Re: [PATCH 1/2] mm/page_alloc: don't spin_trylock() in NMI on UP

[Brendan Jackman]

On Tue Jul 14, 2026 at 9:52 AM UTC, Brendan Jackman wrote:
> On Mon Jul 13, 2026 at 4:15 PM UTC, Harry Yoo wrote:
>>
>>
>> On 7/13/26 11:31 PM, Brendan Jackman wrote:
>>> On Sat Jul 11, 2026 at 12:03 AM UTC, Andrew Morton wrote:
>>>> On Fri, 10 Jul 2026 10:42:20 +0000 Brendan Jackman <jackmanb@xxxxxxxxxx> wrote:
>>>>
>>>>> As noted in can_spin_trylock(), using this is unsafe in this context.
>>>>> commit 620b46ed6ae17 ("mm/page_alloc: return NULL early from
>>>>> alloc_frozen_pages_nolock() in NMI on UP") fixed this on the alloc side
>>>>> but missed the free side.
>>
>> Ouch, do we allow alloc_pages() -> free_pages_nolock()?
>> Didn't notice.
>
> We don't explicitly disallow that but I'd say it's "forbidden by
> default"...

Oh, the BPF arena code does it.

>>>> Also, Sashiko might have found yet more pre-existing issues:
>>>> 	https://sashiko.dev/#/patchset/20260710-spin-trylock-followup-v1-0-affb5fe5ed00@xxxxxxxxxx
>>> 
>>> There are 2 cases here:
>>> 
>>> 1: !pcp_allowed_order() -> This is forbidden by alloc_order_allowed(),
>>>    quite a bad miss from Sashiko IMO.
>>
>> Hmm but alloc_order_allowed() would return true for !pcp_allowed_order()
>> when spinning is allowed, then pages can be freed via
>> free_pages_nolock()?

So yeah Sashiko was right after all.