linux-kernel - Re: [PATCH 1/4] mm/migrate: correct return value of migrate

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Message-ID: <52A1E248.1000204@suse.cz>
Date:	Fri, 06 Dec 2013 15:42:16 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Rafael Aquini <aquini@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <js1304@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/4] mm/migrate: correct return value of migrate_pages()

On 12/06/2013 09:41 AM, Joonsoo Kim wrote:
> migrate_pages() should return number of pages not migrated or error code.
> When unmap_and_move return -EAGAIN, outer loop is re-execution without
> initialising nr_failed. This makes nr_failed over-counted.
>
> So this patch correct it by initialising nr_failed in outer loop.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 3747fcd..1f59ccc 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1102,6 +1102,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>
>   	for(pass = 0; pass < 10 && retry; pass++) {
>   		retry = 0;
> +		nr_failed = 0;
>
>   		list_for_each_entry_safe(page, page2, from, lru) {
>   			cond_resched();
>

If I'm reading the code correctly, unmap_and_move() (and 
unmap_and_move_huge_page() as well) deletes all pages from the 'from' 
list, unless it fails with -EAGAIN. So the only pages you see in 
subsequent passes are those that failed with -EAGAIN and those are not 
counted as nr_failed. So there shouldn't be over-count, but your patch 
could result in under-count.

Perhaps a comment somewhere would clarify this.
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