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

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Message-ID: <20110518075614.GB25632@quack.suse.cz>
Date:	Wed, 18 May 2011 09:56:14 +0200
From:	Jan Kara <jack@...e.cz>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jan Kara <jack@...e.cz>, 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 Tue 17-05-11 11:11:08, Christoph Hellwig wrote:
> 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.
  Well, we can do the non-blocking check at the end of
__block_page_mkwrite() and return some error value (EAGAIN translating to
VM_FAULT_RETRY would look logical, I just have to think off better error
value for VM_FAULT_NOPAGE). But vfs_check_frozen() cannot be in
__block_page_mkwrite() since ext4 needs to call that with a transaction
started so that would create a deadlock and we need to call
vfs_check_frozen() somewhere so that we don't busyloop.

I can call vfs_check_frozen() inside block_page_mkwrite() but it would be a
bit surprising difference from __block_page_mkwrite() to me. Not sure what
the cleanest solution would be here...

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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