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

[Weijie Yang]

On Thu, Nov 13, 2014 at 3:34 AM, Michal Hocko <mhocko@xxxxxxx> 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.
That's really obfuscated : (

> Please do not make this code more obfuscated than it is already...
>
>> Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
>> ---
>>  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
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