Re: [PATCH v3] mm/page_alloc: fix freeing of MIGRATE_RESERVE migratetype pages

Vlastimil Babka
2014-03-21T14:16:35Z

On 03/06/2014 06:35 PM, Bartlomiej Zolnierkiewicz wrote:
> Pages allocated from MIGRATE_RESERVE migratetype pageblocks
> are not freed back to MIGRATE_RESERVE migratetype free
> lists in free_pcppages_bulk()->__free_one_page() if we got
> to free_pcppages_bulk() through drain_[zone_]pages().
> The freeing through free_hot_cold_page() is okay because
> freepage migratetype is set to pageblock migratetype before
> calling free_pcppages_bulk().

I think this is somewhat misleading and got me confused for a while.
It's not about the call path of free_pcppages_bulk(), but about the
fact that rmqueue_bulk() has been called at some point to fill up the
pcp lists, and had to resort to __rmqueue_fallback(). So, going through
free_hot_cold_page() might give you correct migratetype for the last
page freed, but the pcp lists may still contain misplaced pages from
earlier rmqueue_bulk().

> If pages of MIGRATE_RESERVE
> migratetype end up on the free lists of other migratetype
> whole Reserved pageblock may be later changed to the other
> migratetype in __rmqueue_fallback() and it will be never
> changed back to be a Reserved pageblock. Fix the issue by
> moving freepage migratetype setting from rmqueue_bulk() to
> __rmqueue[_fallback]() and preserving freepage migratetype
> as an original pageblock migratetype for MIGRATE_RESERVE
> migratetype pages.

Actually wouldn't the easiest solution to this particular problem to
check current pageblock migratetype in try_to_steal_freepages() and
disallow changing it. However I agree that preventing the misplaced page
in the first place would be even better.

> The problem was introduced in v2.6.31 by commit ed0ae21
> ("page allocator: do not call get_pageblock_migratetype()
> more than necessary").
>
> Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> Reported-by: Yong-Taek Lee <ytk.lee@samsung.com>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Hugh Dickins <hughd@google.com>
> ---
> v2:
> - updated patch description, there is no __zone_pcp_update()
> in newer kernels
> v3:
> - set freepage migratetype in __rmqueue[_fallback]()
> instead of rmqueue_bulk() (per Mel's request)
>
> mm/page_alloc.c | 27 ++++++++++++++++++---------
> 1 file changed, 18 insertions(+), 9 deletions(-)
>
> Index: b/mm/page_alloc.c
> ===================================================================
> --- a/mm/page_alloc.c 2014-03-06 18:10:21.884422983 +0100
> +++ b/mm/page_alloc.c 2014-03-06 18:10:27.016422895 +0100
> @@ -1094,7 +1094,7 @@ __rmqueue_fallback(struct zone *zone, in
> struct free_area *area;
> int current_order;
> struct page *page;
> - int migratetype, new_type, i;
> + int migratetype, new_type, mt = start_migratetype, i;

A better naming would help, "mt" and "migratetype" are the same thing
and it gets too confusing.

>
> /* Find the largest possible block of pages in the other list */
> for (current_order = MAX_ORDER-1; current_order >= order;
> @@ -1125,6 +1125,14 @@ __rmqueue_fallback(struct zone *zone, in
> expand(zone, page, order, current_order, area,
> new_type);
>
> + if (IS_ENABLED(CONFIG_CMA)) {
> + mt = get_pageblock_migratetype(page);
> + if (!is_migrate_cma(mt) &&
> + !is_migrate_isolate(mt))
> + mt = start_migratetype;
> + }
> + set_freepage_migratetype(page, mt);
> +
> trace_mm_page_alloc_extfrag(page, order, current_order,
> start_migratetype, migratetype, new_type);
>
> @@ -1147,7 +1155,9 @@ static struct page *__rmqueue(struct zon
> retry_reserve:
> page = __rmqueue_smallest(zone, order, migratetype);
>
> - if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
> + if (likely(page)) {
> + set_freepage_migratetype(page, migratetype);

Are you sure that here the checking of of CMA and ISOLATE is not needed?
Did the original rmqueue_bulk() have this checking only for the
__rmqueue_fallback() case? Why wouldn't the check already be only in
__rmqueue_fallback() then?

> + } else if (migratetype != MIGRATE_RESERVE) {
> page = __rmqueue_fallback(zone, order, migratetype);
>
> /*
> @@ -1174,7 +1184,7 @@ static int rmqueue_bulk(struct zone *zon
> unsigned long count, struct list_head *list,
> int migratetype, int cold)
> {
> - int mt = migratetype, i;
> + int i;
>
> spin_lock(&zone->lock);
> for (i = 0; i < count; ++i) {
> @@ -1195,16 +1205,15 @@ static int rmqueue_bulk(struct zone *zon
> list_add(&page->lru, list);
> else
> list_add_tail(&page->lru, list);
> + list = &page->lru;
> if (IS_ENABLED(CONFIG_CMA)) {
> - mt = get_pageblock_migratetype(page);
> + int mt = get_pageblock_migratetype(page);
> if (!is_migrate_cma(mt) && !is_migrate_isolate(mt))
> mt = migratetype;
> + if (is_migrate_cma(mt))
> + __mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
> + -(1 << order));
> }
> - set_freepage_migratetype(page, mt);
> - list = &page->lru;
> - if (is_migrate_cma(mt))
> - __mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
> - -(1 << order));
> }
> __mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
> spin_unlock(&zone->lock);
>
> --
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