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

[Bob Liu <lliubbo@gmail.com>]

On Wed, Jul 18, 2012 at 7:40 AM, Minchan Kim <minchan@kernel.org> 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@kernel.org> 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@kernel.org>
>> > ---
>> > 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?

-- 
Thanks,
--Bob

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