Re: [BUGFIX][PATCH -tip] x86: fix text_poke to handle highmem pages

Mathieu Desnoyers
2009-04-06T17:59:41Z

* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Mathieu Desnoyers wrote:
> > * Masami Hiramatsu (mhiramat@redhat.com) wrote:
> >> Fix a bug in text_poke to handle highmem pages, because module
> >> text pages are possible to be highmem pages on x86-32.
> >> In that case, since fixmap can't handle those pages, text_poke
> >> uses kmap_atomic.
> >>
> >
> > Hrm, can you remind me what would be the downside of using kmap_atomic
> > in every scenarios (highmem and non-highmem) then ?
>
> kmap_atomic can handle only highmem pages. If you passes lowmem pages,
> it returns just original vaddr of it. (because kmap is only for
> highmem support)
>

OK, and if we are doing a second kmap_atomic() of a module text page
which is already mapped, does the second kmap_atomic return the vaddr of
the page originally mapped or is it creating a second mapping ?

Because if we ever decide to enforce read-only page mapping for module
text pages, touching highmem pages too, we will run into real trouble if
those happen to be the same page.

Mathieu

> >
> > I would try to avoid "special cases" as much as possible, because they
> > just make problems harder to reproduce.
>
> Actually, this bug is a special case because it happens only on PAE kernel.
>
> >
> > The idea would be to either add fixmap highmem support, or to simply use
> > kmap_atomic() for all cases until we add fixmap highmem support.
> >
>
>
> Thank you,
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
>
> e-mail: mhiramat@redhat.com
>

--
Mathieu Desnoyers
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