Re: [PATCH v1 00/11] Zero page->private when freeing pages

[Christian König]

On 2/23/26 15:14, David Hildenbrand (Arm) wrote:
> On 2/23/26 15:06, Christian König wrote:
>> On 2/23/26 14:46, Christoph Hellwig wrote:
>>> On Sun, Feb 22, 2026 at 10:26:30PM -0500, Zi Yan wrote:
>>>> Hi all,
>>>>
>>>> Based on a recent discussion with David Hildenbrand on page->private
>>>> is not zero when a page is freed[1], this patchset is trying to fix all
>>>> users do not zero ->private when freeing a page and add checks to make
>>>> sure all freed pages have ->private set to zero. For compound pages,
>>>> both head page and tail pages need to have ->private set to zero.
>>>
>>> Requiring the user to clear a field before freeing is just a way to
>>> awkward interface.  Don't do that.
>>
>> Completely agree. This is just asking for trouble.
>>
>> The cache line(s) backing this struct page are most likely accessed anyway on free/alloc. So I don't see much extra overhead.
> 
> I think the question is more around handling non-head pages when freeing larger orders. But maybe the overhead of zeroing page->private it there as well in __free_pages_prepare() is tolerable.

Good point, sounds like that is a bit more than I thought it would be.

> I'll note, though, that we already require page->mapping and page->memcg_data of pages to be zeroed by the caller, so it's not completely crazy. (see page_expected_state)

Well that's not defensive at all, basically everybody which forgets to do that can cause hard to debug trouble. Maybe that practice should be reconsidered.

Regards,
Christian.