Re: [PATCH v3] mm/page_alloc: clear page->private in free_pages_prepare()

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

On 2/9/26 17:33, Zi Yan wrote:
> On 9 Feb 2026, at 11:20, David Hildenbrand (Arm) wrote:
> 
>> On 2/9/26 17:16, David Hildenbrand (Arm) wrote:
>>>
>>> Right. Or someone could use page->private on tail pages and free non- zero ->private that way.
>>>
>>> [...]
>>>
>>>
>>> Thanks.
>>>
>>>
>>> Right.
>>>
>>>
>>> Right. And whether it is okay to have any tail->private be non-zero.
>>>
>>>
>>> Ideally, I guess, we would minimize the clearing of the ->private fields.
>>>
>>> If we could guarantee that *any* pages in the buddy have ->private clear, maybe
>>> prep_compound_tail() could stop clearing it (and check instead).
>>>
>>> So similar to what Vlasta said, maybe we want to (not check but actually clear):
>>>
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index e4104973e22f..4960a36145fe 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -1410,6 +1410,7 @@ __always_inline bool free_pages_prepare(struct page *page,
>>>                                   }
>>>                           }
>>>                           (page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
>>> +                       set_page_private(page + i, 0);
>>>                   }
>>>           }
>>
>> Thinking again, maybe it is indeed better to rework the code to not allow freeing pages with ->private on any page. Then, we only have to zero it out where we actually used it and could check here that all
>> ->private is 0.
>>
>> I guess that's a bit more work, and any temporary fix would likely just do.
> 
> I agree. Silently fixing non zero ->private just moves the work/responsibility
> from users to core mm. They could do better. :)
> 
> We can have a patch or multiple patches to fix users do not zero ->private
> when freeing a page and add the patch below.

Do we know roughly which ones don't zero it out?

> The hassle would be that
> catching all, especially non mm users might not be easy, but we could merge
> the patch below (and obviously fixes) after next merge window is closed and
> let rc tests tell us the remaining one. WDYT?

LGTM, then we can look into stopping to zero for compound pages.

> 
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 24ac34199f95..0c5d117a251e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1411,6 +1411,7 @@ __always_inline bool free_pages_prepare(struct page *page,
>   				}
>   			}
>   			(page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +			VM_WARN_ON_ONCE((page + i)->private);
>   		}
>   	}
>   	if (folio_test_anon(folio)) {
> @@ -1430,6 +1431,7 @@ __always_inline bool free_pages_prepare(struct page *page,
> 
>   	page_cpupid_reset_last(page);
>   	page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +	VM_WARN_ON_ONCE(page->private);
>   	page->private = 0;
>   	reset_page_owner(page, order);
>   	page_table_check_free(page, order);
> 
> 
> Best Regards,
> Yan, Zi


-- 
Cheers,

David