Re: [patch 2.6.13-rc4] fix get_user_pages bug

[Hugh Dickins]

On Tue, 2 Aug 2005, Linus Torvalds wrote:
> On Tue, 2 Aug 2005, Hugh Dickins wrote:
> > 
> > It might not be so bad.  It's going to access the struct page anyway.
> > And clearing dirty from parent and child at fork time could save two
> > set_page_dirtys at exit time.  But I'm not sure that we could batch the
> > the dirty bit clearing into one TLB flush like we do the write protection.
> 
> Yes, good point. If the thing is still marked dirty in the TLB, some other 
> thread might be writing to the page after we've cleared dirty but before 
> we've flushed the TLB - causing the new dirty bit to be lost. I think.

Would that matter?  Yes, if vmscan sneaked in at some point while
page_table_lock is dropped, and wrote away the page with the earlier data.

But I was worrying about the reverse case, that we clear dirty, then
another thread sets it again before we emerge from copy_page_range,
so it gets left behind granting get_user_pages write permission.

Hmm, that implies that the other thread doesn't yet see wrprotect
(because we've not yet flushed TLB), which probably implies it would
still see dirty set: and so not set it again, so not a possible case.
But that's a precarious, processor-dependent argument: I don't think
it's safe to rely on, and your reverse case is already a problem.

I don't believe there's a safe efficient way we could batch clearing
dirty there.  We could make a second pass of the whole mm after the
flush TLB has asserted the wrprotects; but that won't win friends.

I'm thinking of reverting to the old __follow_page, setting write_access
-1 in the get_user_pages special case (to avoid change to all the arches,
in some of which write_access is a boolean, in some a bitflag, but in
none -1), and in that write_access -1 case passing back the special
code to say do_wp_page has done its full job.  Combining your and
Nick's and Andrew's ideas, and letting Martin off the hook.
Or would that disgust you too much?  (We could give -1 a pretty name ;)

Working on it now, but my brain in an even lower power state than ever.

Hugh
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