Re: [PATCH v1 2/4] mm: Expose abnormal new_pte during move_ptes

[Oleg Nesterov <oleg@redhat.com>]

On 05/30, Lorenzo Stoakes wrote:
>
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -237,6 +237,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >
> >  	for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
> >  				   new_pte++, new_addr += PAGE_SIZE) {
> > +		WARN_ON_ONCE(!pte_none(*new_pte));
> > +
>
> I mean, we really really should not ever be seeing a mapped PTE here, so I think
> a WARN_ON_ONCE() is fine.
>
> We unmap anything ahead of time, and only I think this uprobe breakpoint
> installation would ever cause this to be the case.
>
> We can make this a VM_WARN_ON_ONCE() too I suppose, just in case there's
> something we're not thinking of, but I'd say at some point we'd want to change
> it to a WARN_ON_ONCE().

Note also that move_normal_pmd/move_normal_pud use WARN_ON_ONCE(!xxx_none(...)),
not VM_WARN_ON_ONCE().

Oleg.