On Thu, Oct 23 2014, Joonsoo Kim wrote:
> In free_pcppages_bulk(), we use cached migratetype of freepage
> to determine type of buddy list where freepage will be added.
> This information is stored when freepage is added to pcp list, so
> if isolation of pageblock of this freepage begins after storing,
> this cached information could be stale. In other words, it has
> original migratetype rather than MIGRATE_ISOLATE.
>
> There are two problems caused by this stale information. One is that
> we can't keep these freepages from being allocated. Although this
> pageblock is isolated, freepage will be added to normal buddy list
> so that it could be allocated without any restriction. And the other
> problem is incorrect freepage accounting. Freepages on isolate pageblock
> should not be counted for number of freepage.
>
> Following is the code snippet in free_pcppages_bulk().
>
> /* 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);
> }
>
> As you can see above snippet, current code already handle second problem,
> incorrect freepage accounting, by re-fetching pageblock migratetype
> through is_migrate_isolate_page(page). But, because this re-fetched
> information isn't used for __free_one_page(), first problem would not be
> solved. This patch try to solve this situation to re-fetch pageblock
> migratetype before __free_one_page() and to use it for __free_one_page().
>
> In addition to move up position of this re-fetch, this patch use
> optimization technique, re-fetching migratetype only if there is
> isolate pageblock. Pageblock isolation is rare event, so we can
> avoid re-fetching in common case with this optimization.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
> ---
> mm/page_alloc.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
ooo +--<mpn@xxxxxxxxxx>--<xmpp:mina86@xxxxxxxxxx>--ooO--(_)--Ooo--
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