linux-kernel - Re: [PATCH] mm: do not access page->mapping directly on page

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Message-ID: <20170222121100.GA7954@dhcp22.suse.cz>
Date:   Wed, 22 Feb 2017 13:11:00 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Matthew Wilcox <willy@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH] mm: do not access page->mapping directly on page_endio

On Wed 22-02-17 14:39:24, Minchan Kim wrote:
> With rw_page, page_endio is used for completing IO on a page
> and it propagates write error to the address space if the IO
> fails. The problem is it accesses page->mapping directly which
> might be okay for file-backed pages but it shouldn't for
> anonymous page. Otherwise, it can corrupt one of field from
> anon_vma under us and system goes panic randomly.

I was about to say that anonymous pages shouldn't hit that path because
the end_swap_bio_write doesn call page_endio. But then I've noticed that
zram does call this function. On a closer look, though, it doesn't seem
to call it with err != 0 so it cannot hit this path. So I am wondering
whether this actually fixes anything. Why it has been marked for stable?

> 
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  mm/filemap.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 2ba46f410c7c..1944c631e3e6 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1008,9 +1008,12 @@ void page_endio(struct page *page, bool is_write, int err)
>  		unlock_page(page);
>  	} else {
>  		if (err) {
> +			struct address_space *mapping;
> +
>  			SetPageError(page);
> -			if (page->mapping)
> -				mapping_set_error(page->mapping, err);
> +			mapping = page_mapping(page);
> +			if (mapping)
> +				mapping_set_error(mapping, err);
>  		}
>  		end_page_writeback(page);
>  	}
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs