Subject: Re: [PATCH 1/8] mm: Add ptep_try_install() for lockless empty-slot installs

From: Tejun Heo <[email-masked]>
Date: Mon, 18 May 2026 22:58:11 -1000

Hello, David.

On Tue, May 19, 2026 at 10:00:39AM +0200, David Hildenbrand (Arm) wrote:
> Is that really possible? I'd much rather prefer to trylock and retry, unless
> that can really result in deadlocks. But I have the feeling that such deadlocks
> should be impossible here.

I'm not well versed in either mm or BPF, so the BPF folks will have a
better take. But here's a scenario that seemed plausible to me:

1. A bpf prog calls bpf_arena_alloc_pages() on its arena. The kernel
   takes arena->spinlock via raw_res_spin_lock_irqsave().
2. Under the lock, the alloc path goes through bpf_map_alloc_pages()
   -> alloc_pages_node(), which fires trace_mm_page_alloc().
3. A BPF tracepoint program on mm_page_alloc that shares the arena
   starts running with the lock still held.
4. The tracepoint program calls a kfunc, passing an arena pointer
   one entry past the array it meant to touch.
5. The kfunc dereferences. The kernel-side address is unbacked, so
   the CPU faults.

trylock + retry at 5 would A-A deadlock.

> For example, staring at apply_range_set_cb(), what prevents:
>
> (1) apply_range_set_cb() finding pte_none(ptep_get(pte)
> (2) apply_range_set_scratch_cb() succeeding ptep_try_install()
> (3) apply_range_set_cb() overwriting the pte with set_pte_at()
>
> Between (2) and (3) CPUs could access the scratch PTE.

Scratch only gets installed when BPF passes an unallocated arena
address to the kernel side, which is itself the violation, reported
through the program's BPF stream. Behavior at that addr is then
undefined. For scx, the scheduler should be aborted and torn down.

The only requirements are that the kernel doesn't oops and the
violation gets caught. Beyond that, behavior at the address is
unspecified, and which installer wins the race doesn't matter as
long as kernel integrity holds.

Thanks.

--
tejun