Re: [PATCH v1 bpf-next 1/2] bpf: Support BPF_F_MMAPABLE task_local storage

[Martin KaFai Lau <martin.lau@linux.dev>]

On 11/20/23 9:59 AM, Dave Marchevsky wrote:
> diff --git a/include/linux/bpf_local_storage.h b/include/linux/bpf_local_storage.h
> index 173ec7f43ed1..114973f925ea 100644
> --- a/include/linux/bpf_local_storage.h
> +++ b/include/linux/bpf_local_storage.h
> @@ -69,7 +69,17 @@ struct bpf_local_storage_data {
>   	 * the number of cachelines accessed during the cache hit case.
>   	 */
>   	struct bpf_local_storage_map __rcu *smap;
> -	u8 data[] __aligned(8);
> +	/* Need to duplicate smap's map_flags as smap may be gone when
> +	 * it's time to free bpf_local_storage_data
> +	 */
> +	u64 smap_map_flags;
> +	/* If BPF_F_MMAPABLE, this is a void * to separately-alloc'd data
> +	 * Otherwise the actual mapval data lives here
> +	 */
> +	union {
> +		DECLARE_FLEX_ARRAY(u8, data) __aligned(8);
> +		void *actual_data __aligned(8);

The pages (that can be mmap'ed later) feel like a specific kind of kptr.

Have you thought about allowing a kptr (pointing to some pages that can be 
mmap'ed later) to be stored as one of the members of the map's value as a kptr. 
bpf_local_storage_map is one of the maps that supports kptr.

struct normal_and_mmap_value {
     int some_int;
     int __percpu_kptr *some_cnts;

     struct bpf_mmap_page __kptr *some_stats;
};

struct mmap_only_value {
     struct bpf_mmap_page __kptr *some_stats;
};

[ ... ]

> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
> index 146824cc9689..9b3becbcc1a3 100644
> --- a/kernel/bpf/bpf_local_storage.c
> +++ b/kernel/bpf/bpf_local_storage.c
> @@ -15,7 +15,8 @@
>   #include <linux/rcupdate_trace.h>
>   #include <linux/rcupdate_wait.h>
>   
> -#define BPF_LOCAL_STORAGE_CREATE_FLAG_MASK (BPF_F_NO_PREALLOC | BPF_F_CLONE)
> +#define BPF_LOCAL_STORAGE_CREATE_FLAG_MASK \
> +	(BPF_F_NO_PREALLOC | BPF_F_CLONE | BPF_F_MMAPABLE)
>   
>   static struct bpf_local_storage_map_bucket *
>   select_bucket(struct bpf_local_storage_map *smap,
> @@ -24,6 +25,51 @@ select_bucket(struct bpf_local_storage_map *smap,
>   	return &smap->buckets[hash_ptr(selem, smap->bucket_log)];
>   }
>   
> +struct mem_cgroup *bpf_map_get_memcg(const struct bpf_map *map);
> +
> +void *alloc_mmapable_selem_value(struct bpf_local_storage_map *smap)
> +{
> +	struct mem_cgroup *memcg, *old_memcg;
> +	void *ptr;
> +
> +	memcg = bpf_map_get_memcg(&smap->map);
> +	old_memcg = set_active_memcg(memcg);
> +	ptr = bpf_map_area_mmapable_alloc(PAGE_ALIGN(smap->map.value_size),
> +					  NUMA_NO_NODE);
> +	set_active_memcg(old_memcg);
> +	mem_cgroup_put(memcg);
> +
> +	return ptr;
> +}

[ ... ]

> @@ -76,10 +122,19 @@ bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner,
>   		void *value, bool charge_mem, gfp_t gfp_flags)
>   {
>   	struct bpf_local_storage_elem *selem;
> +	void *mmapable_value = NULL;
> +	u32 selem_mem;
>   
> -	if (charge_mem && mem_charge(smap, owner, smap->elem_size))
> +	selem_mem = selem_bytes_used(smap);
> +	if (charge_mem && mem_charge(smap, owner, selem_mem))
>   		return NULL;
>   
> +	if (smap->map.map_flags & BPF_F_MMAPABLE) {
> +		mmapable_value = alloc_mmapable_selem_value(smap);

This probably is not always safe for bpf prog to do. Leaving the gfp_flags was 
not used aside, the bpf local storage is moving to the bpf's memalloc because of 
https://lore.kernel.org/bpf/20221118190109.1512674-1-namhyung@kernel.org/

> +		if (!mmapable_value)
> +			goto err_out;
> +	}
> +