linux-ext4 - Re: [PATCH 3/3] ext4: Block mmapped writes while the fs is frozen

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Date:	Tue, 17 May 2011 11:11:08 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/3] ext4: Block mmapped writes while the fs is frozen

On Wed, May 11, 2011 at 12:29:34AM +0200, Jan Kara wrote:
> We should not allow file modification via mmap while the filesystem is
> frozen. So block in ext4_page_mkwrite() while the filesystem is frozen.
> 
> We have to check for frozen filesystem with the page marked dirty and under
> page lock with which we then return from ext4_page_mkwrite(). Only that way we
> cannot race with writeback done by freezing code - either we mark the page
> dirty after the writeback has started, see freezing in progress and block, or
> writeback will wait for our page lock which is released only when the fault is
> done and then writeback will writeout and writeprotect the page again.

This really should be done in (__)block_page_mkwrite.  I'd also return
VM_FAULT_RETRY instead of retrying inside the block_mkwrite handler
in case you hit the race.

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