Re: [PATCH v8 2/6] mm/memory-failure: surface unhandlable kernel pages as -ENOTRECOVERABLE

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/9/26 04:39, Miaohe Lin wrote:
> On 2026/6/8 22:15, Breno Leitao wrote:
>> On Fri, Jun 05, 2026 at 11:42:53AM +0200, David Hildenbrand (Arm) wrote:
>>>
>>> I mean, any such races can currently already happen one way or the other?
>>>
>>> Really, the only way to not get races is to tryget the (compound)page,
>>> revalidate that the page is still part of the compound page.
>>>
>>> I'm not sure if that's really a good idea.
>>>
>>> But my memory is a bit vague in which scenarios we already hold a page reference
>>> here to prevent any concurrent freeing?
>>
>> No, we don't hold one here in the case that matters.
>>
>> HWPoisonKernelOwned() runs at the very top of get_any_page(), before
>> try_again: and before __get_hwpoison_page(). The first refcount taken in
>> the whole path is the folio_try_get() inside __get_hwpoison_page(), which
>> runs *after* the short-circuit.
>>
>> So get_any_page() itself never holds a reference at the check -- the only way
>> one exists is if the caller passed MF_COUNT_INCREASED (count_increased ==
>> true).
>>
>> So on the MCE/GHES path -- the one this panic option exists for -- no
>> reference is held when HWPoisonKernelOwned() does its compound_head() +
>> PageSlab()/PageTable()/PageLargeKmalloc() checks.
>>
>> Given that, I'd rather keep it racy and take no refcount than add a
>> tryget + revalidate purely for this check. As I've said earleir, an operator
> 
> Would it be acceptable to add a simple recheck? Something like below:
> 
> retry:
> head = compound_head(page);
> PageSlab()/PageTable()/PageLargeKmalloc() checks
> if (head != compound_head(page))
> 	goto retry

Sure. I guess it could still be racy in some weird scenarios where we
free+allocate+free in-between.

-- 
Cheers,

David