Re: [PATCH] mm/swapops: make is_pmd_migration_entry more strict

[Hongchen Zhang]

On 2022/4/29 上午2:41, Peter Xu wrote:
> On Thu, Apr 28, 2022 at 03:35:33PM +0800, Hongchen Zhang wrote:
> > a pmd migration entry should first be a swap pmd,so
> > use is_swap_pmd(pmd) instead of !pmd_present(pmd).
> >
> > On the other hand, some architecture (MIPS for example)
> > may misjudge a pmd_none entry as a pmd migration entry.
> >
> > Signed-off-by: Hongchen Zhang <zhanghongchen@xxxxxxxxxxx>
>
> The change looks reasonable,
>
> Acked-by: Peter Xu <peterx@xxxxxxxxxx>
>
> Two more pure questions..
>
>    (1) is there a real issue to be fixed with it?  Asked because I see most
>        calls to is_pmd_migration_entry() should already make sure it's not
>        none, and,
>
>    (2) why it matters with MIPS?  Firstly this function is not used in arch
>        specific code, and iiuc thp migration is not enabled at all for mips.
>
> Thanks,
>
> > ---
> >   include/linux/swapops.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > index d356ab4..1d16569 100644
> > --- a/include/linux/swapops.h
> > +++ b/include/linux/swapops.h
> > @@ -304,7 +304,7 @@ static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
> >   
> >   static inline int is_pmd_migration_entry(pmd_t pmd)
> >   {
> > -	return !pmd_present(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
> > +	return is_swap_pmd(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
> >   }
> >   #else
> >   static inline void set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
> > --
> > 1.8.3.1
Hello Peter,
  (1) the pmd passed to __split_huge_pmd_locked may be a none pmd,so it 
may be wrongly treated as a pmd migration entry.
  (2) we are preparing to support thp migration on MIPS.

Thanks.