RE: [RFC] Demand faulting for large pages

[Chen, Kenneth W]

Adam Litke wrote on Friday, August 05, 2005 8:22 AM
> Below is a patch to implement demand faulting for huge pages.  The main
> motivation for changing from prefaulting to demand faulting is so that
> huge page allocations can follow the NUMA API.  Currently, huge pages
> are allocated round-robin from all NUMA nodes.   

Chen, Kenneth W wrote on Friday, August 05, 2005 2:34 PM
> Spurious WARN_ON.  Calls to hugetlb_pte_fault() is conditioned upon 
> if (is_vm_hugetlb_page(vma))
> 
> ....
> 
> Broken here.  Return VM_FAULT_SIGBUS when *pte is present??  Why
> can't you move all the logic into hugetlb_pte_fault and simply call
> it directly from handle_mm_fault?


I'm wondering has this patch ever been tested?  More broken bits:
in arch/i386/mm/hugetlbpage.c:huge_pte_offset - with demand paging,
you can't unconditionally walk the page table without checking
existence of pud and pmd.

I haven't looked closely at recent change in free_pgtables(), but
we used to have a need to scrub old pmd mapping before allocate one
for hugetlb pte on x86.  You have to do that in huge_pte_alloc(),
I'm specifically concerned with arch/i386/mm/hugetlbpage.c:huge_pte_alloc()

- Ken

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