[PATCH slab/for-next-fixes v3 0/4] mm/slab: fix unbounded recursion in free path with memalloc profiling

[PATCH slab/for-next-fixes v3 0/4] mm/slab: fix unbounded recursion in free path with memalloc profiling

[Harry Yoo (Oracle)]

This is a follow-up fix after the recent discussion [1].
See patch 4 for the detailed description on the bug.

Based on slab/for-next-fixes (af9ea231c0b45) and is available at
git.kernel.org [2].

Instead preventing cycles by bumping up the allocation size of obj_exts
arrays, it introduces a new kmalloc type called KMALLOC_NO_OBJ_EXT and
disallow formation of cycles between kmalloc types when allocating
obj_exts arrays. obj_exts arrays of normal kmalloc caches are served
from KMALLOC_NO_OBJ_EXT caches (that don't have obj_exts), and all other
obj_exts arrays are served from normal kmalloc caches.

I tried to reuse SLAB_ALLOC_NO_RECURSE to make kmalloc_slab() select
KMALLOC_NO_OBJ_EXT, but it was not great because it does not allow
sheaves for those caches. So I introduced a new slab alloc flag
SLAB_ALLOC_NO_OBJ_EXT.

To avoid huge confusion, I had to decouple "disallowing sheaves"
semantics from SLAB_NO_OBJ_EXT and introduced SLAB_NO_SHEAVES.

While this cannot be directly backported to v6.18 and v6.12 due to lack
of SLAB_ALLOC_* flags and kmalloc_flags(), I don't this will be
particularily challenging to backport it. Instead of a new slab alloc
flag, we can use __GFP_NO_OBJ_EXT to select KMALLOC_NO_OBJ_EXT as
kmalloc caches don't have sheaves in v6.18 anyway.

[1] https://lore.kernel.org/linux-mm/9a139365-28e6-4f1e-b35b-7f6091e9aa14@kernel.org

[2] https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/log/?h=kmalloc-no-objext-v3r1

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

---
Changes in v3 (range-diff included at the end):
- Do not create kmalloc-no-objext-* caches unless required (Vlastimil)
- Introduce mem_alloc_profiling_permanently_disabled() in patch 3,
  to avoid creating KMALLOC_NO_OBJ_EXT caches when mem_profiling is
  permanently disabled.
- Make KMALLOC_NO_OBJ_EXT an alias for KMALLOC_NORMAL if
  CONFIG_SLAB_OBJ_EXT is not defined and avoid ifdefs (refactoring).
- Link to v2: https://lore.kernel.org/r/20260710-kmalloc-no-objext-v2-0-2709afb6a030@kernel.org

Changes in v2:
- Fix a memory leak when kmalloc caches are aliased (patch 1).
  This is included as part of this series as patch 3 makes it
  easier to trigger the leak.

- Fix a warning in MEM_ALLOC_PROFILING_DEBUG by dropping
  mark_obj_codetag_empty() for obj_exts. Sashiko raised this and
  I reproduced it and confirmed that the warnings are gone.
  https://sashiko.dev/#/patchset/20260702-kmalloc-no-objext-v1-0-167175008538%40kernel.org

- Added R-b from Vlastimil on patch 2, thanks!
- Adjusted Vlastimil's feedback on RFC v1
- Link to v1: https://lore.kernel.org/r/20260702-kmalloc-no-objext-v1-0-167175008538@kernel.org
---

$ b4 prep --compare-to v2
1:  26e19124ce7a = 1:  701534464f8e mm/slab: fix a memory leak due to bootstrapping sheaves twice
2:  a4a3a61cea59 = 2:  8c23ba4a36c9 mm/slab: decouple SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT
-:  ------------ > 3:  c285dbfbf6fb lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()
3:  836ba88ba786 ! 4:  8d7f1e6dd334 mm/slab: prevent unbounded recursion in free path with new kmalloc type
    @@ Commit message
         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 when CONFIG_SLAB_OBJ_EXT is
    -    enabled, and they have SLAB_NO_OBJ_EXT flag to prevent allocation
    -    of obj_exts arrays. They remain unused until allocation of obj_exts
    -    arrays for normal kmalloc caches happens.
    +    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
    @@ Commit message
         Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
     
      ## include/linux/slab.h ##
    +@@ include/linux/slab.h: 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_PARTITION_START = KMALLOC_NORMAL,
    + 	KMALLOC_PARTITION_END = KMALLOC_PARTITION_START + KMALLOC_PARTITION_CACHES_NR,
     @@ include/linux/slab.h: enum kmalloc_cache_type {
      #endif
      #ifdef CONFIG_MEMCG
    @@ mm/slab.h: static inline unsigned int size_index_elem(unsigned int bytes)
      	unsigned int index;
     +	enum kmalloc_cache_type type = kmalloc_type(flags, token);
     +
    -+#ifdef CONFIG_SLAB_OBJ_EXT
     +	if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
     +		type = KMALLOC_NO_OBJ_EXT;
    -+#endif
      
      	if (!b)
     -		b = &kmalloc_caches[kmalloc_type(flags, token)];
    @@ mm/slab.h: static inline bool is_kmalloc_normal(struct kmem_cache *s)
      }
      
      bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
    +@@ mm/slab.h: static inline void metadata_access_disable(void)
    + 	kasan_enable_current();
    + }
    + 
    ++/*
    ++ * 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 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
    + 
    + /*
     
      ## mm/slab_common.c ##
     @@ mm/slab_common.c: u8 kmalloc_size_index[24] __ro_after_init = {
    @@ mm/slab_common.c: EXPORT_SYMBOL(kmalloc_size_roundup);
      }
      
     @@ mm/slab_common.c: new_kmalloc_cache(int idx, enum kmalloc_cache_type type)
    - 		flags |= SLAB_NO_MERGE;
    - #endif
    - 
    -+#ifdef CONFIG_SLAB_OBJ_EXT
    -+	if (type == KMALLOC_NO_OBJ_EXT)
    + 			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;
    -+#endif
    -+
    - 	/*
    - 	 * If CONFIG_MEMCG is enabled, disable cache merging for
    - 	 * KMALLOC_NORMAL caches.
    + 	} else if (IS_ENABLED(CONFIG_ZONE_DMA) && (type == KMALLOC_DMA)) {
    + 		flags |= SLAB_CACHE_DMA;
    + 	}
     
      ## mm/slub.c ##
     @@ mm/slub.c: static inline void init_slab_obj_exts(struct slab *slab)
    @@ mm/slub.c: int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
     +	 * However, normal kmalloc caches must allocate them from
     +	 * KMALLOC_NO_OBJ_EXT caches to prevent recursion.
     +	 */
    -+	if (is_kmalloc_normal(s))
    ++	if (is_kmalloc_normal(s)) {
    ++		VM_WARN_ON_ONCE(!need_kmalloc_no_objext());
     +		alloc_flags |= SLAB_ALLOC_NO_OBJ_EXT;
    ++	}
      
     -	sz = obj_exts_alloc_size(s, slab, gfp);
     +	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;

---
Harry Yoo (Oracle) (4):
      mm/slab: fix a memory leak due to bootstrapping sheaves twice
      mm/slab: decouple SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT
      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      |  19 +++++++-
 lib/alloc_tag.c           |   9 ++++
 mm/slab.h                 |  32 +++++++++++--
 mm/slab_common.c          |  19 +++++++-
 mm/slub.c                 | 111 ++++++++++++++++++++--------------------------
 6 files changed, 123 insertions(+), 70 deletions(-)
---
base-commit: 72bb229f9161a1efcd5df32141b69fcc6ae81a13
change-id: 20260702-kmalloc-no-objext-2619c1e06083

Best regards,
-- 
Harry Yoo (Oracle) <harry@kernel.org>

[PATCH slab/for-next-fixes v3 1/4] mm/slab: fix a memory leak due to bootstrapping sheaves twice

[Harry Yoo (Oracle)]

When kmalloc caches are aliased, multiple cache pointers reference
the same kmem_cache. As a result, iterating over kmalloc indices and
bootstrapping sheaves can bootstrap the same cache more than once and
leak memory.

Currently, this could happen when the architecture specifies
minimum alignment for slab caches that is larger than
ARCH_KMALLOC_MINALIGN.

Bootstrap sheaves only when the cache does not have them already.
Add a warning when bootstrap_cache_sheaves() is called for a cache
that already has sheaves enabled.

Cc: stable@vger.kernel.org
Fixes: 913ffd3a1bf5 ("slab: handle kmalloc sheaves bootstrap")
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
---
 mm/slub.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 65febe957886..f9461a0c47d3 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -8497,6 +8497,8 @@ static void __init bootstrap_cache_sheaves(struct kmem_cache *s)
 	bool failed = false;
 	int node, cpu;
 
+	VM_WARN_ON_ONCE(cache_has_sheaves(s));
+
 	capacity = calculate_sheaf_capacity(s, &empty_args);
 
 	/* capacity can be 0 due to debugging or SLUB_TINY */
@@ -8548,8 +8550,11 @@ static void __init bootstrap_kmalloc_sheaves(void)
 
 	for (type = KMALLOC_NORMAL; type <= KMALLOC_PARTITION_END; type++) {
 		for (int idx = 0; idx < KMALLOC_SHIFT_HIGH + 1; idx++) {
-			if (kmalloc_caches[type][idx])
-				bootstrap_cache_sheaves(kmalloc_caches[type][idx]);
+			struct kmem_cache *s = kmalloc_caches[type][idx];
+
+			/* Do not bootstrap twice when caches are aliased */
+			if (s && !cache_has_sheaves(s))
+				bootstrap_cache_sheaves(s);
 		}
 	}
 }

-- 
2.53.0

Re: [PATCH slab/for-next-fixes v3 1/4] mm/slab: fix a memory leak due to bootstrapping sheaves twice

[Suren Baghdasaryan]

On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> When kmalloc caches are aliased, multiple cache pointers reference
> the same kmem_cache. As a result, iterating over kmalloc indices and
> bootstrapping sheaves can bootstrap the same cache more than once and
> leak memory.
>
> Currently, this could happen when the architecture specifies
> minimum alignment for slab caches that is larger than
> ARCH_KMALLOC_MINALIGN.
>
> Bootstrap sheaves only when the cache does not have them already.
> Add a warning when bootstrap_cache_sheaves() is called for a cache
> that already has sheaves enabled.
>
> Cc: stable@vger.kernel.org
> Fixes: 913ffd3a1bf5 ("slab: handle kmalloc sheaves bootstrap")
> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>

nit: Don't know if we really need a warning... I would just do an
early return from bootstrap_cache_sheaves() if cache_has_sheaves() is
true. But that's not critical.

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/slub.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 65febe957886..f9461a0c47d3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -8497,6 +8497,8 @@ static void __init bootstrap_cache_sheaves(struct kmem_cache *s)
>         bool failed = false;
>         int node, cpu;
>
> +       VM_WARN_ON_ONCE(cache_has_sheaves(s));
> +
>         capacity = calculate_sheaf_capacity(s, &empty_args);
>
>         /* capacity can be 0 due to debugging or SLUB_TINY */
> @@ -8548,8 +8550,11 @@ static void __init bootstrap_kmalloc_sheaves(void)
>
>         for (type = KMALLOC_NORMAL; type <= KMALLOC_PARTITION_END; type++) {
>                 for (int idx = 0; idx < KMALLOC_SHIFT_HIGH + 1; idx++) {
> -                       if (kmalloc_caches[type][idx])
> -                               bootstrap_cache_sheaves(kmalloc_caches[type][idx]);
> +                       struct kmem_cache *s = kmalloc_caches[type][idx];
> +
> +                       /* Do not bootstrap twice when caches are aliased */
> +                       if (s && !cache_has_sheaves(s))
> +                               bootstrap_cache_sheaves(s);
>                 }
>         }
>  }
>
> --
> 2.53.0
>

[PATCH slab/for-next-fixes v3 2/4] mm/slab: decouple SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT

[Harry Yoo (Oracle)]

Bootstrap caches are created with SLAB_NO_OBJ_EXT to disallow sheaves
and obj_exts.

To allow disabling obj_exts while allowing sheaves, decouple
SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT. Bootstrap caches now have both
SLAB_NO_SHEAVES and SLAB_NO_OBJ_EXT.

No functional change intended.

Cc: stable@vger.kernel.org
Fixes: e47c897a2949 ("slab: add sheaves to most caches")
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
---
 include/linux/slab.h | 13 +++++++++++--
 mm/slub.c            | 10 ++++++----
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 51f03f18c9a7..08d7b6c9c4d6 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -58,10 +58,13 @@ enum _slab_flag_bits {
 #endif
 	_SLAB_OBJECT_POISON,
 	_SLAB_CMPXCHG_DOUBLE,
+#ifdef CONFIG_SLAB_OBJ_EXT
 	_SLAB_NO_OBJ_EXT,
-#if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT)
+#ifdef CONFIG_64BIT
 	_SLAB_OBJ_EXT_IN_OBJ,
 #endif
+#endif
+	_SLAB_NO_SHEAVES,
 	_SLAB_FLAGS_LAST_BIT
 };
 
@@ -239,8 +242,14 @@ 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
+#define SLAB_NO_OBJ_EXT		__SLAB_FLAG_UNUSED
+#endif
+
+#define SLAB_NO_SHEAVES		__SLAB_FLAG_BIT(_SLAB_NO_SHEAVES)
 
 #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT)
 #define SLAB_OBJ_EXT_IN_OBJ	__SLAB_FLAG_BIT(_SLAB_OBJ_EXT_IN_OBJ)
diff --git a/mm/slub.c b/mm/slub.c
index f9461a0c47d3..abe748b7dddb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7799,12 +7799,12 @@ static unsigned int calculate_sheaf_capacity(struct kmem_cache *s,
 		return 0;
 
 	/*
-	 * Bootstrap caches can't have sheaves for now (SLAB_NO_OBJ_EXT).
+	 * Bootstrap caches can't have sheaves for now (SLAB_NO_SHEAVES).
 	 * SLAB_NOLEAKTRACE caches (e.g., kmemleak's object_cache) must not
 	 * have sheaves to avoid recursion when sheaf allocation triggers
 	 * kmemleak tracking.
 	 */
-	if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
+	if (s->flags & (SLAB_NO_SHEAVES | SLAB_NOLEAKTRACE))
 		return 0;
 
 	/*
@@ -8586,7 +8586,8 @@ void __init kmem_cache_init(void)
 
 	create_boot_cache(kmem_cache_node, "kmem_cache_node",
 			sizeof(struct kmem_cache_node),
-			SLAB_HWCACHE_ALIGN | SLAB_NO_OBJ_EXT, 0, 0);
+			SLAB_HWCACHE_ALIGN | SLAB_NO_SHEAVES | SLAB_NO_OBJ_EXT,
+			0, 0);
 
 	hotplug_node_notifier(slab_memory_callback, SLAB_CALLBACK_PRI);
 
@@ -8596,7 +8597,8 @@ void __init kmem_cache_init(void)
 	create_boot_cache(kmem_cache, "kmem_cache",
 			offsetof(struct kmem_cache, per_node) +
 				nr_node_ids * sizeof(struct kmem_cache_per_node_ptrs),
-			SLAB_HWCACHE_ALIGN | SLAB_NO_OBJ_EXT, 0, 0);
+			SLAB_HWCACHE_ALIGN | SLAB_NO_SHEAVES | SLAB_NO_OBJ_EXT,
+			0, 0);
 
 	kmem_cache = bootstrap(&boot_kmem_cache);
 	kmem_cache_node = bootstrap(&boot_kmem_cache_node);

-- 
2.53.0

Re: [PATCH slab/for-next-fixes v3 2/4] mm/slab: decouple SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT

[Suren Baghdasaryan]

On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> Bootstrap caches are created with SLAB_NO_OBJ_EXT to disallow sheaves
> and obj_exts.
>
> To allow disabling obj_exts while allowing sheaves, decouple
> SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT. Bootstrap caches now have both
> SLAB_NO_SHEAVES and SLAB_NO_OBJ_EXT.
>
> No functional change intended.
>
> Cc: stable@vger.kernel.org
> Fixes: e47c897a2949 ("slab: add sheaves to most caches")
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  include/linux/slab.h | 13 +++++++++++--
>  mm/slub.c            | 10 ++++++----
>  2 files changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 51f03f18c9a7..08d7b6c9c4d6 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -58,10 +58,13 @@ enum _slab_flag_bits {
>  #endif
>         _SLAB_OBJECT_POISON,
>         _SLAB_CMPXCHG_DOUBLE,
> +#ifdef CONFIG_SLAB_OBJ_EXT
>         _SLAB_NO_OBJ_EXT,
> -#if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT)
> +#ifdef CONFIG_64BIT
>         _SLAB_OBJ_EXT_IN_OBJ,
>  #endif
> +#endif
> +       _SLAB_NO_SHEAVES,
>         _SLAB_FLAGS_LAST_BIT
>  };
>
> @@ -239,8 +242,14 @@ 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
> +#define SLAB_NO_OBJ_EXT                __SLAB_FLAG_UNUSED
> +#endif
> +
> +#define SLAB_NO_SHEAVES                __SLAB_FLAG_BIT(_SLAB_NO_SHEAVES)
>
>  #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT)
>  #define SLAB_OBJ_EXT_IN_OBJ    __SLAB_FLAG_BIT(_SLAB_OBJ_EXT_IN_OBJ)
> diff --git a/mm/slub.c b/mm/slub.c
> index f9461a0c47d3..abe748b7dddb 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7799,12 +7799,12 @@ static unsigned int calculate_sheaf_capacity(struct kmem_cache *s,
>                 return 0;
>
>         /*
> -        * Bootstrap caches can't have sheaves for now (SLAB_NO_OBJ_EXT).
> +        * Bootstrap caches can't have sheaves for now (SLAB_NO_SHEAVES).
>          * SLAB_NOLEAKTRACE caches (e.g., kmemleak's object_cache) must not
>          * have sheaves to avoid recursion when sheaf allocation triggers
>          * kmemleak tracking.
>          */
> -       if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
> +       if (s->flags & (SLAB_NO_SHEAVES | SLAB_NOLEAKTRACE))
>                 return 0;
>
>         /*
> @@ -8586,7 +8586,8 @@ void __init kmem_cache_init(void)
>
>         create_boot_cache(kmem_cache_node, "kmem_cache_node",
>                         sizeof(struct kmem_cache_node),
> -                       SLAB_HWCACHE_ALIGN | SLAB_NO_OBJ_EXT, 0, 0);
> +                       SLAB_HWCACHE_ALIGN | SLAB_NO_SHEAVES | SLAB_NO_OBJ_EXT,
> +                       0, 0);
>
>         hotplug_node_notifier(slab_memory_callback, SLAB_CALLBACK_PRI);
>
> @@ -8596,7 +8597,8 @@ void __init kmem_cache_init(void)
>         create_boot_cache(kmem_cache, "kmem_cache",
>                         offsetof(struct kmem_cache, per_node) +
>                                 nr_node_ids * sizeof(struct kmem_cache_per_node_ptrs),
> -                       SLAB_HWCACHE_ALIGN | SLAB_NO_OBJ_EXT, 0, 0);
> +                       SLAB_HWCACHE_ALIGN | SLAB_NO_SHEAVES | SLAB_NO_OBJ_EXT,
> +                       0, 0);
>
>         kmem_cache = bootstrap(&boot_kmem_cache);
>         kmem_cache_node = bootstrap(&boot_kmem_cache_node);
>
> --
> 2.53.0
>

[PATCH slab/for-next-fixes v3 3/4] lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()

[Harry Yoo (Oracle)]

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.

Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
Cc: stable@vger.kernel.org
Signed-off-by: Harry Yoo (Oracle) <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 068ba2e77c5d..6ed9f82e639f 100644
--- a/include/linux/alloc_tag.h
+++ b/include/linux/alloc_tag.h
@@ -134,6 +134,8 @@ static inline bool mem_alloc_profiling_enabled(void)
 				   &mem_alloc_profiling_key);
 }
 
+bool 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 };
@@ -239,6 +241,7 @@ static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag)
 
 #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 d9be1cf5187d..e5b218176c5a 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -26,6 +26,15 @@ static bool mem_profiling_support = true;
 static bool mem_profiling_support;
 #endif
 
+/*
+ * Memory allocation profiling is permanently disabled and cannot be enabled.
+ * Must be called after setup_early_mem_profiling().
+ */
+bool mem_alloc_profiling_permanently_disabled(void)
+{
+	return !mem_profiling_support;
+}
+
 static struct codetag_type *alloc_tag_cttype;
 
 #ifdef CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU

-- 
2.53.0

Re: [PATCH slab/for-next-fixes v3 3/4] lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()

[Suren Baghdasaryan]

On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> 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.

nit: I would call it mem_alloc_profiling_supported() but this name would do.

>
> Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
> Cc: stable@vger.kernel.org
> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>

Acked-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  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 068ba2e77c5d..6ed9f82e639f 100644
> --- a/include/linux/alloc_tag.h
> +++ b/include/linux/alloc_tag.h
> @@ -134,6 +134,8 @@ static inline bool mem_alloc_profiling_enabled(void)
>                                    &mem_alloc_profiling_key);
>  }
>
> +bool 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 };
> @@ -239,6 +241,7 @@ static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag)
>
>  #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 d9be1cf5187d..e5b218176c5a 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -26,6 +26,15 @@ static bool mem_profiling_support = true;
>  static bool mem_profiling_support;
>  #endif
>
> +/*
> + * Memory allocation profiling is permanently disabled and cannot be enabled.
> + * Must be called after setup_early_mem_profiling().
> + */
> +bool mem_alloc_profiling_permanently_disabled(void)
> +{
> +       return !mem_profiling_support;
> +}
> +
>  static struct codetag_type *alloc_tag_cttype;
>
>  #ifdef CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU
>
> --
> 2.53.0
>

Re: [PATCH slab/for-next-fixes v3 3/4] lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()

[Vlastimil Babka (SUSE)]

On 7/13/26 17:43, Suren Baghdasaryan wrote:
> On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>>
>> 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.
> 
> nit: I would call it mem_alloc_profiling_supported() but this name would do.

I wouldn't like that name, because _supported to me sounds like those
CONFIG_ARCH_SUPPORTS... things and not something that was just disabled.
I suggested mem_alloc_profiling_unavailable(), but it's not ideal either,
naming is hard.

>>
>> Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
> 
> Acked-by: Suren Baghdasaryan <surenb@google.com>
> 
>> ---
>>  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 068ba2e77c5d..6ed9f82e639f 100644
>> --- a/include/linux/alloc_tag.h
>> +++ b/include/linux/alloc_tag.h
>> @@ -134,6 +134,8 @@ static inline bool mem_alloc_profiling_enabled(void)
>>                                    &mem_alloc_profiling_key);
>>  }
>>
>> +bool 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 };
>> @@ -239,6 +241,7 @@ static inline bool alloc_tag_is_inaccurate(struct alloc_tag *tag)
>>
>>  #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 d9be1cf5187d..e5b218176c5a 100644
>> --- a/lib/alloc_tag.c
>> +++ b/lib/alloc_tag.c
>> @@ -26,6 +26,15 @@ static bool mem_profiling_support = true;
>>  static bool mem_profiling_support;
>>  #endif
>>
>> +/*
>> + * Memory allocation profiling is permanently disabled and cannot be enabled.
>> + * Must be called after setup_early_mem_profiling().
>> + */
>> +bool mem_alloc_profiling_permanently_disabled(void)
>> +{
>> +       return !mem_profiling_support;
>> +}
>> +
>>  static struct codetag_type *alloc_tag_cttype;
>>
>>  #ifdef CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU
>>
>> --
>> 2.53.0
>>

Re: [PATCH slab/for-next-fixes v3 3/4] lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()

[Harry Yoo]

[-- Attachment #1.1: Type: text/plain, Size: 1585 bytes --]



On 7/14/26 1:15 AM, Vlastimil Babka (SUSE) wrote:
> On 7/13/26 17:43, Suren Baghdasaryan wrote:
>> On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>>>
>>> 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.
>>
>> nit: I would call it mem_alloc_profiling_supported() but this name would do.
> 
> I wouldn't like that name, because _supported to me sounds like those
> CONFIG_ARCH_SUPPORTS... things and not something that was just disabled.

Agreed!

Actually, last year I wasn't sure what "_support" in
mem_profiling_support means and had to look at the code while fixing a
bug [1].

This year I forgot what it meant and had to look at the code again :)

> I suggested mem_alloc_profiling_unavailable(), but it's not ideal either,

Hehe, in terms of 'the meaning is obvious without reading the code',
it's not very straightforward...

> naming is hard.

Agreed!

[1]
https://lore.kernel.org/linux-mm/20250624072513.84219-1-harry.yoo@oracle.com

-- 
Cheers,
Harry / Hyeonggon


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

Re: [PATCH slab/for-next-fixes v3 3/4] lib/alloc_tag: introduce mem_alloc_profiling_permanently_disabled()

[Suren Baghdasaryan]

On Mon, Jul 13, 2026 at 9:28 AM Harry Yoo <harry@kernel.org> wrote:
>
>
>
> On 7/14/26 1:15 AM, Vlastimil Babka (SUSE) wrote:
> > On 7/13/26 17:43, Suren Baghdasaryan wrote:
> >> On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
> >>>
> >>> 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.
> >>
> >> nit: I would call it mem_alloc_profiling_supported() but this name would do.
> >
> > I wouldn't like that name, because _supported to me sounds like those
> > CONFIG_ARCH_SUPPORTS... things and not something that was just disabled.
>
> Agreed!
>
> Actually, last year I wasn't sure what "_support" in
> mem_profiling_support means and had to look at the code while fixing a
> bug [1].
>
> This year I forgot what it meant and had to look at the code again :)
>
> > I suggested mem_alloc_profiling_unavailable(), but it's not ideal either,
>
> Hehe, in terms of 'the meaning is obvious without reading the code',
> it's not very straightforward...
>
> > naming is hard.
>
> Agreed!

I'll let you pick one :)

>
> [1]
> https://lore.kernel.org/linux-mm/20250624072513.84219-1-harry.yoo@oracle.com
>
> --
> Cheers,
> Harry / Hyeonggon
>

[PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Harry Yoo (Oracle)]

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>
---
 include/linux/slab.h |  6 ++++
 mm/slab.h            | 32 ++++++++++++++++--
 mm/slab_common.c     | 19 ++++++++++-
 mm/slub.c            | 92 +++++++++++++++++++---------------------------------
 4 files changed, 87 insertions(+), 62 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 08d7b6c9c4d6..3a14df50766a 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -708,6 +708,9 @@ 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_PARTITION_START = KMALLOC_NORMAL,
 	KMALLOC_PARTITION_END = KMALLOC_PARTITION_START + KMALLOC_PARTITION_CACHES_NR,
@@ -721,6 +724,9 @@ 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 281a65233795..f5e336b6b6b0 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -22,6 +22,7 @@
 #define SLAB_ALLOC_NOLOCK	0x01 /* a kmalloc_nolock() allocation */
 #define SLAB_ALLOC_NEW_SLAB	0x02 /* a flag for alloc_slab_obj_exts() */
 #define SLAB_ALLOC_NO_RECURSE	0x04 /* prevent kmalloc() recursion */
+#define SLAB_ALLOC_NO_OBJ_EXT	0x08 /* prevent obj_exts array allocation */
 
 static inline bool alloc_flags_allow_spinning(const unsigned int alloc_flags)
 {
@@ -386,12 +387,17 @@ static inline unsigned int size_index_elem(unsigned int bytes)
  * KMALLOC_MAX_CACHE_SIZE and the caller must check that.
  */
 static inline struct kmem_cache *
-kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token)
+kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token,
+	     unsigned int alloc_flags)
 {
 	unsigned int index;
+	enum kmalloc_cache_type type = kmalloc_type(flags, token);
+
+	if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
+		type = KMALLOC_NO_OBJ_EXT;
 
 	if (!b)
-		b = &kmalloc_caches[kmalloc_type(flags, token)];
+		b = &kmalloc_caches[type];
 	if (size <= 192)
 		index = kmalloc_size_index[size_index_elem(size)];
 	else
@@ -426,7 +432,8 @@ 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));
 }
 
 bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
@@ -529,6 +536,25 @@ static inline void metadata_access_disable(void)
 	kasan_enable_current();
 }
 
+/*
+ * 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 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 b6426d7ceec9..03ecac12cd86 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -783,11 +783,15 @@ u8 kmalloc_size_index[24] __ro_after_init = {
 size_t kmalloc_size_roundup(size_t size)
 {
 	if (size && size <= KMALLOC_MAX_CACHE_SIZE) {
+		struct kmem_cache *s;
+
 		/*
 		 * The flags don't matter since size_index is common to all.
 		 * Neither does the caller for just getting ->object_size.
 		 */
-		return kmalloc_slab(size, NULL, GFP_KERNEL, __kmalloc_token(0))->object_size;
+		s = kmalloc_slab(size, NULL, GFP_KERNEL, __kmalloc_token(0),
+				 SLAB_ALLOC_DEFAULT);
+		return s->object_size;
 	}
 
 	/* Above the smaller buckets, size is a multiple of page size. */
@@ -843,6 +847,12 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
 #define KMALLOC_PARTITION_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,	\
@@ -850,6 +860,7 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
 	KMALLOC_CGROUP_NAME(__short_size)			\
 	KMALLOC_DMA_NAME(__short_size)				\
 	KMALLOC_PARTITION_NAME(KMALLOC_PARTITION_CACHES_NR, __short_size)	\
+	KMALLOC_NO_OBJ_EXT_NAME(__short_size)			\
 	.size = __size,						\
 }
 
@@ -957,6 +968,12 @@ 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 abe748b7dddb..a34f9b8770dc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2123,42 +2123,6 @@ static inline void init_slab_obj_exts(struct slab *slab)
 	slab->obj_exts = 0;
 }
 
-/*
- * Calculate the allocation size for slabobj_ext array.
- *
- * When memory allocation profiling is enabled, the obj_exts array
- * could be allocated from the same slab cache it's being allocated for.
- * This would prevent the slab from ever being freed because it would
- * always contain at least one allocated object (its own obj_exts array).
- *
- * To avoid this, increase the allocation size when we detect the array
- * may come from the same cache, forcing it to use a different cache.
- */
-static inline size_t obj_exts_alloc_size(struct kmem_cache *s,
-					 struct slab *slab, gfp_t gfp)
-{
-	size_t sz = sizeof(struct slabobj_ext) * slab->objects;
-	struct kmem_cache *obj_exts_cache;
-
-	if (sz > KMALLOC_MAX_CACHE_SIZE)
-		return sz;
-
-	if (!is_kmalloc_normal(s))
-		return sz;
-
-	obj_exts_cache = kmalloc_slab(sz, NULL, gfp, __kmalloc_token(0));
-	/*
-	 * We can't simply compare s with obj_exts_cache, because partitioned kmalloc
-	 * caches have multiple caches per size, selected by caller address or type.
-	 * Since caller address or type may differ between kmalloc_slab() and actual
-	 * allocation, bump size when sizes are equal.
-	 */
-	if (s->object_size == obj_exts_cache->object_size)
-		return obj_exts_cache->object_size + 1;
-
-	return sz;
-}
-
 int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 			gfp_t gfp, unsigned int alloc_flags)
 {
@@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 	unsigned long new_exts;
 	unsigned long old_exts;
 	struct slabobj_ext *vec;
-	size_t sz;
+	size_t sz = sizeof(struct slabobj_ext) * slab->objects;
 
 	gfp &= ~OBJCGS_CLEAR_MASK;
-	/* Prevent recursive extension vector allocation */
-	alloc_flags |= SLAB_ALLOC_NO_RECURSE;
-	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
+	/*
+	 * 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)) {
+		VM_WARN_ON_ONCE(!need_kmalloc_no_objext());
+		alloc_flags |= SLAB_ALLOC_NO_OBJ_EXT;
+	}
 
-	sz = obj_exts_alloc_size(s, slab, gfp);
+	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
 
 	/* This will use kmalloc_nolock() if alloc_flags say so */
 	vec = kmalloc_flags(sz, gfp | __GFP_ZERO, alloc_flags, slab_nid(slab));
@@ -2193,8 +2163,21 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		return -ENOMEM;
 	}
 
-	VM_WARN_ON_ONCE(virt_to_slab(vec) != NULL &&
-			virt_to_slab(vec)->slab_cache == s);
+	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
@@ -2217,7 +2200,6 @@ 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_obj_codetag_empty(vec);
 		if (unlikely(!allow_spin))
 			kfree_nolock(vec);
 		else
@@ -2253,14 +2235,6 @@ static inline void free_slab_obj_exts(struct slab *slab, bool allow_spin)
 		return;
 	}
 
-	/*
-	 * obj_exts was created with SLAB_ALLOC_NO_RECURSE 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_obj_codetag_empty(obj_exts);
 	if (allow_spin)
 		kfree(obj_exts);
 	else
@@ -5356,7 +5330,7 @@ void *__do_kmalloc_node(kmem_buckets *b, gfp_t flags, int node,
 	if (unlikely(!size))
 		return ZERO_SIZE_PTR;
 
-	s = kmalloc_slab(size, b, flags, token);
+	s = kmalloc_slab(size, b, flags, token, ac->alloc_flags);
 
 	ret = slab_alloc_node(s, flags, node, ac);
 	ret = kasan_kmalloc(s, ret, size, flags);
@@ -5419,7 +5393,9 @@ static void *__kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_f
 retry:
 	if (unlikely(size > KMALLOC_MAX_CACHE_SIZE))
 		return NULL;
-	s = kmalloc_slab(size, NULL, gfp_flags, PASS_TOKEN_PARAM(token));
+
+	s = kmalloc_slab(size, NULL, gfp_flags, PASS_TOKEN_PARAM(token),
+			 ac->alloc_flags);
 
 	if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
 		/*
@@ -7979,10 +7955,10 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
 		s->allocflags |= __GFP_RECLAIMABLE;
 
 	/*
-	 * For KMALLOC_NORMAL caches we enable sheaves later by
-	 * bootstrap_kmalloc_sheaves() to avoid recursion
+	 * For kmalloc caches we enable sheaves later by
+	 * bootstrap_kmalloc_sheaves() to avoid recursion.
 	 */
-	if (!is_kmalloc_normal(s))
+	if (!is_kmalloc_cache(s))
 		s->sheaf_capacity = calculate_sheaf_capacity(s, args);
 
 	/*
@@ -8548,7 +8524,7 @@ static void __init bootstrap_kmalloc_sheaves(void)
 {
 	enum kmalloc_cache_type type;
 
-	for (type = KMALLOC_NORMAL; type <= KMALLOC_PARTITION_END; type++) {
+	for (type = KMALLOC_NORMAL; type < NR_KMALLOC_TYPES; type++) {
 		for (int idx = 0; idx < KMALLOC_SHIFT_HIGH + 1; idx++) {
 			struct kmem_cache *s = kmalloc_caches[type][idx];
 

-- 
2.53.0

Re: [PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Suren Baghdasaryan]

On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> 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>
> ---
>  include/linux/slab.h |  6 ++++
>  mm/slab.h            | 32 ++++++++++++++++--
>  mm/slab_common.c     | 19 ++++++++++-
>  mm/slub.c            | 92 +++++++++++++++++++---------------------------------
>  4 files changed, 87 insertions(+), 62 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 08d7b6c9c4d6..3a14df50766a 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -708,6 +708,9 @@ 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_PARTITION_START = KMALLOC_NORMAL,
>         KMALLOC_PARTITION_END = KMALLOC_PARTITION_START + KMALLOC_PARTITION_CACHES_NR,
> @@ -721,6 +724,9 @@ 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 281a65233795..f5e336b6b6b0 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -22,6 +22,7 @@
>  #define SLAB_ALLOC_NOLOCK      0x01 /* a kmalloc_nolock() allocation */
>  #define SLAB_ALLOC_NEW_SLAB    0x02 /* a flag for alloc_slab_obj_exts() */
>  #define SLAB_ALLOC_NO_RECURSE  0x04 /* prevent kmalloc() recursion */
> +#define SLAB_ALLOC_NO_OBJ_EXT  0x08 /* prevent obj_exts array allocation */
>
>  static inline bool alloc_flags_allow_spinning(const unsigned int alloc_flags)
>  {
> @@ -386,12 +387,17 @@ static inline unsigned int size_index_elem(unsigned int bytes)
>   * KMALLOC_MAX_CACHE_SIZE and the caller must check that.
>   */
>  static inline struct kmem_cache *
> -kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token)
> +kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token,
> +            unsigned int alloc_flags)
>  {
>         unsigned int index;
> +       enum kmalloc_cache_type type = kmalloc_type(flags, token);
> +
> +       if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
> +               type = KMALLOC_NO_OBJ_EXT;

Why not let kmalloc_type() handle alloc_flags? Other users (there are
only 4 of them) can pass SLAB_ALLOC_DEFAULT. That seems cleaner to me
and more robust.

>
>         if (!b)
> -               b = &kmalloc_caches[kmalloc_type(flags, token)];
> +               b = &kmalloc_caches[type];
>         if (size <= 192)
>                 index = kmalloc_size_index[size_index_elem(size)];
>         else
> @@ -426,7 +432,8 @@ 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));
>  }
>
>  bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
> @@ -529,6 +536,25 @@ static inline void metadata_access_disable(void)
>         kasan_enable_current();
>  }
>
> +/*
> + * 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 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 b6426d7ceec9..03ecac12cd86 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -783,11 +783,15 @@ u8 kmalloc_size_index[24] __ro_after_init = {
>  size_t kmalloc_size_roundup(size_t size)
>  {
>         if (size && size <= KMALLOC_MAX_CACHE_SIZE) {
> +               struct kmem_cache *s;
> +
>                 /*
>                  * The flags don't matter since size_index is common to all.
>                  * Neither does the caller for just getting ->object_size.
>                  */
> -               return kmalloc_slab(size, NULL, GFP_KERNEL, __kmalloc_token(0))->object_size;
> +               s = kmalloc_slab(size, NULL, GFP_KERNEL, __kmalloc_token(0),
> +                                SLAB_ALLOC_DEFAULT);
> +               return s->object_size;
>         }
>
>         /* Above the smaller buckets, size is a multiple of page size. */
> @@ -843,6 +847,12 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
>  #define KMALLOC_PARTITION_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,      \
> @@ -850,6 +860,7 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
>         KMALLOC_CGROUP_NAME(__short_size)                       \
>         KMALLOC_DMA_NAME(__short_size)                          \
>         KMALLOC_PARTITION_NAME(KMALLOC_PARTITION_CACHES_NR, __short_size)       \
> +       KMALLOC_NO_OBJ_EXT_NAME(__short_size)                   \
>         .size = __size,                                         \
>  }
>
> @@ -957,6 +968,12 @@ 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) {

Hmm, you have to check IS_ENABLED(CONFIG_SLAB_OBJ_EXT) here because
KMALLOC_NO_OBJ_EXT can be aliased with KMALLOC_NORMAL... Could we
instead have a helper function like this (maybe with a better name):

#ifdef CONFIG_SLAB_OBJ_EXT
bool is_kmalloc_no_obj_ext_type(type) { return type == KMALLOC_NO_OBJ_EXT; }
#else
bool is_kmalloc_no_obj_ext_type(type) { return false; }
#endif

?

> +               if (!need_kmalloc_no_objext()) {
> +                       kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];

Could kmalloc_caches[KMALLOC_NORMAL][idx] be NULL here?
In general why do we special-case and do an early exit here? Can we do instead:

if (need_kmalloc_no_objext())
    flags |= SLAB_NO_OBJ_EXT | SLAB_NO_MERGE;

and use the common path?

> +                       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 abe748b7dddb..a34f9b8770dc 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2123,42 +2123,6 @@ static inline void init_slab_obj_exts(struct slab *slab)
>         slab->obj_exts = 0;
>  }
>
> -/*
> - * Calculate the allocation size for slabobj_ext array.
> - *
> - * When memory allocation profiling is enabled, the obj_exts array
> - * could be allocated from the same slab cache it's being allocated for.
> - * This would prevent the slab from ever being freed because it would
> - * always contain at least one allocated object (its own obj_exts array).
> - *
> - * To avoid this, increase the allocation size when we detect the array
> - * may come from the same cache, forcing it to use a different cache.
> - */
> -static inline size_t obj_exts_alloc_size(struct kmem_cache *s,
> -                                        struct slab *slab, gfp_t gfp)
> -{
> -       size_t sz = sizeof(struct slabobj_ext) * slab->objects;
> -       struct kmem_cache *obj_exts_cache;
> -
> -       if (sz > KMALLOC_MAX_CACHE_SIZE)
> -               return sz;
> -
> -       if (!is_kmalloc_normal(s))
> -               return sz;
> -
> -       obj_exts_cache = kmalloc_slab(sz, NULL, gfp, __kmalloc_token(0));
> -       /*
> -        * We can't simply compare s with obj_exts_cache, because partitioned kmalloc
> -        * caches have multiple caches per size, selected by caller address or type.
> -        * Since caller address or type may differ between kmalloc_slab() and actual
> -        * allocation, bump size when sizes are equal.
> -        */
> -       if (s->object_size == obj_exts_cache->object_size)
> -               return obj_exts_cache->object_size + 1;
> -
> -       return sz;
> -}
> -
>  int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>                         gfp_t gfp, unsigned int alloc_flags)
>  {
> @@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>         unsigned long new_exts;
>         unsigned long old_exts;
>         struct slabobj_ext *vec;
> -       size_t sz;
> +       size_t sz = sizeof(struct slabobj_ext) * slab->objects;
>
>         gfp &= ~OBJCGS_CLEAR_MASK;
> -       /* Prevent recursive extension vector allocation */
> -       alloc_flags |= SLAB_ALLOC_NO_RECURSE;
> -       alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
> +       /*
> +        * 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.

For debugging it would have been convenient to allocate all obj_ext
vectors from dedicated caches... Maybe we can do that for
CONFIG_DEBUG_VM or someday when we add CONFIG_OBJ_EXT_DEBUG? Anyway,
not really a complaint but a wish.

> +        */
> +       if (is_kmalloc_normal(s)) {
> +               VM_WARN_ON_ONCE(!need_kmalloc_no_objext());
> +               alloc_flags |= SLAB_ALLOC_NO_OBJ_EXT;
> +       }
>
> -       sz = obj_exts_alloc_size(s, slab, gfp);
> +       alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
>
>         /* This will use kmalloc_nolock() if alloc_flags say so */
>         vec = kmalloc_flags(sz, gfp | __GFP_ZERO, alloc_flags, slab_nid(slab));
> @@ -2193,8 +2163,21 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>                 return -ENOMEM;
>         }
>
> -       VM_WARN_ON_ONCE(virt_to_slab(vec) != NULL &&
> -                       virt_to_slab(vec)->slab_cache == s);
> +       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
> @@ -2217,7 +2200,6 @@ 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_obj_codetag_empty(vec);
>                 if (unlikely(!allow_spin))
>                         kfree_nolock(vec);
>                 else
> @@ -2253,14 +2235,6 @@ static inline void free_slab_obj_exts(struct slab *slab, bool allow_spin)
>                 return;
>         }
>
> -       /*
> -        * obj_exts was created with SLAB_ALLOC_NO_RECURSE 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_obj_codetag_empty(obj_exts);
>         if (allow_spin)
>                 kfree(obj_exts);
>         else
> @@ -5356,7 +5330,7 @@ void *__do_kmalloc_node(kmem_buckets *b, gfp_t flags, int node,
>         if (unlikely(!size))
>                 return ZERO_SIZE_PTR;
>
> -       s = kmalloc_slab(size, b, flags, token);
> +       s = kmalloc_slab(size, b, flags, token, ac->alloc_flags);
>
>         ret = slab_alloc_node(s, flags, node, ac);
>         ret = kasan_kmalloc(s, ret, size, flags);
> @@ -5419,7 +5393,9 @@ static void *__kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_f
>  retry:
>         if (unlikely(size > KMALLOC_MAX_CACHE_SIZE))
>                 return NULL;
> -       s = kmalloc_slab(size, NULL, gfp_flags, PASS_TOKEN_PARAM(token));
> +
> +       s = kmalloc_slab(size, NULL, gfp_flags, PASS_TOKEN_PARAM(token),
> +                        ac->alloc_flags);
>
>         if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
>                 /*
> @@ -7979,10 +7955,10 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
>                 s->allocflags |= __GFP_RECLAIMABLE;
>
>         /*
> -        * For KMALLOC_NORMAL caches we enable sheaves later by
> -        * bootstrap_kmalloc_sheaves() to avoid recursion
> +        * For kmalloc caches we enable sheaves later by
> +        * bootstrap_kmalloc_sheaves() to avoid recursion.
>          */
> -       if (!is_kmalloc_normal(s))
> +       if (!is_kmalloc_cache(s))
>                 s->sheaf_capacity = calculate_sheaf_capacity(s, args);
>
>         /*
> @@ -8548,7 +8524,7 @@ static void __init bootstrap_kmalloc_sheaves(void)
>  {
>         enum kmalloc_cache_type type;
>
> -       for (type = KMALLOC_NORMAL; type <= KMALLOC_PARTITION_END; type++) {
> +       for (type = KMALLOC_NORMAL; type < NR_KMALLOC_TYPES; type++) {
>                 for (int idx = 0; idx < KMALLOC_SHIFT_HIGH + 1; idx++) {
>                         struct kmem_cache *s = kmalloc_caches[type][idx];
>
>
> --
> 2.53.0
>

Re: [PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Harry Yoo]

[-- Attachment #1.1: Type: text/plain, Size: 5461 bytes --]


On 7/14/26 2:08 AM, Suren Baghdasaryan wrote:
> On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>> @@ -386,12 +387,17 @@ static inline unsigned int size_index_elem(unsigned int bytes)
>>   * KMALLOC_MAX_CACHE_SIZE and the caller must check that.
>>   */
>>  static inline struct kmem_cache *
>> -kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token)
>> +kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token,
>> +            unsigned int alloc_flags)
>>  {
>>         unsigned int index;
>> +       enum kmalloc_cache_type type = kmalloc_type(flags, token);
>> +
>> +       if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
>> +               type = KMALLOC_NO_OBJ_EXT;

Hi Suren, thanks for the reviews.
It's indeed helpful to have an eye for those bugfixes.

> Why not let kmalloc_type() handle alloc_flags?

Good point!

> Other users (there are
> only 4 of them) can pass SLAB_ALLOC_DEFAULT. That seems cleaner to me
> and more robust.

Hmm, there was a reason... *checks notes*, oh, there is no note.
IIRC I was afraid of exposing SLAB_ALLOC_* flags to arbitrary users.

should probably fine as long as it's not used in
kmalloc/kmem_cache_alloc() APIs, not sure.

>>         if (!b)
>> -               b = &kmalloc_caches[kmalloc_type(flags, token)];
>> +               b = &kmalloc_caches[type];
>>         if (size <= 192)
>>                 index = kmalloc_size_index[size_index_elem(size)];
>>         else
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index b6426d7ceec9..03ecac12cd86 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -957,6 +968,12 @@ 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) {
> 
> Hmm, you have to check IS_ENABLED(CONFIG_SLAB_OBJ_EXT) here because
> KMALLOC_NO_OBJ_EXT can be aliased with KMALLOC_NORMAL... Could we
> instead have a helper function like this (maybe with a better name):

Hmm that's fine, but I think that bit should not be part of -stable
fixes at least. Here I tried to make it consistent with KMALLOC_RECLAIM
and KMALLOC_DMA :)

> #ifdef CONFIG_SLAB_OBJ_EXT
> bool is_kmalloc_no_obj_ext_type(type) { return type == KMALLOC_NO_OBJ_EXT; }
> #else
> bool is_kmalloc_no_obj_ext_type(type) { return false; }
> #endif
> ?

is_kmalloc_no_obj_ext_type(),
kmalloc_type_is_no_obj_ext(),
is_kmalloc_type_no_obj_ext(),
...

naming is hard, ugh :)

> 
>> +               if (!need_kmalloc_no_objext()) {
>> +                       kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
> 
> Could kmalloc_caches[KMALLOC_NORMAL][idx] be NULL here?

No. KMALLOC_NORMAL caches are created before all other kmalloc
caches.

IIRC checking if kmalloc_caches[KMALLOC_NORMAL][idx] is NULL was added
by commit 963e84b0f262 ("mm/slab: limit kmalloc() minimum alignment
to dma_get_cache_alignment()") to avoid creating kmalloc caches of same
type and size due to minimum alignment.

> In general why do we special-case and do an early exit here?
>
> Can we do instead:
> 
> if (need_kmalloc_no_objext())
>     flags |= SLAB_NO_OBJ_EXT | SLAB_NO_MERGE;
> 
> and use the common path?

Hmm, but even without SLAB_NO_MERGE, we often end up not merging
kmalloc caches e.g.) because of non-zero s->usersize.

I think that's why we do special-case and an early exit?

We could probably do some refactoring to change that, but in general
I'm afraid of backporting refactoring work to -stable because I fear
introducing very subtle behavioral changes that nobody would notice.

>> +                       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 abe748b7dddb..a34f9b8770dc 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>>         unsigned long new_exts;
>>         unsigned long old_exts;
>>         struct slabobj_ext *vec;
>> -       size_t sz;
>> +       size_t sz = sizeof(struct slabobj_ext) * slab->objects;
>>
>>         gfp &= ~OBJCGS_CLEAR_MASK;
>> -       /* Prevent recursive extension vector allocation */
>> -       alloc_flags |= SLAB_ALLOC_NO_RECURSE;
>> -       alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
>> +       /*
>> +        * 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.
> 
> For debugging it would have been convenient to allocate all obj_ext
> vectors from dedicated caches... Maybe we can do that for
> CONFIG_DEBUG_VM or someday when we add CONFIG_OBJ_EXT_DEBUG? Anyway,
> not really a complaint but a wish.

Vlastimil and I had a conversation on always (even w/o debug options)
having dedicated caches (primarily to separate lifetime and for
simplicity), it would be interesting to explore.

https://lore.kernel.org/all/2436707a-b6ab-45ec-98e5-538e18589462@kernel.org

-- 
Cheers,
Harry / Hyeonggon

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Re: [PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Vlastimil Babka (SUSE)]

On 7/14/26 07:17, Harry Yoo wrote:
> 
> On 7/14/26 2:08 AM, Suren Baghdasaryan wrote:
>> On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>>> @@ -386,12 +387,17 @@ static inline unsigned int size_index_elem(unsigned int bytes)
>>>   * KMALLOC_MAX_CACHE_SIZE and the caller must check that.
>>>   */
>>>  static inline struct kmem_cache *
>>> -kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token)
>>> +kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token,
>>> +            unsigned int alloc_flags)
>>>  {
>>>         unsigned int index;
>>> +       enum kmalloc_cache_type type = kmalloc_type(flags, token);
>>> +
>>> +       if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
>>> +               type = KMALLOC_NO_OBJ_EXT;
> 
> Hi Suren, thanks for the reviews.
> It's indeed helpful to have an eye for those bugfixes.
> 
>> Why not let kmalloc_type() handle alloc_flags?
> 
> Good point!
> 
>> Other users (there are
>> only 4 of them) can pass SLAB_ALLOC_DEFAULT. That seems cleaner to me
>> and more robust.
> 
> Hmm, there was a reason... *checks notes*, oh, there is no note.
> IIRC I was afraid of exposing SLAB_ALLOC_* flags to arbitrary users.

Hm good point, they are in mm/slab.h and kmalloc_type() needs to be in the
public include/linux/slab.h. Given its usage in __builtin_constant_p(size)
paths of kmalloc[_node] I would rather not complicate it (even if a
hardcoded SLAB_ALLOC_DEFAULT should eliminate the extra code).
So I'd rather keep KMALLOC_NO_OBJ_EXT handling hidden in mm/ as long as it's
possible.

> should probably fine as long as it's not used in
> kmalloc/kmem_cache_alloc() APIs, not sure.
> 
>>>         if (!b)
>>> -               b = &kmalloc_caches[kmalloc_type(flags, token)];
>>> +               b = &kmalloc_caches[type];
>>>         if (size <= 192)
>>>                 index = kmalloc_size_index[size_index_elem(size)];
>>>         else
>>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>>> index b6426d7ceec9..03ecac12cd86 100644
>>> --- a/mm/slab_common.c
>>> +++ b/mm/slab_common.c
>>> @@ -957,6 +968,12 @@ 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) {
>> 
>> Hmm, you have to check IS_ENABLED(CONFIG_SLAB_OBJ_EXT) here because
>> KMALLOC_NO_OBJ_EXT can be aliased with KMALLOC_NORMAL... Could we
>> instead have a helper function like this (maybe with a better name):
> 
> Hmm that's fine, but I think that bit should not be part of -stable
> fixes at least. Here I tried to make it consistent with KMALLOC_RECLAIM
> and KMALLOC_DMA :)

Agreed.

>> #ifdef CONFIG_SLAB_OBJ_EXT
>> bool is_kmalloc_no_obj_ext_type(type) { return type == KMALLOC_NO_OBJ_EXT; }
>> #else
>> bool is_kmalloc_no_obj_ext_type(type) { return false; }
>> #endif
>> ?
> 
> is_kmalloc_no_obj_ext_type(),
> kmalloc_type_is_no_obj_ext(),
> is_kmalloc_type_no_obj_ext(),
> ...
> 
> naming is hard, ugh :)
> 
>> 
>>> +               if (!need_kmalloc_no_objext()) {
>>> +                       kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
>> 
>> Could kmalloc_caches[KMALLOC_NORMAL][idx] be NULL here?
> 
> No. KMALLOC_NORMAL caches are created before all other kmalloc
> caches.
> 
> IIRC checking if kmalloc_caches[KMALLOC_NORMAL][idx] is NULL was added
> by commit 963e84b0f262 ("mm/slab: limit kmalloc() minimum alignment
> to dma_get_cache_alignment()") to avoid creating kmalloc caches of same
> type and size due to minimum alignment.
> 
>> In general why do we special-case and do an early exit here?
>>
>> Can we do instead:
>> 
>> if (need_kmalloc_no_objext())
>>     flags |= SLAB_NO_OBJ_EXT | SLAB_NO_MERGE;
>> 
>> and use the common path?
> 
> Hmm, but even without SLAB_NO_MERGE, we often end up not merging
> kmalloc caches e.g.) because of non-zero s->usersize.
> 
> I think that's why we do special-case and an early exit?
> 
> We could probably do some refactoring to change that, but in general
> I'm afraid of backporting refactoring work to -stable because I fear
> introducing very subtle behavioral changes that nobody would notice.

Agreed, let's keep hotfixes as minimal as possible and refactor later.

>>> +                       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 abe748b7dddb..a34f9b8770dc 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>>>         unsigned long new_exts;
>>>         unsigned long old_exts;
>>>         struct slabobj_ext *vec;
>>> -       size_t sz;
>>> +       size_t sz = sizeof(struct slabobj_ext) * slab->objects;
>>>
>>>         gfp &= ~OBJCGS_CLEAR_MASK;
>>> -       /* Prevent recursive extension vector allocation */
>>> -       alloc_flags |= SLAB_ALLOC_NO_RECURSE;
>>> -       alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
>>> +       /*
>>> +        * 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.
>> 
>> For debugging it would have been convenient to allocate all obj_ext
>> vectors from dedicated caches... Maybe we can do that for
>> CONFIG_DEBUG_VM or someday when we add CONFIG_OBJ_EXT_DEBUG? Anyway,
>> not really a complaint but a wish.
> 
> Vlastimil and I had a conversation on always (even w/o debug options)
> having dedicated caches (primarily to separate lifetime and for
> simplicity), it would be interesting to explore.
> 
> https://lore.kernel.org/all/2436707a-b6ab-45ec-98e5-538e18589462@kernel.org
> 

Re: [PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Suren Baghdasaryan]

On Tue, Jul 14, 2026 at 2:12 AM Vlastimil Babka (SUSE)
<vbabka@kernel.org> wrote:
>
> On 7/14/26 07:17, Harry Yoo wrote:
> >
> > On 7/14/26 2:08 AM, Suren Baghdasaryan wrote:
> >> On Mon, Jul 13, 2026 at 7:29 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
> >>> @@ -386,12 +387,17 @@ static inline unsigned int size_index_elem(unsigned int bytes)
> >>>   * KMALLOC_MAX_CACHE_SIZE and the caller must check that.
> >>>   */
> >>>  static inline struct kmem_cache *
> >>> -kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token)
> >>> +kmalloc_slab(size_t size, kmem_buckets *b, gfp_t flags, kmalloc_token_t token,
> >>> +            unsigned int alloc_flags)
> >>>  {
> >>>         unsigned int index;
> >>> +       enum kmalloc_cache_type type = kmalloc_type(flags, token);
> >>> +
> >>> +       if (alloc_flags & SLAB_ALLOC_NO_OBJ_EXT)
> >>> +               type = KMALLOC_NO_OBJ_EXT;
> >
> > Hi Suren, thanks for the reviews.
> > It's indeed helpful to have an eye for those bugfixes.
> >
> >> Why not let kmalloc_type() handle alloc_flags?
> >
> > Good point!
> >
> >> Other users (there are
> >> only 4 of them) can pass SLAB_ALLOC_DEFAULT. That seems cleaner to me
> >> and more robust.
> >
> > Hmm, there was a reason... *checks notes*, oh, there is no note.
> > IIRC I was afraid of exposing SLAB_ALLOC_* flags to arbitrary users.
>
> Hm good point, they are in mm/slab.h and kmalloc_type() needs to be in the
> public include/linux/slab.h. Given its usage in __builtin_constant_p(size)
> paths of kmalloc[_node] I would rather not complicate it (even if a
> hardcoded SLAB_ALLOC_DEFAULT should eliminate the extra code).
> So I'd rather keep KMALLOC_NO_OBJ_EXT handling hidden in mm/ as long as it's
> possible.
>
> > should probably fine as long as it's not used in
> > kmalloc/kmem_cache_alloc() APIs, not sure.
> >
> >>>         if (!b)
> >>> -               b = &kmalloc_caches[kmalloc_type(flags, token)];
> >>> +               b = &kmalloc_caches[type];
> >>>         if (size <= 192)
> >>>                 index = kmalloc_size_index[size_index_elem(size)];
> >>>         else
> >>> diff --git a/mm/slab_common.c b/mm/slab_common.c
> >>> index b6426d7ceec9..03ecac12cd86 100644
> >>> --- a/mm/slab_common.c
> >>> +++ b/mm/slab_common.c
> >>> @@ -957,6 +968,12 @@ 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) {
> >>
> >> Hmm, you have to check IS_ENABLED(CONFIG_SLAB_OBJ_EXT) here because
> >> KMALLOC_NO_OBJ_EXT can be aliased with KMALLOC_NORMAL... Could we
> >> instead have a helper function like this (maybe with a better name):
> >
> > Hmm that's fine, but I think that bit should not be part of -stable
> > fixes at least. Here I tried to make it consistent with KMALLOC_RECLAIM
> > and KMALLOC_DMA :)
>
> Agreed.
>
> >> #ifdef CONFIG_SLAB_OBJ_EXT
> >> bool is_kmalloc_no_obj_ext_type(type) { return type == KMALLOC_NO_OBJ_EXT; }
> >> #else
> >> bool is_kmalloc_no_obj_ext_type(type) { return false; }
> >> #endif
> >> ?
> >
> > is_kmalloc_no_obj_ext_type(),
> > kmalloc_type_is_no_obj_ext(),
> > is_kmalloc_type_no_obj_ext(),
> > ...
> >
> > naming is hard, ugh :)
> >
> >>
> >>> +               if (!need_kmalloc_no_objext()) {
> >>> +                       kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
> >>
> >> Could kmalloc_caches[KMALLOC_NORMAL][idx] be NULL here?
> >
> > No. KMALLOC_NORMAL caches are created before all other kmalloc
> > caches.
> >
> > IIRC checking if kmalloc_caches[KMALLOC_NORMAL][idx] is NULL was added
> > by commit 963e84b0f262 ("mm/slab: limit kmalloc() minimum alignment
> > to dma_get_cache_alignment()") to avoid creating kmalloc caches of same
> > type and size due to minimum alignment.
> >
> >> In general why do we special-case and do an early exit here?
> >>
> >> Can we do instead:
> >>
> >> if (need_kmalloc_no_objext())
> >>     flags |= SLAB_NO_OBJ_EXT | SLAB_NO_MERGE;
> >>
> >> and use the common path?
> >
> > Hmm, but even without SLAB_NO_MERGE, we often end up not merging
> > kmalloc caches e.g.) because of non-zero s->usersize.
> >
> > I think that's why we do special-case and an early exit?
> >
> > We could probably do some refactoring to change that, but in general
> > I'm afraid of backporting refactoring work to -stable because I fear
> > introducing very subtle behavioral changes that nobody would notice.
>
> Agreed, let's keep hotfixes as minimal as possible and refactor later.

Sounds good. Feel free to add:

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

>
> >>> +                       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 abe748b7dddb..a34f9b8770dc 100644
> >>> --- a/mm/slub.c
> >>> +++ b/mm/slub.c
> >>> @@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >>>         unsigned long new_exts;
> >>>         unsigned long old_exts;
> >>>         struct slabobj_ext *vec;
> >>> -       size_t sz;
> >>> +       size_t sz = sizeof(struct slabobj_ext) * slab->objects;
> >>>
> >>>         gfp &= ~OBJCGS_CLEAR_MASK;
> >>> -       /* Prevent recursive extension vector allocation */
> >>> -       alloc_flags |= SLAB_ALLOC_NO_RECURSE;
> >>> -       alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
> >>> +       /*
> >>> +        * 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.
> >>
> >> For debugging it would have been convenient to allocate all obj_ext
> >> vectors from dedicated caches... Maybe we can do that for
> >> CONFIG_DEBUG_VM or someday when we add CONFIG_OBJ_EXT_DEBUG? Anyway,
> >> not really a complaint but a wish.
> >
> > Vlastimil and I had a conversation on always (even w/o debug options)
> > having dedicated caches (primarily to separate lifetime and for
> > simplicity), it would be interesting to explore.
> >
> > https://lore.kernel.org/all/2436707a-b6ab-45ec-98e5-538e18589462@kernel.org
> >
>

Re: [PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Vlastimil Babka (SUSE)]

On 7/13/26 16:28, Harry Yoo (Oracle) wrote:
> @@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  	unsigned long new_exts;
>  	unsigned long old_exts;
>  	struct slabobj_ext *vec;
> -	size_t sz;
> +	size_t sz = sizeof(struct slabobj_ext) * slab->objects;
>  
>  	gfp &= ~OBJCGS_CLEAR_MASK;
> -	/* Prevent recursive extension vector allocation */
> -	alloc_flags |= SLAB_ALLOC_NO_RECURSE;
> -	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
> +	/*
> +	 * 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)) {
> +		VM_WARN_ON_ONCE(!need_kmalloc_no_objext());

As discussed off-list, this can false positively trigger due to a race with
shutdown_mem_profiling() so I'll remove it.

> +		alloc_flags |= SLAB_ALLOC_NO_OBJ_EXT;
> +	}
>  
> -	sz = obj_exts_alloc_size(s, slab, gfp);
> +	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
>  
>  	/* This will use kmalloc_nolock() if alloc_flags say so */
>  	vec = kmalloc_flags(sz, gfp | __GFP_ZERO, alloc_flags, slab_nid(slab));

Re: [PATCH slab/for-next-fixes v3 0/4] mm/slab: fix unbounded recursion in free path with memalloc profiling

[Vlastimil Babka (SUSE)]

On 7/13/26 16:28, Harry Yoo (Oracle) wrote:
> This is a follow-up fix after the recent discussion [1].
> See patch 4 for the detailed description on the bug.
> 
> Based on slab/for-next-fixes (af9ea231c0b45) and is available at
> git.kernel.org [2].
> 
> Instead preventing cycles by bumping up the allocation size of obj_exts
> arrays, it introduces a new kmalloc type called KMALLOC_NO_OBJ_EXT and
> disallow formation of cycles between kmalloc types when allocating
> obj_exts arrays. obj_exts arrays of normal kmalloc caches are served
> from KMALLOC_NO_OBJ_EXT caches (that don't have obj_exts), and all other
> obj_exts arrays are served from normal kmalloc caches.
> 
> I tried to reuse SLAB_ALLOC_NO_RECURSE to make kmalloc_slab() select
> KMALLOC_NO_OBJ_EXT, but it was not great because it does not allow
> sheaves for those caches. So I introduced a new slab alloc flag
> SLAB_ALLOC_NO_OBJ_EXT.
> 
> To avoid huge confusion, I had to decouple "disallowing sheaves"
> semantics from SLAB_NO_OBJ_EXT and introduced SLAB_NO_SHEAVES.
> 
> While this cannot be directly backported to v6.18 and v6.12 due to lack
> of SLAB_ALLOC_* flags and kmalloc_flags(), I don't this will be
> particularily challenging to backport it. Instead of a new slab alloc
> flag, we can use __GFP_NO_OBJ_EXT to select KMALLOC_NO_OBJ_EXT as
> kmalloc caches don't have sheaves in v6.18 anyway.
> 
> [1] https://lore.kernel.org/linux-mm/9a139365-28e6-4f1e-b35b-7f6091e9aa14@kernel.org
> 
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/log/?h=kmalloc-no-objext-v3r1
> 
> To: Vlastimil Babka <vbabka@kernel.org>
> To: Andrew Morton <akpm@linux-foundation.org>
> To: Hao Li <hao.li@linux.dev>
> To: Christoph Lameter <cl@gentwo.org>
> To: David Rientjes <rientjes@google.com>
> To: Roman Gushchin <roman.gushchin@linux.dev>
> To: Suren Baghdasaryan <surenb@google.com>
> To: Hao Ge <hao.ge@linux.dev>
> To: Kees Cook <kees@kernel.org>
> To: Pedro Falcato <pfalcato@suse.de>
> To: Shakeel Butt <shakeel.butt@linux.dev>
> To: Danielle Constantino <dcostantino@meta.com>
> To: Liam R. Howlett <liam@infradead.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>

Added to slab/for-next-fixes, thanks!

I've removed Fixes: and Cc: stable from patches 2 and 3 as I don't believe
it's the way to handle the dependencies.