Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[Kirill A. Shutemov]

On Fri, Dec 17, 2021 at 12:45:45PM -0800, Linus Torvalds wrote:
> On Fri, Dec 17, 2021 at 12:42 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > > Then somebody else modified that page, and you got exactly what you
> > > asked for - a COW event. The original R/O pin has the original page
> > > that it asked for, and can read it just fine.
> >
> > Where in the code did I ask for a COW event? I asked for a R/O pin, not
> > any kind of memory protection.
> 
> Why didn't you ask for a shared pin, if that is what you want?
> 
> We already support that.
> 
> If you don't like the read-only pins, don't use them. It's that simple.

So you are saying that if a GUP user wants to see changes made by
userspace to the page after the GUP it must ask for FOLL_WRITE, even if it
doesn't have intend to write to the page?

That's news to me.

Or did I misunderstand you?

-- 
 Kirill A. Shutemov