Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault()

[zhangpeng (AS)]

On 2023/11/24 14:05, Matthew Wilcox wrote:

> On Wed, Nov 22, 2023 at 10:00:52PM +0800, Peng Zhang wrote:
> > From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> >
> > The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE)
> > in application, which leading to an unexpected performance issue[1].
> >
> > This caused by temporarily cleared pte during a read/modify/write update
> > of the pte, eg, do_numa_page()/change_pte_range().
> What I haven't quite understood yet is why we need to set the pte to
> zero on x86 in the specific case of do_numa_page().  I understand that
> ppc needs to.

I'm also curious. Could ptep_modify_prot_start() of other architectures
(except ppc) not clear pte? We are mainly concerned with arm64 and x86.

> Could someone explain why the _default_ definition of
> ptep_modify_prot_start() is not:
>
> +++ b/include/linux/pgtable.h
> @@ -1074,7 +1074,7 @@ static inline pte_t ptep_modify_prot_start(struct vm_area_struct *vma,
>                                             unsigned long addr,
>                                             pte_t *ptep)
>   {
> -       return __ptep_modify_prot_start(vma, addr, ptep);
> +       return *ptep;
>   }
>
>   /*
--
Best Regards,
Peng