Re: [PATCH bpf-next v2 2/7] mm/percpu.c: introduce pcpu_alloc_size()

[Dennis Zhou <dennis@kernel.org>]

On Wed, Oct 18, 2023 at 07:33:38PM +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> Introduce pcpu_alloc_size() to get the size of the dynamic per-cpu
> area. It will be used by bpf memory allocator in the following patches.
> BPF memory allocator maintains per-cpu area caches for multiple area
> sizes and its free API only has the to-be-freed per-cpu pointer, so it
> needs the size of dynamic per-cpu area to select the corresponding cache
> when bpf program frees the dynamic per-cpu pointer.
> 
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
>  include/linux/percpu.h |  1 +
>  mm/percpu.c            | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index 68fac2e7cbe6..8c677f185901 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -132,6 +132,7 @@ extern void __init setup_per_cpu_areas(void);
>  extern void __percpu *__alloc_percpu_gfp(size_t size, size_t align, gfp_t gfp) __alloc_size(1);
>  extern void __percpu *__alloc_percpu(size_t size, size_t align) __alloc_size(1);
>  extern void free_percpu(void __percpu *__pdata);
> +extern size_t pcpu_alloc_size(void __percpu *__pdata);
>  
>  DEFINE_FREE(free_percpu, void __percpu *, free_percpu(_T))
>  
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 76b9c5e63c56..b0cea2dc16a9 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -2244,6 +2244,36 @@ static void pcpu_balance_workfn(struct work_struct *work)
>  	mutex_unlock(&pcpu_alloc_mutex);
>  }
>  
> +/**
> + * pcpu_alloc_size - the size of the dynamic percpu area
> + * @ptr: pointer to the dynamic percpu area
> + *
> + * Return the size of the dynamic percpu area @ptr.
> + *

Alexei, can you modify the above comment to:

Returns the size of the @ptr allocation.  This is undefined for statically
defined percpu variables as there is no corresponding chunk->bound_map.

> + * RETURNS:
> + * The size of the dynamic percpu area.
> + *
> + * CONTEXT:
> + * Can be called from atomic context.
> + */
> +size_t pcpu_alloc_size(void __percpu *ptr)
> +{
> +	struct pcpu_chunk *chunk;
> +	unsigned long bit_off, end;
> +	void *addr;
> +
> +	if (!ptr)
> +		return 0;
> +
> +	addr = __pcpu_ptr_to_addr(ptr);
> +	/* No pcpu_lock here: ptr has not been freed, so chunk is still alive */
> +	chunk = pcpu_chunk_addr_search(addr);
> +	bit_off = (addr - chunk->base_addr) / PCPU_MIN_ALLOC_SIZE;
> +	end = find_next_bit(chunk->bound_map, pcpu_chunk_map_bits(chunk),
> +			    bit_off + 1);
> +	return (end - bit_off) * PCPU_MIN_ALLOC_SIZE;
> +}
> +
>  /**
>   * free_percpu - free percpu area
>   * @ptr: pointer to area to free
> -- 
> 2.29.2
> 
> 

Acked-by: Dennis Zhou <dennis@kernel.org>

Thanks,
Dennis