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

[Oleg Nesterov]

On 09/22, Peter Xu wrote:
>
> On Tue, Sep 22, 2020 at 06:53:55PM +0200, Oleg Nesterov wrote:
> > On 09/22, Peter Xu wrote:
> > >
> > > On Tue, Sep 22, 2020 at 05:48:46PM +0200, Oleg Nesterov wrote:
> > > > > 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.
> > > >
> > > > Well, if copy_one_pte(src_pte) hits a swap entry and returns entry.val != 0, then
> > > > pte_none(*src_pte) is not possible after restart? This means that copy_one_pte()
> > > > will be called at least once.
> > >
> > > Note that we've released the page table locks, so afaict the old swp entry can
> > > be gone under us when we go back to the "do" loop... :)
> >
> > But how?
> >
> > I am just curious, I don't understand this code enough.
>
> Me neither.
>
> The point is I think we can't assume *src_pte will read the same if we have
> released the src_ptl in copy_pte_range(),

This is clear.

But I still think that !pte_none() -> pte_none() transition is not possible
under mmap_write_lock()...

OK, let me repeat I don't understans these code paths enough, let me reword:
I don't see how this transition is possible.

Oleg.