Re: [patch 2.6.13-rc4] fix get_user_pages bug

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

> > Any chance you can change the __follow_page test to account for
> > writeable clean ptes? Something like
> >
> >       if (write && !pte_dirty(pte) && !pte_write(pte))
> >               goto out;
> >
> > And then you would re-add the set_page_dirty logic further on.
>
> Hmm.. That should be possible. I wanted to do the simplest possible code
> sequence, but yeah, I guess there's nothing wrong with allowing the code
> to dirty the page.
>
> Somebody want to send me a proper patch? Also, I haven't actually heard
> from whoever actually noticed the problem in the first place (Robin?)
> whether the fix does fix it. It "obviously does", but testing is always
> good ;)

Why do we require the !pte_dirty(pte) check? I don't get it. If a writeable
clean pte is just fine then why do we check the dirty bit at all? Doesn't
pte_dirty() imply pte_write()?

With the additional !pte_write(pte) check (and if I haven't overlooked
something which is not unlikely) s390 should work fine even without the
software-dirty bit hack.

blue skies,
   Martin

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH


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