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Date: Wed, 18 Jul 2012 12:56:06 +0900
From: Minchan Kim <minchan@...nel.org>
To: Bob Liu <lliubbo@...il.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC 3/3] memory-hotplug: bug fix race between isolation and
allocation
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@...nel.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@...nel.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@...nel.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@...nel.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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