Re: [PATCH v5 08/14] mm/gup: do not allow zero page for pinned pages

[Jason Gunthorpe]

On Tue, Jan 19, 2021 at 01:34:26PM -0500, Pavel Tatashin wrote:
> On Tue, Jan 19, 2021 at 1:30 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Mon, Jan 18, 2021 at 11:39:14PM -0500, Pavel Tatashin wrote:
> > > Zero page should not be used for long term pinned pages. Once pages
> > > are pinned their physical addresses cannot changed until they are unpinned.
> > >
> > > Guarantee to always return real pages when they are pinned by adding
> > > FOLL_WRITE.
> > >
> > > Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> > >  mm/gup.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > No, this will definitely break things
> 
> What will break

Things assuming GUP doesn't break COW, making all GUP WRITE was
already tried and revered for some other reason

> > Why does the zero page have to be movable?
> 
> It is not even about being movable, we can't cow pinned pages returned
> by GUP call, how can we use zero page for that?

The zero page is always zero, it is never written to. What does cow
matter?

Jason