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

[Peter Xu]

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(), because imho the src_ptl is the only
thing to protect it.  Or to be more explicit, we need pte_alloc_map_lock() to
read a stable pmd/pte or before update (since src_ptl itself could change too).

Thanks,

-- 
Peter Xu