On 6/2/26 05:08, Miaohe Lin wrote:
> On 2026/6/1 21:22, David Hildenbrand (Arm) wrote:
>> On 6/1/26 14:28, Miaohe Lin wrote:
>>>
>>> Thanks for your patch.
>>>
>>>
>>> Once shake_page finds a lightweight range-based way to shrink slab, slab
>>> pages could be freed
>>> into buddy and above PageSlab test should be removed then. Maybe add a TODO
>>> or XXX here?
>>>
>>>
>>> I'm not sure but is it safe or a common way to test PageReserved, PageSlab,
>>> PageTable and PageLargeKmalloc without extra page refcnt?
>>
>> Checking typed pages in a racy fashion is fine (PageSlab, PageTable,
>> PageLargeKmalloc).
>
> Got it. Thanks.
>
>> Checking PageReserved in a racy fashion is fine as well. TESTPAGEFLAG() will
>> allow checking it on compound pages.
>
> It seems PageReserved is not intended to be set on compound pages. I see
> there are PF_NO_COMPOUND
> in its definition: PAGEFLAG(Reserved, reserved, PF_NO_COMPOUND).
>
>>
>> For PageLargeKmalloc, we would want to check the head page, though. The page
>> type is only stored for the head page.
>
> Maybe we should check the head page for PageSlab and PageTable too?
> alloc_slab_page only
> set PageSlab on the head page and __pagetable_ctor uses __folio_set_pgtable
> to set PageTable
> on folio.
>
>>
>> So maybe we want to lookup the compound head (if any) and perform the type
>> checks against that?
>
> Maybe we should or we might miss some pages that could have been handled. And
> if compound head is required, should we hold an extra page refcnt to guard
> against
> possible folio split race?
Races are fine. We might miss some pages, but that can happen on races either
way.
I'd just do something like
if (PageReserved(page))
return true;
head = compound_head(page);
return PageSlab(head) || ...;
--
Cheers,
David
