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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