Re: [PATCH 1/2] mm: page_isolation: check pfn validity before access

[Michal Hocko <mhocko@suse.cz>]

On Thu 13-11-14 09:46:34, Weijie Yang wrote:
> On Thu, Nov 13, 2014 at 3:34 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > On Thu 06-11-14 16:08:02, Weijie Yang wrote:
> >> In the undo path of start_isolate_page_range(), we need to check
> >> the pfn validity before access its page, or it will trigger an
> >> addressing exception if there is hole in the zone.
> >
> > This looks a bit fishy to me. I am not familiar with the code much but
> > at least __offline_pages zone = page_zone(pfn_to_page(start_pfn)) so it
> > would blow up before we got here. Same applies to the other caller
> > alloc_contig_range. So either both need a fix and then
> > start_isolate_page_range doesn't need more checks or this is all
> > unnecessary.
> 
> Thanks for your suggestion.
> If start_isolate_page_range()'s user can ensure there isn't hole in
> the [start_pfn, end_pfn) range, we can remove the checks. But if we
> cann't, I think it's better reserve these "unnecessary" code.

I am not sure I understand you correctly but my point was that we do not
need check at start_isolate_page_range level but rather than in the
caller (or do not rely on pfn_to_page at that level). 

> That's really obfuscated : (
> 
> > Please do not make this code more obfuscated than it is already...
> >
> >> Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
> >> ---
> >>  mm/page_isolation.c |    7 +++++--
> >>  1 files changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> >> index d1473b2..3ddc8b3 100644
> >> --- a/mm/page_isolation.c
> >> +++ b/mm/page_isolation.c
> >> @@ -137,8 +137,11 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
> >>  undo:
> >>       for (pfn = start_pfn;
> >>            pfn < undo_pfn;
> >> -          pfn += pageblock_nr_pages)
> >> -             unset_migratetype_isolate(pfn_to_page(pfn), migratetype);
> >> +          pfn += pageblock_nr_pages) {
> >> +             page = __first_valid_page(pfn, pageblock_nr_pages);
> >> +             if (page)
> >> +                     unset_migratetype_isolate(page, migratetype);
> >> +     }
> >>
> >>       return -EBUSY;
> >>  }
> >> --
> >> 1.7.0.4
> >>
> >>
> >> --
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> >
> > --
> > Michal Hocko
> > SUSE Labs

-- 
Michal Hocko
SUSE Labs