Re: [RFC] PAGE_RW Should be added to PAGE_COPY ?

[Nick Piggin]

(adding linux-mm)

Hugh Dickins wrote:

> > but for the app to use a shared mapping instead of a private.
>
>
> But that suggestion wasn't helpful: you'd much prefer not to
> restrict what areas of userspace are used in this way.
>
> The problem, as I now see it, is precisely with do_wp_page()'s
> TestSetPageLocked, as you first said.  There is indeed a small
> but real chance that will fail.  At some time in the past I did
> realize that, but pushed it to the back of my mind, waiting for
> someone actually to complain: now you have.
>
> Yes, it would be good if we could do that check in some other,
> reliable way.  The problem is that can_share_swap_page has to
> check page_mapcount (and PageSwapCache) and page_swapcount in
> an atomic way: the page lock is what we have used to guard the
> movement between mapcount and swapcount.
>
> I'll try to think whether we can do that better,
> but not until next week.

I don't think TestSetPageLocked is the problem. Indeed you may be
able to get around a few specific cases say, by turning that into
a plain lock_page()... but the problem is still fundamentally COW.

In other words, one should always be able to return 0 from that
can_share_swap_page and have the system continue to work... right?
Because even if you hadn't done that mprotect trick, you may still
have a problem because the page may *have* to be copied on write
if it is shared over fork.

So if we filled in the missing mm/ implementation of VM_DONTCOPY
(and call it MAP_DONTCOPY rather than the confusing MAP_DONTFORK)
such that it withstands such an mprotect sequence, we can then ask
that all userspace drivers do their get_user_pages memory on these
types of vmas.

Would that work?

--
SUSE Labs, Novell Inc.
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