Re: [PATCH] slab: fix the type of the index on freelist index accessor

[Steven King]

On Friday 18 April 2014 12:24:09 am Joonsoo Kim wrote:
> commit 8dcc774 (slab: introduce byte sized index for the freelist of
> a slab) changes the size of freelist index and also changes prototype
> of accessor function to freelist index. And there was a mistake.
>
> The mistake is that although it changes the size of freelist index
> correctly, it changes the size of the index of freelist index incorrectly.
> With patch, freelist index can be 1 byte or 2 bytes, that means that
> num of object on on a slab can be more than 255. So we need more than 1
> byte for the index to find the index of free object on freelist. But,
> above patch makes this index type 1 byte, so slab which have more than
> 255 objects cannot work properly and in consequence of it, the system
> cannot boot.
>
> This issue was reported by Steven King on m68knommu which would use
> 2 bytes freelist index. Please refer following link.
>
> https://lkml.org/lkml/2014/4/16/433
>
> To fix it is so easy. To change the type of the index of freelist index
> on accessor functions is enough to fix this bug. Although 2 bytes is
> enough, I use 4 bytes since it have no bad effect and make things
> more easier. This fix was suggested and tested by Steven in his
> original report.
>
> Reported-by: Steven King <sfking@xxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
> Hello, Pekka.
>
> Could you send this for v3.15-rc2?
> Without this patch, many architecture using 2 bytes freelist index cannot
> work properly, I guess.
>
> This patch is based on v3.15-rc1.
>
> Thanks.
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 388cb1a..d7f9f44 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2572,13 +2572,13 @@ static void *alloc_slabmgmt(struct kmem_cache
> *cachep, return freelist;
>  }
>
> -static inline freelist_idx_t get_free_obj(struct page *page, unsigned char
> idx) +static inline freelist_idx_t get_free_obj(struct page *page, unsigned
> int idx) {
>  	return ((freelist_idx_t *)page->freelist)[idx];
>  }
>
>  static inline void set_free_obj(struct page *page,
> -					unsigned char idx, freelist_idx_t val)
> +					unsigned int idx, freelist_idx_t val)
>  {
>  	((freelist_idx_t *)(page->freelist))[idx] = val;
>  }

Acked-by: Steven King <sfking@xxxxxxxxx>
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