Re: [PATCH v4 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc

[Song Liu]

On Wed, Oct 2, 2024 at 11:10 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> The bpf_get_kmem_cache() is to get a slab cache information from a
> virtual address like virt_to_cache().  If the address is a pointer
> to a slab object, it'd return a valid kmem_cache pointer, otherwise
> NULL is returned.
>
> It doesn't grab a reference count of the kmem_cache so the caller is
> responsible to manage the access.  The intended use case for now is to
> symbolize locks in slab objects from the lock contention tracepoints.
>
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx> (mm/*)
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> #mm/slab
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  kernel/bpf/helpers.c |  1 +
>  mm/slab_common.c     | 19 +++++++++++++++++++
>  2 files changed, 20 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 4053f279ed4cc7ab..3709fb14288105c6 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3090,6 +3090,7 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
>  BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
> +BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_RET_NULL)
>  BTF_KFUNCS_END(common_btf_ids)
>
>  static const struct btf_kfunc_id_set common_kfunc_set = {
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 7443244656150325..5484e1cd812f698e 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1322,6 +1322,25 @@ size_t ksize(const void *objp)
>  }
>  EXPORT_SYMBOL(ksize);
>
> +#ifdef CONFIG_BPF_SYSCALL
> +#include <linux/btf.h>
> +
> +__bpf_kfunc_start_defs();
> +
> +__bpf_kfunc struct kmem_cache *bpf_get_kmem_cache(u64 addr)
> +{
> +       struct slab *slab;
> +
> +       if (!virt_addr_valid(addr))
> +               return NULL;
> +
> +       slab = virt_to_slab((void *)(long)addr);
> +       return slab ? slab->slab_cache : NULL;
> +}

Do we need to hold a refcount to the slab_cache? Given
we make this kfunc available everywhere, including
sleepable contexts, I think it is necessary.

Thanks
Song

> +
> +__bpf_kfunc_end_defs();
> +#endif /* CONFIG_BPF_SYSCALL */
> +
>  /* Tracepoints definitions. */
>  EXPORT_TRACEPOINT_SYMBOL(kmalloc);
>  EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
> --
> 2.46.1.824.gd892dcdcdd-goog
>