Re: [RFC 3/3] memory-hotplug: bug fix race between isolation and allocation

[Bob Liu]

On Wed, Jul 18, 2012 at 10:41 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Wed, Jul 18, 2012 at 10:12:57AM +0800, Bob Liu wrote:
>> On Wed, Jul 18, 2012 at 7:40 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> > Hi Bob,
>> >
>> > On Tue, Jul 17, 2012 at 06:13:17PM +0800, Bob Liu wrote:
>> >> Hi Minchan,
>> >>
>> >> On Tue, Jul 17, 2012 at 3:01 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> >> > Like below, memory-hotplug makes race between page-isolation
>> >> > and page-allocation so it can hit BUG_ON in __offline_isolated_pages.
>> >> >
>> >> >         CPU A                                   CPU B
>> >> >
>> >> > start_isolate_page_range
>> >> > set_migratetype_isolate
>> >> > spin_lock_irqsave(zone->lock)
>> >> >
>> >> >                                 free_hot_cold_page(Page A)
>> >> >                                 /* without zone->lock */
>> >> >                                 migratetype = get_pageblock_migratetype(Page A);
>> >> >                                 /*
>> >> >                                  * Page could be moved into MIGRATE_MOVABLE
>> >> >                                  * of per_cpu_pages
>> >> >                                  */
>> >> >                                 list_add_tail(&page->lru, &pcp->lists[migratetype]);
>> >> >
>> >> > set_pageblock_isolate
>> >> > move_freepages_block
>> >> > drain_all_pages
>> >> >
>> >> >                                 /* Page A could be in MIGRATE_MOVABLE of free_list. */
>> >> >
>> >> > check_pages_isolated
>> >> > __test_page_isolated_in_pageblock
>> >> > /*
>> >> >  * We can't catch freed page which
>> >> >  * is free_list[MIGRATE_MOVABLE]
>> >> >  */
>> >> > if (PageBuddy(page A))
>> >> >         pfn += 1 << page_order(page A);
>> >> >
>> >> >                                 /* So, Page A could be allocated */
>> >> >
>> >> > __offline_isolated_pages
>> >> > /*
>> >> >  * BUG_ON hit or offline page
>> >> >  * which is used by someone
>> >> >  */
>> >> > BUG_ON(!PageBuddy(page A));
>> >> >
>> >> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
>> >> > ---
>> >> > I found this problem during code review so please confirm it.
>> >> > Kame?
>> >> >
>> >> >  mm/page_isolation.c |    5 ++++-
>> >> >  1 file changed, 4 insertions(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> >> > index acf65a7..4699d1f 100644
>> >> > --- a/mm/page_isolation.c
>> >> > +++ b/mm/page_isolation.c
>> >> > @@ -196,8 +196,11 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn)
>> >> >                         continue;
>> >> >                 }
>> >> >                 page = pfn_to_page(pfn);
>> >> > -               if (PageBuddy(page))
>> >> > +               if (PageBuddy(page)) {
>> >> >                         pfn += 1 << page_order(page);
>> >> > +                       if (get_page_migratetype(page) != MIGRATE_ISOLATE)
>> >> > +                               break;
>> >> > +               }
>> >>
>> >> test_page_isolated() already have check
>> >> get_pageblock_migratetype(page) != MIGRATE_ISOLATE.
>> >>
>> >
>> > That's why I send a patch.
>> > As I describe in description, pageblock migration type of get_page_migratetype(page)
>> > is inconsistent with free_list[migrationtype].
>> > I mean get_pageblock_migratetype(page) will return MIGRATE_ISOLATE but the page would be
>> > in free_list[MIGRATE_MOVABLE] so it could be allocated for someone if that race happens.
>> >
>>
>> Sorry, I'm still not get the situation how this race happens.
>>
>> set_pageblock_isolate
>> move_freepages_block
>> drain_all_pages
>>
>>                                 /* Page A could be in MIGRATE_MOVABLE
>> of free_list. */
>>
>> I think move_freepages_block() will call list_move() to move Page A to
>> free_list[MIGRATE_ISOLATE], so this case can't happen?
>
> move_freepages_block handles only pages in free_list but Page A is on pcp, not free_list.
>

Got it, then why not just drain pcp pages before move_freepages_block() ?

And I didn't see the effect by adding the check if
(get_page_migratetype(page) != MIGRATE_ISOLATE) for this race.
Since set_pageblock_isolate() have been called by CPU A, this check
will always false which cause CPU A still consider Page A isolated,
then PAGE A still can be allocated by CPU B from pcp.

-- 
Regards,
--Bob
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