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

[Pavel Tatashin]

On Tue, Jan 19, 2021 at 1:47 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> 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?

Hi Jason,

I was thinking about a use case where userland would pin an address
without FOLL_WRITE, because the PTE for that address is not going to
be writable, but some device via DMA will write to it. Now, if we got
a zero page we have a problem... If this usecase is not valid then the
fix for movable zero page is make the zero page always come from a
non-movable zone so we do not need to isolate it during migration, and
so the memory can be offlined later.

Pasha

>
> Jason