Re: get_user_pages() with write=1 and force=1 gets read-only pages.

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

Robin Holt wrote:
> Should there be a check to ensure we don't return VM_FAULT_RACE when the
> pte which was inserted is exactly the same one we would have inserted?

That would slow down the do_xxx_fault fastpaths, though.

Considering VM_FAULT_RACE will only make any difference to get_user_pages
(ie. not the page fault fastpath), and only then in rare cases of a racing
fault on the same pte, I don't think the extra test would be worthwhile.

> Could we generalize that more to the point of only returning VM_FAULT_RACE
> when write access was requested but the racing pte was not writable?
> 

I guess get_user_pages could be changed to retry on VM_FAULT_RACE only if
it is attempting write access... is that worthwhile? I guess so...

> Most of the test cases I have thrown at this have gotten the writer
> faulting first which did not result in problems.  I would hate to slow
> things down if not necessary.  I am unaware of more issues than the one
> I have been tripping.
> 

I think the VM_FAULT_RACE patch as-is should be fairly unintrusive to the
page fault fastpaths. I think weighing down get_user_pages is preferable to
putting logic in the general fault path - though I don't think there should
be too much overhead introduced even there...

Do you think the patch (or at least, the idea) looks like a likely solution
to your problem? Obviously the !i386 architecture specific parts still need
to be filled in...

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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