On 2026/6/2 17:41, David Hildenbrand (Arm) wrote: > 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);
If @head is split just after compound_head. And then @head is freed into buddy and re-allocated as slab page while @page is still in the buddy. We would panic on this scene as @head is PageSlab. But we were supposed to successfully handle @page. Or am I miss something? Thanks. . > return PageSlab(head) || ...; > >
