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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] NUMA Mempolicy Bypass in bpf_map_alloc_page_sleepable() and __bpf_alloc_page()
--

--- Patch [1]: [PATCH bpf-next 1/3] bpf: Add a sleepable page allocator for map memory ---
commit 7a6023ef7f19fc884e94ffe457ae83ff0d4b5e1e
Author: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>

bpf: Add a sleepable page allocator for map memory

This commit introduces bpf_map_alloc_page_sleepable() to provide a blocking
page allocator for callers known to be in a sleepable context, such as user
page fault handlers. This avoids the conservative, non-blocking behavior of
can_alloc_pages() which is always false under PREEMPT_RT.

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> --- 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);

[Severity: Medium]
This isn't a bug introduced by this patch, but does using alloc_pages_node()
here bypass the task's NUMA mempolicy?

Because alloc_pages_node() ignores task mempolicy, pages might land on the
current CPU's local NUMA node instead of the node requested by the task.
Could this branch to use a mempolicy-aware allocation when nid is
NUMA_NO_NODE?

>  }
[ ... ]
> +/*
> + * 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);

[Severity: Medium]
Since this function is intended for user space page fault handlers, will
unconditionally calling alloc_pages_node() cause the allocation to ignore
the user task's mempolicy?

Similar to __bpf_alloc_page(), if nid is NUMA_NO_NODE, replacing this with a
mempolicy-aware allocation might prevent silent performance regressions from
misaligned memory placement during page faults.

> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260727062521.376231-1-jiayuan.chen@xxxxxxxxx?part=1