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Message-ID: <53E38E81.3030301@suse.cz>
Date: Thu, 07 Aug 2014 16:34:41 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>
CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
Johannes Weiner <hannes@...xchg.org>,
Minchan Kim <minchan@...nel.org>,
Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
Zhang Yanfei <zhangyanfei@...fujitsu.com>,
"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
Tang Chen <tangchen@...fujitsu.com>,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
Wen Congyang <wency@...fujitsu.com>,
Marek Szyprowski <m.szyprowski@...sung.com>,
Michal Nazarewicz <mina86@...a86.com>,
Laura Abbott <lauraa@...eaurora.org>,
Heesub Shin <heesub.shin@...sung.com>,
"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
Ritesh Harjani <ritesh.list@...il.com>,
t.stanislaws@...sung.com, Gioh Kim <gioh.kim@....com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] mm/isolation: close the two race problems related
to pageblock isolation
On 08/06/2014 09:18 AM, Joonsoo Kim wrote:
> We got migratetype of the freeing page without holding the zone lock so
> it could be racy. There are two cases of this race.
>
> 1. pages are added to isolate buddy list after restoring original
> migratetype.
> 2. pages are added to normal buddy list while pageblock is isolated.
>
> If case 1 happens, we can't allocate freepages on isolate buddy list
> until next pageblock isolation occurs.
> In case of 2, pages could be merged with pages on isolate buddy list and
> located on normal buddy list. This makes freepage counting incorrect
> and break the property of pageblock isolation.
>
> One solution to this problem is checking pageblock migratetype with
> holding zone lock in __free_one_page() and I posted it before, but,
> it didn't get welcome since it needs the hook in zone lock critical
> section on freepath.
>
> This is another solution to this problem and impose most overhead on
> pageblock isolation logic. Following is how this solution works.
>
> 1. Extends irq disabled period on freepath to call
> get_pfnblock_migratetype() with irq disabled. With this, we can be
> sure that future freed pages will see modified pageblock migratetype
> after certain synchronization point so we don't need to hold the zone
> lock to get correct pageblock migratetype. Although it extends irq
> disabled period on freepath, I guess it is marginal and better than
> adding the hook in zone lock critical section.
>
> 2. #1 requires IPI for synchronization and we can't hold the zone lock
It would be better to explain here that the synchronization point is
pcplists draining.
> during processing IPI. In this time, some pages could be moved from buddy
> list to pcp list on page allocation path and later it could be moved again
> from pcp list to buddy list. In this time, this page would be on isolate
It is difficult to understand the problem just by reading this. I guess
the timelines you included while explaining the problem to me, would
help here :)
> pageblock, so, the hook is required on free_pcppages_bulk() to prevent
More clearly, a recheck for pageblock's migratetype would be needed in
free_pcppages_bulk(), which would again impose overhead outside isolation.
> misplacement. To remove this possibility, disabling and draining pcp
> list is needed during isolation. It guaratees that there is no page on pcp
> list on all cpus while isolation, so misplacement problem can't happen.
>
> Note that this doesn't fix freepage counting problem. To fix it,
> we need more logic. Following patches will do it.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
> mm/internal.h | 2 ++
> mm/page_alloc.c | 27 ++++++++++++++++++++-------
> mm/page_isolation.c | 45 +++++++++++++++++++++++++++++++++------------
> 3 files changed, 55 insertions(+), 19 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index a1b651b..81b8884 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -108,6 +108,8 @@ extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
> /*
> * in mm/page_alloc.c
> */
> +extern void zone_pcp_disable(struct zone *zone);
> +extern void zone_pcp_enable(struct zone *zone);
> extern void __free_pages_bootmem(struct page *page, unsigned int order);
> extern void prep_compound_page(struct page *page, unsigned long order);
> #ifdef CONFIG_MEMORY_FAILURE
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3e1e344..4517b1d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -726,11 +726,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> /* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
> __free_one_page(page, page_to_pfn(page), zone, 0, mt);
> trace_mm_page_pcpu_drain(page, 0, mt);
> - if (likely(!is_migrate_isolate_page(page))) {
> - __mod_zone_page_state(zone, NR_FREE_PAGES, 1);
> - if (is_migrate_cma(mt))
> - __mod_zone_page_state(zone, NR_FREE_CMA_PAGES, 1);
> - }
> + __mod_zone_freepage_state(zone, 1, mt);
Could be worth mentioning that this can now be removed as it was an
incomplete attempt to fix freepage counting, but didn't address the
misplacement.
> } while (--to_free && --batch_free && !list_empty(list));
> }
> spin_unlock(&zone->lock);
> @@ -789,8 +785,8 @@ static void __free_pages_ok(struct page *page, unsigned int order)
> if (!free_pages_prepare(page, order))
> return;
>
> - migratetype = get_pfnblock_migratetype(page, pfn);
> local_irq_save(flags);
> + migratetype = get_pfnblock_migratetype(page, pfn);
> __count_vm_events(PGFREE, 1 << order);
> set_freepage_migratetype(page, migratetype);
> free_one_page(page_zone(page), page, pfn, order, migratetype);
> @@ -1410,9 +1406,9 @@ void free_hot_cold_page(struct page *page, bool cold)
> if (!free_pages_prepare(page, 0))
> return;
>
> + local_irq_save(flags);
> migratetype = get_pfnblock_migratetype(page, pfn);
> set_freepage_migratetype(page, migratetype);
> - local_irq_save(flags);
> __count_vm_event(PGFREE);
Maybe add comments to these two to make it clear that this cannot be
moved outside of the irq disabled part, in case anyone considers it
(again) in the future?
> @@ -55,20 +56,32 @@ int set_migratetype_isolate(struct page *page, bool skip_hwpoisoned_pages)
> */
>
> out:
> - if (!ret) {
> - unsigned long nr_pages;
> - int migratetype = get_pageblock_migratetype(page);
> + if (ret) {
> + spin_unlock_irqrestore(&zone->lock, flags);
> + return ret;
> + }
> on pcplists
> - set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> - nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE);
> + migratetype = get_pageblock_migratetype(page);
> + set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> + spin_unlock_irqrestore(&zone->lock, flags);
>
> - __mod_zone_freepage_state(zone, -nr_pages, migratetype);
> - }
> + zone_pcp_disable(zone);
> +
> + /*
> + * After this point, freed pages will see MIGRATE_ISOLATE as
> + * their pageblock migratetype on all cpus. And pcp list has
> + * no free page.
> + */
> + on_each_cpu(drain_local_pages, NULL, 1);
Is there any difference between drain_all_pages() and this, or why
didn't you use drain_all_pages()?
--
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