linux-ext4 - Re: [PATCH 3/5 resend] VFS: Fix s

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Date:	Wed, 7 Dec 2011 16:49:20 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Kamal Mostafa <kamal@...onical.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Randy Dunlap <rdunlap@...otime.net>,
	Theodore Tso <tytso@....edu>, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Surbhi Palande <csurbhi@...il.com>,
	Valerie Aurora <val@...consulting.com>,
	Christopher Chaltain <christopher.chaltain@...onical.com>,
	"Peter M. Petrakis" <peter.petrakis@...onical.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Miao Xie <miaox@...fujitsu.com>
Subject: Re: [PATCH 3/5 resend] VFS: Fix s_umount thaw/write deadlock

On Thu, Dec 08, 2011 at 12:16:58AM +0100, Jan Kara wrote:
> On Tue 06-12-11 06:35:44, Christoph Hellwig wrote:
> > We make sure to not dirty any new inodes after the first phase of the
> > freeze, so this should be a BUG_ON/WARN_ON.
>   This is not really true in presence of mmaped writes. To block mmaped
> writes on a frozen filesystem, we need some synchronization between
> page_mkwrite() and freezing code. Currently, to avoid any additional
> locking overhead, we set page dirty and *then* check for filesystem being
> frozen. Only this order can make sure either the page is written (and
> write-protected) or the frozen check triggers and we wait... (see the
> comment in block_page_mkwrite()). The nasty sideeffect of this is that
> there can be dirty pages & inodes on a frozen filesystem. We are blocked in
> the page fault of these pages so user cannot write any data to these pages
> but still they are marked dirty.

Can't we make the mmaps read-only during the freezing process?

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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