linux-kernel - Re: [RFC] mm: change mm_advise

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Date:	Fri, 27 Feb 2015 22:02:33 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>
Cc:	'Minchan Kim' <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Shaohua Li <shli@...nel.org>
Subject: Re: [RFC] mm: change mm_advise_free to clear page dirty

On Fri 27-02-15 11:37:18, Wang, Yalin wrote:
> This patch add ClearPageDirty() to clear AnonPage dirty flag,
> the Anonpage mapcount must be 1, so that this page is only used by
> the current process, not shared by other process like fork().
> if not clear page dirty for this anon page, the page will never be
> treated as freeable.

Very well spotted! I haven't noticed that during the review.

> Signed-off-by: Yalin Wang <yalin.wang@...ymobile.com>
> ---
>  mm/madvise.c | 15 +++++----------
>  1 file changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6d0fcb8..257925a 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -297,22 +297,17 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  			continue;
>  
>  		page = vm_normal_page(vma, addr, ptent);
> -		if (!page)
> +		if (!page || !PageAnon(page) || !trylock_page(page))
>  			continue;

PageAnon check seems to be redundant because we are not allowing
MADV_FREE on any !anon private mappings AFAIR.

>  
>  		if (PageSwapCache(page)) {
> -			if (!trylock_page(page))
> +			if (!try_to_free_swap(page))
>  				continue;

You need to unlock the page here.

> -
> -			if (!try_to_free_swap(page)) {
> -				unlock_page(page);
> -				continue;
> -			}
> -
> -			ClearPageDirty(page);
> -			unlock_page(page);
>  		}
>  
> +		if (page_mapcount(page) == 1)
> +			ClearPageDirty(page);

Please add a comment about why we need to ClearPageDirty even
!PageSwapCache. Anon pages are usually not marked dirty AFAIR. The
reason seem to be racing try_to_free_swap which sets the page that way
(although I do not seem to remember why are we doing that in the first
place...)

> +		unlock_page(page);
>  		/*
>  		 * Some of architecture(ex, PPC) don't update TLB
>  		 * with set_pte_at and tlb_remove_tlb_entry so for
> -- 
> 2.2.2

-- 
Michal Hocko
SUSE Labs
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