[PATCH] mm/slab: simplify SLAB_* flag handling

[PATCH] mm/slab: simplify SLAB_* flag handling

[Kevin Brodsky]

SLUB is the only remaining allocator. We can therefore get rid of
the logic for allocator-specific flags:

* Merge SLAB_CACHE_FLAGS into SLAB_CORE_FLAGS.

* Remove SLAB_FLAGS_PERMITTED (effectively the same as
  CACHE_CREATE_MASK aside from debug flags being unconditionally
  included).

While at it also remove misleading comments that suggest that
multiple allocators are available.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---
 mm/slab.h        | 28 +++++-----------------------
 mm/slab_common.c | 12 ------------
 2 files changed, 5 insertions(+), 35 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 632fedd71fea..392f4763dee8 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -457,10 +457,12 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
 	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
 }
 
-/* Legal flag mask for kmem_cache_create(), for various configurations */
 #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
 			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
-			 SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS )
+			 SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS | \
+			 SLAB_NOLEAKTRACE | SLAB_RECLAIM_ACCOUNT | \
+			 SLAB_TEMPORARY | SLAB_ACCOUNT | \
+			 SLAB_NO_USER_FLAGS | SLAB_KMALLOC | SLAB_NO_MERGE)
 
 #ifdef CONFIG_SLUB_DEBUG
 #define SLAB_DEBUG_FLAGS (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
@@ -469,27 +471,7 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
 #define SLAB_DEBUG_FLAGS (0)
 #endif
 
-#define SLAB_CACHE_FLAGS (SLAB_NOLEAKTRACE | SLAB_RECLAIM_ACCOUNT | \
-			  SLAB_TEMPORARY | SLAB_ACCOUNT | \
-			  SLAB_NO_USER_FLAGS | SLAB_KMALLOC | SLAB_NO_MERGE)
-
-/* Common flags available with current configuration */
-#define CACHE_CREATE_MASK (SLAB_CORE_FLAGS | SLAB_DEBUG_FLAGS | SLAB_CACHE_FLAGS)
-
-/* Common flags permitted for kmem_cache_create */
-#define SLAB_FLAGS_PERMITTED (SLAB_CORE_FLAGS | \
-			      SLAB_RED_ZONE | \
-			      SLAB_POISON | \
-			      SLAB_STORE_USER | \
-			      SLAB_TRACE | \
-			      SLAB_CONSISTENCY_CHECKS | \
-			      SLAB_NOLEAKTRACE | \
-			      SLAB_RECLAIM_ACCOUNT | \
-			      SLAB_TEMPORARY | \
-			      SLAB_ACCOUNT | \
-			      SLAB_KMALLOC | \
-			      SLAB_NO_MERGE | \
-			      SLAB_NO_USER_FLAGS)
+#define CACHE_CREATE_MASK (SLAB_CORE_FLAGS | SLAB_DEBUG_FLAGS)
 
 bool __kmem_cache_empty(struct kmem_cache *);
 int __kmem_cache_shutdown(struct kmem_cache *);
diff --git a/mm/slab_common.c b/mm/slab_common.c
index a29457bef626..3b07cdaac3ae 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -305,18 +305,6 @@ struct kmem_cache *__kmem_cache_create_args(const char *name,
 		goto out_unlock;
 	}
 
-	/* Refuse requests with allocator specific flags */
-	if (flags & ~SLAB_FLAGS_PERMITTED) {
-		err = -EINVAL;
-		goto out_unlock;
-	}
-
-	/*
-	 * Some allocators will constraint the set of valid flags to a subset
-	 * of all flags. We expect them to define CACHE_CREATE_MASK in this
-	 * case, and we'll just provide them with a sanitized version of the
-	 * passed flags.
-	 */
 	flags &= CACHE_CREATE_MASK;
 
 	/* Fail closed on bad usersize of useroffset values. */

base-commit: 9bffa1ad25b8b3b95d8f463e5c24dabe3c87d54d
-- 
2.47.0

Re: [PATCH] mm/slab: simplify SLAB_* flag handling

[Christoph Lameter (Ampere)]

On Fri, 17 Jan 2025, Kevin Brodsky wrote:

> index a29457bef626..3b07cdaac3ae 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -305,18 +305,6 @@ struct kmem_cache *__kmem_cache_create_args(const char *name,
>  		goto out_unlock;
>  	}
>
> -	/* Refuse requests with allocator specific flags */
> -	if (flags & ~SLAB_FLAGS_PERMITTED) {
> -		err = -EINVAL;
> -		goto out_unlock;
> -	}

I think we should keep checking for invalid flags.
 -
> -	/*
> -	 * Some allocators will constraint the set of valid flags to a subset
> -	 * of all flags. We expect them to define CACHE_CREATE_MASK in this
> -	 * case, and we'll just provide them with a sanitized version of the
> -	 * passed flags.
> -	 */
>  	flags &= CACHE_CREATE_MASK;

This would silently clear some flags instead of creating an error.

Re: [PATCH] mm/slab: simplify SLAB_* flag handling

[Kevin Brodsky]

On 17/01/2025 23:13, Christoph Lameter (Ampere) wrote:
> On Fri, 17 Jan 2025, Kevin Brodsky wrote:
>
>> index a29457bef626..3b07cdaac3ae 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -305,18 +305,6 @@ struct kmem_cache *__kmem_cache_create_args(const char *name,
>>  		goto out_unlock;
>>  	}
>>
>> -	/* Refuse requests with allocator specific flags */
>> -	if (flags & ~SLAB_FLAGS_PERMITTED) {
>> -		err = -EINVAL;
>> -		goto out_unlock;
>> -	}
> I think we should keep checking for invalid flags.

The commit that introduced this check [1] aimed to ensure that no
allocator-specific flag is passed to kmem_cache_create(), so it seemed
to me it was no longer needed now that allocator-specific flags are gone.

Having said that, we could keep it in order to reject flags that are not
supposed to be passed to kmem_cache_create() (e.g. SLAB_SKIP_KFENCE).
With that approach we'd just need to clear SLAB_DEBUG_FLAGS below if
!CONFIG_SLUB_DEBUG (and get rid of CACHE_CREATE_MASK).

- Kevin

[1]
https://lore.kernel.org/all/1478553075-120242-2-git-send-email-thgarnie@google.com/

>> -	/*
>> -	 * Some allocators will constraint the set of valid flags to a subset
>> -	 * of all flags. We expect them to define CACHE_CREATE_MASK in this
>> -	 * case, and we'll just provide them with a sanitized version of the
>> -	 * passed flags.
>> -	 */
>>  	flags &= CACHE_CREATE_MASK;
> This would silently clear some flags instead of creating an error.

Re: [PATCH] mm/slab: simplify SLAB_* flag handling

[Vlastimil Babka]

On 1/21/25 11:46, Kevin Brodsky wrote:
> On 17/01/2025 23:13, Christoph Lameter (Ampere) wrote:
>> On Fri, 17 Jan 2025, Kevin Brodsky wrote:
>>
>>> index a29457bef626..3b07cdaac3ae 100644
>>> --- a/mm/slab_common.c
>>> +++ b/mm/slab_common.c
>>> @@ -305,18 +305,6 @@ struct kmem_cache *__kmem_cache_create_args(const char *name,
>>>  		goto out_unlock;
>>>  	}
>>>
>>> -	/* Refuse requests with allocator specific flags */
>>> -	if (flags & ~SLAB_FLAGS_PERMITTED) {
>>> -		err = -EINVAL;
>>> -		goto out_unlock;
>>> -	}
>> I think we should keep checking for invalid flags.
> 
> The commit that introduced this check [1] aimed to ensure that no
> allocator-specific flag is passed to kmem_cache_create(), so it seemed
> to me it was no longer needed now that allocator-specific flags are gone.
> 
> Having said that, we could keep it in order to reject flags that are not
> supposed to be passed to kmem_cache_create() (e.g. SLAB_SKIP_KFENCE).
> With that approach we'd just need to clear SLAB_DEBUG_FLAGS below if
> !CONFIG_SLUB_DEBUG (and get rid of CACHE_CREATE_MASK).

Sounds like a good plan to me, thanks!

> - Kevin
> 
> [1]
> https://lore.kernel.org/all/1478553075-120242-2-git-send-email-thgarnie@google.com/
> 
>>> -	/*
>>> -	 * Some allocators will constraint the set of valid flags to a subset
>>> -	 * of all flags. We expect them to define CACHE_CREATE_MASK in this
>>> -	 * case, and we'll just provide them with a sanitized version of the
>>> -	 * passed flags.
>>> -	 */
>>>  	flags &= CACHE_CREATE_MASK;
>> This would silently clear some flags instead of creating an error.
> 
> 
>