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SubjectRe: [PATCH 2/4] mm: prevent double decrease of nr_reserved_highatomic
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On 10/07/2016 07:45 AM, Minchan Kim wrote:
> There is race between page freeing and unreserved highatomic.
>
> CPU 0 CPU 1
>
> free_hot_cold_page
> mt = get_pfnblock_migratetype

so here mt == MIGRATE_HIGHATOMIC?

> set_pcppage_migratetype(page, mt)
> unreserve_highatomic_pageblock
> spin_lock_irqsave(&zone->lock)
> move_freepages_block
> set_pageblock_migratetype(page)
> spin_unlock_irqrestore(&zone->lock)
> free_pcppages_bulk
> __free_one_page(mt) <- mt is stale
>
> By above race, a page on CPU 0 could go non-highorderatomic free list
> since the pageblock's type is changed.
> By that, unreserve logic of
> highorderatomic can decrease reserved count on a same pageblock
> several times and then it will make mismatch between
> nr_reserved_highatomic and the number of reserved pageblock.

Hmm I see.

> So, this patch verifies whether the pageblock is highatomic or not
> and decrease the count only if the pageblock is highatomic.

Yeah I guess that's the easiest solution.

> Signed-off-by: Minchan Kim <minchan@kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/page_alloc.c | 24 ++++++++++++++++++------
> 1 file changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e7cbb3cc22fa..d110cd640264 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2133,13 +2133,25 @@ static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
> continue;
>
> /*
> - * It should never happen but changes to locking could
> - * inadvertently allow a per-cpu drain to add pages
> - * to MIGRATE_HIGHATOMIC while unreserving so be safe
> - * and watch for underflows.
> + * In page freeing path, migratetype change is racy so
> + * we can counter several free pages in a pageblock
> + * in this loop althoug we changed the pageblock type
> + * from highatomic to ac->migratetype. So we should
> + * adjust the count once.
> */
> - zone->nr_reserved_highatomic -= min(pageblock_nr_pages,
> - zone->nr_reserved_highatomic);
> + if (get_pageblock_migratetype(page) ==
> + MIGRATE_HIGHATOMIC) {
> + /*
> + * It should never happen but changes to
> + * locking could inadvertently allow a per-cpu
> + * drain to add pages to MIGRATE_HIGHATOMIC
> + * while unreserving so be safe and watch for
> + * underflows.
> + */
> + zone->nr_reserved_highatomic -= min(
> + pageblock_nr_pages,
> + zone->nr_reserved_highatomic);
> + }
>
> /*
> * Convert to ac->migratetype and avoid the normal
>

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