linux-kernel - Re: [rfc patch] mm: protect set_page

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Message-Id: <20141127015054.6368af49.akpm@linux-foundation.org>
Date:	Thu, 27 Nov 2014 01:50:54 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [rfc patch] mm: protect set_page_dirty() from ongoing
 truncation

On Thu, 27 Nov 2014 10:40:06 +0100 Jan Kara <jack@...e.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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