Re: [PATCH v2 2/3] mm: add kmem_cache_create_rcu()

[Vlastimil Babka <vbabka@suse.cz>]

On 8/27/24 17:59, Christian Brauner wrote:
> When a kmem cache is created with SLAB_TYPESAFE_BY_RCU the free pointer
> must be located outside of the object because we don't know what part of
> the memory can safely be overwritten as it may be needed to prevent
> object recycling.
> 
> That has the consequence that SLAB_TYPESAFE_BY_RCU may end up adding a
> new cacheline. This is the case for .e.g, struct file. After having it
> shrunk down by 40 bytes and having it fit in three cachelines we still
> have SLAB_TYPESAFE_BY_RCU adding a fourth cacheline because it needs to
> accomodate the free pointer and is hardware cacheline aligned.
> 
> I tried to find ways to rectify this as struct file is pretty much
> everywhere and having it use less memory is a good thing. So here's a
> proposal.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>

So logistically patch 3 needs stuff in the vfs tree and having 1+2 in slab
tree and 3 in vfs that depends on 1+2 elsewhere is infeasible, so it will be
easiest for whole series to be in vfs, right?

> ---
>  include/linux/slab.h |   9 ++++
>  mm/slab.h            |   1 +
>  mm/slab_common.c     | 133 ++++++++++++++++++++++++++++++++++++---------------
>  mm/slub.c            |  17 ++++---
>  4 files changed, 114 insertions(+), 46 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index eb2bf4629157..5b2da2cf31a8 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -212,6 +212,12 @@ enum _slab_flag_bits {
>  #define SLAB_NO_OBJ_EXT		__SLAB_FLAG_UNUSED
>  #endif
>  
> +/*
> + * freeptr_t represents a SLUB freelist pointer, which might be encoded
> + * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
> + */
> +typedef struct { unsigned long v; } freeptr_t;
> +
>  /*
>   * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
>   *
> @@ -242,6 +248,9 @@ struct kmem_cache *kmem_cache_create_usercopy(const char *name,
>  			slab_flags_t flags,
>  			unsigned int useroffset, unsigned int usersize,
>  			void (*ctor)(void *));
> +struct kmem_cache *kmem_cache_create_rcu(const char *name, unsigned int size,
> +					 unsigned int freeptr_offset,
> +					 slab_flags_t flags);
>  void kmem_cache_destroy(struct kmem_cache *s);
>  int kmem_cache_shrink(struct kmem_cache *s);
>  
> diff --git a/mm/slab.h b/mm/slab.h
> index dcdb56b8e7f5..b05512a14f07 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -261,6 +261,7 @@ struct kmem_cache {
>  	unsigned int object_size;	/* Object size without metadata */
>  	struct reciprocal_value reciprocal_size;
>  	unsigned int offset;		/* Free pointer offset */
> +	unsigned int rcu_freeptr_offset; /* Specific free pointer requested */

More precisely something like:

				  Specific offset requested (if not
				  UINT_MAX)

?

>  #ifdef CONFIG_SLUB_CPU_PARTIAL
>  	/* Number of per cpu partial objects to keep around */
>  	unsigned int cpu_partial;
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index c8dd7e08c5f6..c4beff642fff 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -202,9 +202,10 @@ struct kmem_cache *find_mergeable(unsigned int size, unsigned int align,
>  }
>  
>  static struct kmem_cache *create_cache(const char *name,
> -		unsigned int object_size, unsigned int align,
> -		slab_flags_t flags, unsigned int useroffset,
> -		unsigned int usersize, void (*ctor)(void *))
> +		unsigned int object_size, unsigned int freeptr_offset,
> +		unsigned int align, slab_flags_t flags,
> +		unsigned int useroffset, unsigned int usersize,
> +		void (*ctor)(void *))
>  {
>  	struct kmem_cache *s;
>  	int err;
> @@ -212,6 +213,12 @@ static struct kmem_cache *create_cache(const char *name,
>  	if (WARN_ON(useroffset + usersize > object_size))
>  		useroffset = usersize = 0;
>  
> +	err = -EINVAL;
> +	if (freeptr_offset < UINT_MAX &&

freeptr_offset != UINT_MAX to be more obvious and match has_freeptr_offset() ?

> +	    (freeptr_offset >= object_size ||
> +	     (freeptr_offset && !(flags & SLAB_TYPESAFE_BY_RCU))))

and here drop the "freeptr_offset &&" as zero is a valid value

instead we could want alignment to sizeof(freeptr_t) if we were paranoid?

> +		goto out;

The rest seems good to me now.