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

[Michal Hocko]

On Fri 24-02-17 08:26:09, Minchan Kim wrote:
[...]
> >From 9efb87a873db67a9e6ebf44fdabf7d05fe4b4e21 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@xxxxxxxxxx>
> Date: Fri, 4 Nov 2016 09:12:39 +0900
> Subject: [PATCH v2] mm: do not access page->mapping directly on page_endio
> 
> 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.
> 
> swap_writepage
>   bdev_writepage
>     ops->rw_page
> 
> I encountered the BUG during developing new zram feature and
> it was really hard to figure it out because it made random
> crash, somtime mmap_sem lockdep, sometime other places where
> places never related to zram/zsmalloc, and not reproducible
> with some configuration.
> 
> When I consider how that bug is subtle and people do fast-swap
> test with brd, it's worth to add stable mark, I think.
> 
> Fixes: dd6bd0d9c7db ("swap: use bdev_read_page() / bdev_write_page()")
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks for the chagelog update

> ---
> * from v1
>   * add more detailed description with Fix tag - Michal
> 
>  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