Re: [PATCH bpf-next 1/3] bpf: Add a sleepable page allocator for map memory

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Mon Jul 27, 2026 at 2:24 AM EDT, Jiayuan Chen wrote:
> bpf_map_alloc_pages() picks the allocator via can_alloc_pages(), a
> conservative guess for BPF program context that is always false under
> PREEMPT_RT. So even a caller that really is sleepable gets the
> non-blocking allocator, which never reclaims and never engages the OOM
> machinery.
>
> Add bpf_map_alloc_page_sleepable() for callers that know they are
> sleepable. The next patch uses it from the arena page fault handler.
>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  include/linux/bpf.h  |  1 +
>  kernel/bpf/syscall.c | 21 +++++++++++++++++----
>  2 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index d9542127dfdf..9ba03622709b 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2779,6 +2779,7 @@ struct bpf_prog *bpf_prog_get_curr_or_next(u32 *id);
>  
>  int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
>  			unsigned long nr_pages, struct page **page_array);
> +struct page *bpf_map_alloc_page_sleepable(const struct bpf_map *map, int nid);
>  #ifdef CONFIG_MEMCG
>  void bpf_map_memcg_enter(const struct bpf_map *map, struct mem_cgroup **old_memcg,
>  			 struct mem_cgroup **new_memcg);
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 0ff9e3aa293d..cec88450e4ce 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -602,15 +602,14 @@ static bool can_alloc_pages(void)
>  		!IS_ENABLED(CONFIG_PREEMPT_RT);
>  }
>  
> +#define BPF_PAGE_GFP (GFP_KERNEL | __GFP_ZERO | __GFP_ACCOUNT | __GFP_NOWARN)
> +
>  static struct page *__bpf_alloc_page(int nid)
>  {
>  	if (!can_alloc_pages())
>  		return alloc_pages_nolock(__GFP_ACCOUNT, nid, 0);
>  
> -	return alloc_pages_node(nid,
> -				GFP_KERNEL | __GFP_ZERO | __GFP_ACCOUNT
> -				| __GFP_NOWARN,
> -				0);
> +	return alloc_pages_node(nid, BPF_PAGE_GFP, 0);
>  }
>  
>  int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
> @@ -636,6 +635,20 @@ int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
>  	return ret;
>  }
>  
> +/*
> + * For callers that know they run in a sleepable context, e.g. a user page
> + * fault handler. can_alloc_pages() is a conservative guess made for BPF
> + * program context - notably it is always false on PREEMPT_RT - so going
> + * through bpf_map_alloc_pages() there would needlessly pick the
> + * non-blocking allocator, which never reclaims and never engages the OOM
> + * machinery.
> + */
> +struct page *bpf_map_alloc_page_sleepable(const struct bpf_map *map, int nid)
> +{
> +	might_sleep();
> +	return alloc_pages_node(nid, BPF_PAGE_GFP, 0);
> +}
> +
>  
>  static int btf_field_cmp(const void *a, const void *b)
>  {