(exts_cache) &&
+					!(exts_cache->flags & SLAB_NO_OBJ_EXT));
+		}
+	}
+
 	new_exts = (unsigned long)vec;
 #ifdef CONFIG_MEMCG
 	new_exts |= MEMCG_DATA_OBJEXTS;
@@ -2034,7 +2055,6 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		 * assign slabobj_exts in parallel. In this case the existing
 		 * objcg vector should be reused.
 		 */
-		mark_objexts_empty(vec);
 		kfree(vec);
 		return 0;
 	} else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
@@ -2061,14 +2081,6 @@ static inline void free_slab_obj_exts(struct slab *slab)
 		return;
 	}
 
-	/*
-	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
-	 * corresponding extension will be NULL. alloc_tag_sub() will throw a
-	 * warning if slab has extensions but the extension of an object is
-	 * NULL, therefore replace NULL with CODETAG_EMPTY to indicate that
-	 * the extension for obj_exts is expected to be NULL.
-	 */
-	mark_objexts_empty(obj_exts);
 	kfree(obj_exts);
 	slab->obj_exts = 0;
 }

-- 
2.53.0

Re: [PATCH 6.12.y 0/2] mm/slab: fix unbounded recursion in free path with memalloc profiling

[Sasha Levin]

On Thu, Aug 06, 2026 at 05:37:40AM +0000, Harry Yoo (Oracle) wrote:
> This is backport of "mm/slab: fix unbounded recursion in free path with
> memalloc profiling" series [1] for 6.12.y.

Queued the series for 6.12, thanks.

-- 
Thanks,
Sasha