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

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Date:   Wed, 22 Feb 2017 15:53:16 +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 23:35:17, Minchan Kim wrote:
> On Wed, Feb 22, 2017 at 01:11:00PM +0100, Michal Hocko wrote:
> > 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
> 
> No. For driver to support rw_page, every swap_writepage calls rw_page.
> 
> swap_writepage
>   bdev_writepage
>     ops->rw_page

Ohh, you are right, I have missed this option. I was looking at the
normal swapout path which uses bio.

> > 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?
> 
> Look at other drivers to support rw_page, not zram, esp, brd.
> They can be used for swap device and then can hit the case.
> 
> In fact, I encountered the BUG during zram development(i.e., it doesn't
> land to upstream) and it was really hard to figure it out because it made
> random crash, sometime mmap_sem lockdep, sometime other places where
> places never related to zram/zsmalloc, sometime not reproducible.
> 
> When I consider how that bug is subtle and people do fast-swap test as brd,
> it's worth to add stable mark, I think.

Sure, could you add this to the changelog. Along with Fixes tag? I
suspect it is dd6bd0d9c7db ("swap: use bdev_read_page() /
bdev_write_page()") which has introduced this but I didn't look too
close. The patch is trivially correct.

-- 
Michal Hocko
SUSE Labs