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

[Minchan Kim <minchan@kernel.org>]

On Wed, Jul 18, 2012 at 11:12:27AM +0800, Bob Liu wrote:
> On Wed, Jul 18, 2012 at 10:41 AM, Minchan Kim <minchan@kernel.org> 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@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?
> >
> > 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() ?

        CPU A                   CPU B

drain_all_pages
lock(zone->lock);
                                free_hot_cold_page
                                MIGRATE_MOVABLE = get_pageblock_migratetype(page);
                                list_add(&page->lru, &pcp->lists[migratetype])
set_pageblock_isolate
move_free_pages_block
unlock(zone->lock);

We can't make it atomic.

> 
> 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.

Please don't confuse get_page_migratetype and get_pageblock_migratetype.
get_page_migratetype returns migratetype inforamtion of *page* which is
in free_list while get_pageblock_migratetype returns *pageblock*'s migratetype.

> 
> -- 
> Regards,
> --Bob
> 
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-- 
Kind regards,
Minchan Kim

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