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

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Date:   Fri, 24 Feb 2017 08:26:09 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...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, Feb 22, 2017 at 03:53:16PM +0100, Michal Hocko wrote:
> 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.

Sure. Thanks for the review.

Andrew, Could you change description with this?

>From 9efb87a873db67a9e6ebf44fdabf7d05fe4b4e21 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
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@...nel.org>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Minchan Kim <minchan@...nel.org>
---
* 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