Re: [PATCH 2/8] bpf: Recover arena kernel faults with scratch page

[Alexei Starovoitov]

On Sun May 31, 2026 at 10:47 AM PDT, Tejun Heo wrote:
> Hello,
>
> I posted the check removal [1], and Sashiko's review flagged a
> break-before-make problem with it [2] that I think is real.

ohh. interesting.

> So instead of just dropping the check, the install should route through an
> invalid entry rather than overwrite in place:
>
> 	while (!ptep_try_set(pte, mk_pte(page, PAGE_KERNEL))) {
> 		old = ptep_get(pte);
> 		if (pte_none(old))
> 			continue;
> 		if (WARN_ON_ONCE(pte_page(old) != arena->scratch_page))
> 			return -EBUSY;
> 		ptep_get_and_clear(&init_mm, addr, pte);
> 		broke_scratch = true;
> 	}
>
> ptep_try_set() only fills a none slot, so the slot goes scratch->none->page
> and never valid->valid, and the loop copes with a concurrent fault
> re-scratching it. This also closes the set_pte_at()-vs-ptep_try_set() race
> I raised earlier, since both sides are now cmpxchg. A broken scratch entry
> was live, so the caller flush_tlb_kernel_range()s those pages when
> broke_scratch is set, like arena_free_pages() already does after clearing.

Makes sense to me.