Re: [BUGFIX][PATCH 1/3] memory hotplug: fix next block calculation in is_removable

[Hiroyuki Kamezawa]

2010/9/6 Mel Gorman <mel@xxxxxxxxx>:
> On Mon, Sep 06, 2010 at 02:42:28PM +0900, KAMEZAWA Hiroyuki wrote:
>>
>> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>>
>> next_active_pageblock() is for finding next _used_ freeblock. It skips
>> several blocks when it finds there are a chunk of free pages lager than
>> pageblock. But it has 2 bugs.
>>
>>   1. We have no lock. page_order(page) - pageblock_order can be minus.
>>   2. pageblocks_stride += is wrong. it should skip page_order(p) of pages.
>>
>> CC: stable@xxxxxxxxxx
>> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>> ---
>>  mm/memory_hotplug.c |   16 ++++++++--------
>>  1 file changed, 8 insertions(+), 8 deletions(-)
>>
>> Index: kametest/mm/memory_hotplug.c
>> ===================================================================
>> --- kametest.orig/mm/memory_hotplug.c
>> +++ kametest/mm/memory_hotplug.c
>> @@ -584,19 +584,19 @@ static inline int pageblock_free(struct
>>  /* Return the start of the next active pageblock after a given page */
>>  static struct page *next_active_pageblock(struct page *page)
>>  {
>> -     int pageblocks_stride;
>> -
>>       /* Ensure the starting page is pageblock-aligned */
>>       BUG_ON(page_to_pfn(page) & (pageblock_nr_pages - 1));
>>
>> -     /* Move forward by at least 1 * pageblock_nr_pages */
>> -     pageblocks_stride = 1;
>> -
>>       /* If the entire pageblock is free, move to the end of free page */
>> -     if (pageblock_free(page))
>> -             pageblocks_stride += page_order(page) - pageblock_order;
>> +     if (pageblock_free(page)) {
>> +             int order;
>> +             /* be careful. we don't have locks, page_order can be changed.*/
>> +             order = page_order(page);
>> +             if (order > pageblock_order)
>> +                     return page + (1 << order);
>> +     }
>
> As you note in your changelog, page_order() is unsafe because we do not have
> the zone lock but you don't check if order is somewhere between pageblock_order
> and MAX_ORDER_NR_PAGES. How is this safer?
>
Ah, I missed that.

if ((pageblock_order <= order) && (order < MAX_ORDER))
          return page + (1 << order);
ok ?

Thanks,
-Kame
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