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Date: Tue, 12 May 2015 09:51:56 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>
CC: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Mel Gorman <mgorman@...e.de>,
Johannes Weiner <hannes@...xchg.org>,
Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/3] mm/page_alloc: don't break highest order freepage
if steal
On 04/27/2015 09:23 AM, Joonsoo Kim wrote:
> When we steal whole pageblock, we don't need to break highest order
> freepage. Perhaps, there is small order freepage so we can use it.
>
> This also gives us some code size reduction because expand() which
> is used in __rmqueue_fallback() and inlined into __rmqueue_fallback()
> is removed.
>
> text data bss dec hex filename
> 37413 1440 624 39477 9a35 mm/page_alloc.o
> 37249 1440 624 39313 9991 mm/page_alloc.o
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
> mm/page_alloc.c | 40 +++++++++++++++++++++-------------------
> 1 file changed, 21 insertions(+), 19 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ed0f1c6..044f16c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1239,14 +1239,14 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
> }
>
> /* Remove an element from the buddy allocator from the fallback list */
This is no longer accurate description.
> -static inline struct page *
> +static inline bool
> __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
> {
> struct free_area *area;
> unsigned int current_order;
> struct page *page;
> int fallback_mt;
> - bool can_steal;
> + bool can_steal_pageblock;
>
> /* Find the largest possible block of pages in the other list */
> for (current_order = MAX_ORDER-1;
> @@ -1254,26 +1254,24 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
> --current_order) {
> area = &(zone->free_area[current_order]);
> fallback_mt = find_suitable_fallback(area, current_order,
> - start_migratetype, false, &can_steal);
> + start_migratetype, false,
> + &can_steal_pageblock);
> if (fallback_mt == -1)
> continue;
>
> page = list_entry(area->free_list[fallback_mt].next,
> struct page, lru);
> - if (can_steal)
> + BUG_ON(!page);
Please no new BUG_ON. VM_BUG_ON maybe for debugging, otherwise just let
it panic on null pointer exception accessing page->lru later on.
> +
> + if (can_steal_pageblock)
> steal_suitable_fallback(zone, page, start_migratetype);
>
> - /* Remove the page from the freelists */
> - area->nr_free--;
> - list_del(&page->lru);
> - rmv_page_order(page);
> + list_move(&page->lru, &area->free_list[start_migratetype]);
This list_move is redundant if we are stealing whole pageblock, right?
Just put it in an else of the if above, and explain in comment?
> - expand(zone, page, order, current_order, area,
> - start_migratetype);
> /*
> * The freepage_migratetype may differ from pageblock's
> * migratetype depending on the decisions in
> - * try_to_steal_freepages(). This is OK as long as it
> + * find_suitable_fallback(). This is OK as long as it
> * does not differ for MIGRATE_CMA pageblocks. For CMA
> * we need to make sure unallocated pages flushed from
> * pcp lists are returned to the correct freelist.
The whole thing with set_freepage_migratetype(page, start_migratetype);
below this comment is now redundant, as rmqueue_smallest will do it too.
The comment itself became outdated and misplaced too. I guess
MIGRATE_CMA is now handled just by the fact that is is not set as
fallback in the fallbacks array?
> @@ -1283,10 +1281,10 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
> trace_mm_page_alloc_extfrag(page, order, current_order,
> start_migratetype, fallback_mt);
>
> - return page;
> + return true;
> }
>
> - return NULL;
> + return false;
> }
>
> /*
> @@ -1297,28 +1295,32 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
> int migratetype)
> {
> struct page *page;
> + bool steal_fallback;
>
> -retry_reserve:
> +retry:
> page = __rmqueue_smallest(zone, order, migratetype);
>
> if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
> if (migratetype == MIGRATE_MOVABLE)
> page = __rmqueue_cma_fallback(zone, order);
>
> - if (!page)
> - page = __rmqueue_fallback(zone, order, migratetype);
> + if (page)
> + goto out;
> +
> + steal_fallback = __rmqueue_fallback(zone, order, migratetype);
>
> /*
> * Use MIGRATE_RESERVE rather than fail an allocation. goto
> * is used because __rmqueue_smallest is an inline function
> * and we want just one call site
> */
> - if (!page) {
> + if (!steal_fallback)
> migratetype = MIGRATE_RESERVE;
> - goto retry_reserve;
> - }
> +
> + goto retry;
> }
>
> +out:
> trace_mm_page_alloc_zone_locked(page, order, migratetype);
> return page;
> }
>
--
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