Re: [PATCH bpf-next v2 3/4] bpf: arena: make arena kfuncs any context safe

[Puranjay Mohan <puranjay@kernel.org>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Fri, Nov 14, 2025 at 3:17 AM Puranjay Mohan <puranjay@kernel.org> wrote:
>>
>>
>> +       init_llist_head(&free_pages);
>> +       /* clear ptes and collect struct pages */
>> +       apply_to_existing_page_range(&init_mm, kaddr, page_cnt << PAGE_SHIFT,
>> +                                    apply_range_clear_cb, &free_pages);
>> +
>> +       /* drop the lock to do the tlb flush and zap pages */
>> +       raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
>> +
>> +       /* ensure no stale TLB entries */
>> +       flush_tlb_kernel_range(kaddr, kaddr + (page_cnt * PAGE_SIZE));
>> +
>>         if (page_cnt > 1)
>>                 /* bulk zap if multiple pages being freed */
>>                 zap_pages(arena, full_uaddr, page_cnt);
>>
>> -       kaddr = bpf_arena_get_kern_vm_start(arena) + uaddr;
>> -       for (i = 0; i < page_cnt; i++, kaddr += PAGE_SIZE, full_uaddr += PAGE_SIZE) {
>> -               page = vmalloc_to_page((void *)kaddr);
>> -               if (!page)
>> -                       continue;
>> +       llist_for_each_safe(pos, t, llist_del_all(&free_pages)) {
>
> llist_del_all() ?! Why? it's a variable on stack. There is no race.

Yeah, I should have used __llist_del_all() which doesn't do an xchg() or
in this case I can just use free_pages.first