Date: Thu, 27 Nov 2014 01:50:54 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>,
        Hugh Dickins <hughd@google.com>, Michel Lespinasse <walken@google.com>,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [rfc patch] mm: protect set_page_dirty() from ongoing
 truncation
Message-Id: <20141127015054.6368af49.akpm@linux-foundation.org>
In-Reply-To: <20141127094006.GC30152@quack.suse.cz>
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	<20141126140006.d6f71f447b69cd4fadc42c26@linux-foundation.org>
	<20141127094006.GC30152@quack.suse.cz>
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On Thu, 27 Nov 2014 10:40:06 +0100 Jan Kara <jack@suse.cz> wrote:

> > so we no longer require that the address_space be stabilized after
> > lock_page().  Of course something needs to protect the bdi and I'm not
> > sure what that is, but we're talking about umount and that quiesces and
> > evicts lots of things before proceeding, so surely there's something in
> > there which will save us ;)
>   In do_wp_page() the process doing the fault and ending in
> balance_dirty_pages() has to have the page mapped, thus it has to have the
> file open => no umount.

Actually, umount isn't enough to kill the backing_dev_info.  It's an
attribute of the device itself (for blockdevs it's a field in
request_queue) so I assume it will be stable until device hot-unplug,
losetup -d, rmmod, etc.  If the backing_dev can go away in the middle
of a pagefault against that device then we have bigger problems ;)


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