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

[Harry Yoo <harry@kernel.org>]

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On 7/14/26 6:52 PM, 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@google.com> 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"...
> 
> But I think that's unrelated? It doesn't mean you can't use
> free_pages_nolock() from NMI, right? (Would be weird to alloc from
> another context and then free in NMI, but I don't think it's "forbidden
> by default" in the way that using unmatched APIs is).

If you can't free pages that are not allocated via alloc_pages_nolock(),
and if alloc_pages_nolock() always fails, you can't really use
free_pages_nolock().

But yeah BPF seems to do that and also the comment says:

/*
 * Can be called while holding raw_spin_lock or from IRQ and NMI for any
 * page type (not only those that came from alloc_pages_nolock)
 */

-- 
Cheers,
Harry / Hyeonggon


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