linux-kernel - Re: [PATCH v2 2/7] mm/page_owner: initialize page owner without holding the zone lock

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Date:	Fri, 3 Jun 2016 12:23:06 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:	mgorman@...hsingularity.net, Minchan Kim <minchan@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 2/7] mm/page_owner: initialize page owner without
 holding the zone lock

On 05/26/2016 04:37 AM, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> It's not necessary to initialized page_owner with holding the zone lock.
> It would cause more contention on the zone lock although it's not
> a big problem since it is just debug feature. But, it is better
> than before so do it. This is also preparation step to use stackdepot
> in page owner feature. Stackdepot allocates new pages when there is no
> reserved space and holding the zone lock in this case will cause deadlock.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>  mm/compaction.c     | 3 +++
>  mm/page_alloc.c     | 2 --
>  mm/page_isolation.c | 9 ++++++---
>  3 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 8e013eb..6043ef8 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -20,6 +20,7 @@
>  #include <linux/kasan.h>
>  #include <linux/kthread.h>
>  #include <linux/freezer.h>
> +#include <linux/page_owner.h>
>  #include "internal.h"
>
>  #ifdef CONFIG_COMPACTION
> @@ -80,6 +81,8 @@ static void map_pages(struct list_head *list)
>  		arch_alloc_page(page, order);
>  		kernel_map_pages(page, nr_pages, 1);
>  		kasan_alloc_pages(page, order);
> +
> +		set_page_owner(page, order, __GFP_MOVABLE);
>  		if (order)
>  			split_page(page, order);
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5134f46..1b1ca57 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2507,8 +2507,6 @@ int __isolate_free_page(struct page *page, unsigned int order)
>  	zone->free_area[order].nr_free--;
>  	rmv_page_order(page);
>
> -	set_page_owner(page, order, __GFP_MOVABLE);
> -
>  	/* Set the pageblock if the isolated page is at least a pageblock */
>  	if (order >= pageblock_order - 1) {
>  		struct page *endpage = page + (1 << order) - 1;
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 612122b..927f5ee 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -7,6 +7,7 @@
>  #include <linux/pageblock-flags.h>
>  #include <linux/memory.h>
>  #include <linux/hugetlb.h>
> +#include <linux/page_owner.h>
>  #include "internal.h"
>
>  #define CREATE_TRACE_POINTS
> @@ -108,8 +109,6 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  			if (pfn_valid_within(page_to_pfn(buddy)) &&
>  			    !is_migrate_isolate_page(buddy)) {
>  				__isolate_free_page(page, order);
> -				kernel_map_pages(page, (1 << order), 1);
> -				set_page_refcounted(page);
>  				isolated_page = page;
>  			}
>  		}
> @@ -128,8 +127,12 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  	zone->nr_isolate_pageblock--;
>  out:
>  	spin_unlock_irqrestore(&zone->lock, flags);
> -	if (isolated_page)
> +	if (isolated_page) {
> +		kernel_map_pages(page, (1 << order), 1);

So why we don't need the other stuff done by e.g. map_pages()? For example 
arch_alloc_page() and kasan_alloc_pages(). Maybe kasan_free_pages() (called 
below via __free_pages() I assume) now doesn't check if the allocation part was 
done. But maybe it will start doing that?

See how the multiple places doing similar stuff is fragile? :(

> +		set_page_refcounted(page);
> +		set_page_owner(page, order, __GFP_MOVABLE);
>  		__free_pages(isolated_page, order);

This mixing of "isolated_page" and "page" is not a bug, but quite ugly. Can't 
isolated_page variable just be a bool?

> +	}
>  }
>
>  static inline struct page *
>