Re: [PATCH 3/3] mm, slab, kasan: replace kasan_never_merge() with SLAB_NO_MERGE

[Andrey Konovalov]

On Tue, Feb 20, 2024 at 5:58 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> The SLAB_KASAN flag prevents merging of caches in some configurations,
> which is handled in a rather complicated way via kasan_never_merge().
> Since we now have a generic SLAB_NO_MERGE flag, we can instead use it
> for KASAN caches in addition to SLAB_KASAN in those configurations,
> and simplify the SLAB_NEVER_MERGE handling.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  include/linux/kasan.h |  6 ------
>  mm/kasan/generic.c    | 16 ++++------------
>  mm/slab_common.c      |  2 +-
>  3 files changed, 5 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index dbb06d789e74..70d6a8f6e25d 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -429,7 +429,6 @@ struct kasan_cache {
>  };
>
>  size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object);
> -slab_flags_t kasan_never_merge(void);
>  void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>                         slab_flags_t *flags);
>
> @@ -446,11 +445,6 @@ static inline size_t kasan_metadata_size(struct kmem_cache *cache,
>  {
>         return 0;
>  }
> -/* And thus nothing prevents cache merging. */
> -static inline slab_flags_t kasan_never_merge(void)
> -{
> -       return 0;
> -}
>  /* And no cache-related metadata initialization is required. */
>  static inline void kasan_cache_create(struct kmem_cache *cache,
>                                       unsigned int *size,
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index df6627f62402..d8b78d273b9f 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -334,14 +334,6 @@ DEFINE_ASAN_SET_SHADOW(f3);
>  DEFINE_ASAN_SET_SHADOW(f5);
>  DEFINE_ASAN_SET_SHADOW(f8);
>
> -/* Only allow cache merging when no per-object metadata is present. */
> -slab_flags_t kasan_never_merge(void)
> -{
> -       if (!kasan_requires_meta())
> -               return 0;
> -       return SLAB_KASAN;
> -}
> -
>  /*
>   * Adaptive redzone policy taken from the userspace AddressSanitizer runtime.
>   * For larger allocations larger redzones are used.
> @@ -372,13 +364,13 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>         /*
>          * SLAB_KASAN is used to mark caches that are sanitized by KASAN
>          * and that thus have per-object metadata.
> -        * Currently this flag is used in two places:
> +        * Currently this flag is used in one place:
>          * 1. In slab_ksize() to account for per-object metadata when
>          *    calculating the size of the accessible memory within the object.
> -        * 2. In slab_common.c via kasan_never_merge() to prevent merging of
> -        *    caches with per-object metadata.

Let's reword this to:

SLAB_KASAN is used to mark caches that are sanitized by KASAN and that
thus have per-object metadata. Currently, this flag is used in
slab_ksize() to account for per-object metadata when calculating the
size of the accessible memory within the object.

> +        * Additionally, we use SLAB_NO_MERGE to prevent merging of caches
> +        * with per-object metadata.
>          */
> -       *flags |= SLAB_KASAN;
> +       *flags |= SLAB_KASAN | SLAB_NO_MERGE;
>
>         ok_size = *size;
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 238293b1dbe1..7cfa2f1ce655 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -50,7 +50,7 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
>   */
>  #define SLAB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
>                 SLAB_TRACE | SLAB_TYPESAFE_BY_RCU | SLAB_NOLEAKTRACE | \
> -               SLAB_FAILSLAB | SLAB_NO_MERGE | kasan_never_merge())
> +               SLAB_FAILSLAB | SLAB_NO_MERGE)
>
>  #define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
>                          SLAB_CACHE_DMA32 | SLAB_ACCOUNT)
>
> --
> 2.43.1
>

Otherwise, looks good to me.

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Thanks!