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

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Date:	Thu, 01 Oct 2009 17:37:49 +0530
From:	Suresh Jayaraman <sjayaraman@...e.de>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
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

Hugh Dickins wrote:
> On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
>> Yeah, perhaps. I stumbled upon one more of such error - a NULL pointer
>> dereference in blkdev_issue_discard() called from get_swap_page() when I ran
>> memhog, a simple program to generate a memory hog with Swap over NFS patches.
>>
>> 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..

> So I don't think this second patch should be necessary: you did have
> your first applied when you found this?

yes, I had the first patch applied when it oopsed and I don't use SSD at
all.

> I wonder if there's a funny little issue like si->lowest_alloc not
> being reset to 0 where it should be.  Were you switching between
> NFS swap and SSD swap in your testing?

No.

Thanks,

-- 
Suresh Jayaraman
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