linux-kernel - Re: [PATCH] swapfile: avoid NULL pointer dereference in swapon when s

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Date:	Tue, 6 Oct 2009 22:03:40 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Suresh Jayaraman <sjayaraman@...e.de>
cc:	Rafael Wysocki <rjw@...k.pl>, Jens Axboe <axboe@...nel.dk>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] swapfile: avoid NULL pointer dereference in swapon when
 s_bdev is NULL

On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
> Hugh Dickins wrote:
> > On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
> >>
> >> The call sequence is add_to_swap() -> get_swap_page() ->  scan_swap_map()
> >> -> discard_swap_cluster() -> blkdev_issue_discard().
> >>
> >> Wrapping the code around a NULL check fixes the Oops for me.
> > 
> > That's odd: scan_swap_map() should only discard_swap_cluster() if
> > SWP_DISCARDABLE got set, and your first patch made sure that it wasn't.
> 
> I forgot to mention, this is not on loopback NFS mount but an remote NFS
> mount (so possibly s_bdev is not NULL) when doing swapon. The oops was
> triggered when memhog program tries to use the swap space on the newly
> created swapfile on NFS. I have not completely investigated the issue,
> perhaps s_bdev is not being set when it ought to be..

I'm happy to see your first patch already in 2.6.32-rc3, but still
suspicious of this second patch you sent afterwards.  A quick skim
through your patchset suggests 23/31 is probably responsible:

--- mmotm.orig/include/linux/swap.h
+++ mmotm/include/linux/swap.h
@@ -120,6 +120,7 @@ struct swap_extent {
 enum {
 	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
 	SWP_WRITEOK	= (1 << 1),	/* ok to write to this swap?	*/
+	SWP_FILE	= (1 << 2),	/* file swap area */
 	SWP_DISCARDABLE = (1 << 2),	/* blkdev supports discard */
 	SWP_DISCARDING	= (1 << 3),	/* now discarding a free cluster */
 	SWP_SOLIDSTATE	= (1 << 4),	/* blkdev seeks are cheap */

Hugh
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