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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:

> On Sat, 16 Sep 2006, Nick Piggin wrote:
>
> > ... but the problem is still fundamentally COW.
>
> Well, yes, we wouldn't have all these problems if we didn't have
> to respect COW.  But generally a process can, one way or another,
> make sure it won't get into those problems: Yingchao is concerned
> with the way the TestSetPageLocked unpredictably upsets correctness.
> I'd say it's a more serious error than the general problems with COW.

But correctness is no more upset here than with any other reason that
the page gets COWed.

> > 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.
>
> Most processes won't fork, and exec has freed them from sharing
> their parents pages, and their private file mappings aren't being
> used as buffers.  Maybe Yingchao will later have to worry about
> those cases, but for now it seems not.

So we should still solve it for once and for all just by turning off
COW completely.

> > 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.
>
> (madvise MADV_DONTFORK)
>
> For the longest time I couldn't understand you there at all, perhaps
> distracted by your parenthetical line: at last I think you're proposing
> we tweak mprotect to behave differently on a VM_DONTCOPY area.
>
> But differently in what way?  Allow it to ignore Copy-On-Write?

Well I think that we should have a flag that just prevents copy
on write from ever happening. Maybe that would mean it be easiest
to implement in mmap rather than as madvise, but that should be
OK.

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