Re: [PATCH splitout] mm: memory-failure: serialize TestSetPageHWPoison with zone->lock

[David Hildenbrand (Arm)]

On 6/9/26 20:10, Andrew Morton wrote:
> On Tue, 9 Jun 2026 06:12:49 -0400 "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> 
>> TestSetPageHWPoison() is called without zone->lock, so its atomic
>> update to page->flags can race with non-atomic flag operations
>> that run under zone->lock in the buddy allocator.
>>
>> In particular, __free_pages_prepare() does:
>>
>>     page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
>>
>> This non-atomic read-modify-write, while correctly excluding
>> __PG_HWPOISON from the mask, can still lose a concurrent
>> TestSetPageHWPoison if the read happens before the poison bit
>> is set and the write happens after.  Will only get worse if/when
>> we add more non-atomic flag operations.
>>
>> Fix by acquiring zone->lock around TestSetPageHWPoison and
>> around ClearPageHWPoison in the retry path.  This
>> serializes with all buddy flag manipulation.  The cost is
>> negligible: one lock/unlock in an extremely rare path
>> (hardware memory errors).
>>
>> Note: SetPageHWPoison and TestClearPageHWPoison calls elsewhere
>> in this file operate on pages already removed from the buddy
>> allocator or on non-buddy pages (DAX, hugetlb), so they do not
>> need zone->lock protection.
> 
> Sashiko is saying this doesn't do anything "Because
> __free_pages_prepare() executes entirely locklessly".  Did it goof?
> 
> https://sashiko.dev/#/patchset/df06b66fe4ff8e925ee0714955abc2183a727b90.1780998980.git.mst@xxxxxxxxxx

Battle of the bots: it's right.

-- 
Cheers,

David