Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()

Peter Xu
2020-09-22T15:36:21Z

On Tue, Sep 22, 2020 at 12:18:16PM +0200, Oleg Nesterov wrote:
> On 09/22, Oleg Nesterov wrote:
> >
> > On 09/21, Peter Xu wrote:
> > >
> > > @@ -866,13 +877,18 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > pte_unmap_unlock(orig_dst_pte, dst_ptl);
> > > cond_resched();
> > >
> > > - if (entry.val) {
> > > - if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
> > > + switch (copy_ret) {
> > > + case COPY_MM_SWAP_CONT:
> > > + if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0)
> > > return -ENOMEM;
> > > - progress = 0;
> > > + break;
> >
> > Note that you didn't clear copy_ret, it is still COPY_MM_SWAP_CONT,
> >
> > > + default:
> > > + break;
> > > }
> > > +
> > > if (addr != end)
> > > goto again;
> >
> > After that the main loop can stop again because of need_resched(), and
> > in this case add_swap_count_continuation(data.entry) will be called again?
>
> No, this is not possible, copy_one_pte() should be called at least once,
> progress = 0 before restart. Sorry for noise.

Oh wait, I think you're right... when we get a COPY_MM_SWAP_CONT, goto "again",
then if there're 32 pte_none() ptes _plus_ an need_resched(), then we might
reach again at the same add_swap_count_continuation() with the same swp entry.

However since I didn't change this logic in this patch, it probably means this
bug is also in the original code before this series... I'm thinking maybe I
should prepare a standalone patch to clear the swp_entry_t and cc stable.

Thanks,

--
Peter Xu