Re: [PATCH RFC 04/29] mm/page_alloc: allow for making page types sticky until freed

[Zi Yan <ziy@nvidia.com>]

On 23 Jun 2025, at 11:26, David Hildenbrand wrote:

> On 18.06.25 20:08, Zi Yan wrote:
>> On 18 Jun 2025, at 14:04, Zi Yan wrote:
>>
>>> On 18 Jun 2025, at 13:39, David Hildenbrand wrote:
>>>
>>>> Let's allow for not clearing a page type before freeing a page to the
>>>> buddy.
>>>>
>>>> We'll focus on having a type set on the first page of a larger
>>>> allocation only.
>>>>
>>>> With this change, we can reliably identify typed folios even though
>>>> they might be in the process of getting freed, which will come in handy
>>>> in migration code (at least in the transition phase).
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>   mm/page_alloc.c | 3 +++
>>>>   1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 858bc17653af9..44e56d31cfeb1 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -1380,6 +1380,9 @@ __always_inline bool free_pages_prepare(struct page *page,
>>>>   			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
>>>>   		page->mapping = NULL;
>>>>   	}
>>>> +	if (unlikely(page_has_type(page)))
>>>> +		page->page_type = UINT_MAX;
>>>> +
>>>>   	if (is_check_pages_enabled()) {
>>>>   		if (free_page_is_bad(page))
>>>>   			bad++;
>>>> -- 
>>>> 2.49.0
>>>
>>> How does this preserve page type? Isn’t page->page_type = UINT_MAX clearing
>>> page_type?
>>
>> OK, next patch explains it. free_pages_prepare() clears page_type,
>> so that caller does not need to.
>>
>> I think the message is better to be
>>
>> mm/page_alloc: clear page_type at page free time
>>
>> page_type is no longer needed to be cleared before a page is freed, as
>> page free code does that.
>>
>> With this change, we can reliably identify typed folios even though
>> they might be in the process of getting freed, which will come in handy
>> in migration code (at least in the transition phase).
>
>
> I'll change it to
>
>     mm/page_alloc: let page freeing clear any set page type
>         Currently, any user of page types must clear that type before freeing
>     a page back to the buddy, otherwise we'll run into mapcount related
>     sanity checks (because the page type currently overlays the page
>     mapcount).
>         Let's allow for not clearing the page type by page type users by letting
>     the buddy handle it instead.
>         We'll focus on having a page type set on the first page of a larger
>     allocation only.
>         With this change, we can reliably identify typed folios even though
>     they might be in the process of getting freed, which will come in handy
>     in migration code (at least in the transition phase).

Thanks.

Reviewed-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi