Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

[Linus Torvalds]

On Fri, Jan 8, 2021 at 10:31 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> Can we just remove vmsplice() support?  We could make it do a normal
> copy, thereby getting rid of a fair amount of nastiness and potential
> attacks.  Even ignoring issues relating to the length of time that the
> vmsplice reference is alive, we also have whatever problems could be
> caused by a malicious or misguided user vmsplice()ing some memory and
> then modifying it.

Well, that "misguided user" is kind of the point, originally. That's
what zero-copying is all about.

But we could certainly remove it in favor of copying, because
zero-copy has seldom really been a huge advantage in practice outside
of benchmarks.

That said, I continue to not buy into Andrea's argument that
page_count() is wrong.

Instead, the argument is:

 (1) COW can never happen "too much": the definition of a private
mapping is that you have your own copy of the data.

 (2) the one counter case I feel is valid is page pinning when used
for a special "pseudo-shared memory" thing and that's basically what
FOLL_GUP does.

So _regardless_ of any vmsplice issues, I actually think that those
two basic rules should be our guiding principle.

And the corollary to (2) is that COW must absolutely NEVER re-use too
little. And that _was_ the bug with vmsplice, in that it allowed
re-use that it shouldn't have.

           Linus