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Date: Wed, 7 Mar 2007 12:46:58 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [patch 8/8] consolidate generic_writepages and mpage_writepages
 fix
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On Tue, 06 Mar 2007 19:04:51 +0100
Miklos Szeredi <miklos@szeredi.hu> wrote:

> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> Fix NULL pointer dereference in __mpage_writepage.  This probably
> doesn't matter in practice, but this is the original behavior.
> 
> Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
> ---
> 
> Index: linux/fs/mpage.c
> ===================================================================
> --- linux.orig/fs/mpage.c	2007-03-06 17:30:09.000000000 +0100
> +++ linux/fs/mpage.c	2007-03-06 17:30:33.000000000 +0100
> @@ -662,7 +662,7 @@ confused:
>  	if (bio)
>  		bio = mpage_bio_submit(WRITE, bio);
>  
> -	if (mpd->use_writepage) {
> +	if (mpd->use_writepage && mapping->a_ops->writepage) {
>  		ret = mapping->a_ops->writepage(page, wbc);
>  	} else {
>  		ret = -EAGAIN;
> 
> --

I'm inclined to leave the code as-is, as an optimisation.  If it
blows up then it'd be cleaner to change the caller (mpage_writepages)
to do

	.use_writepage = (writepage != NULL),

?