linux-ext4 - Re: your mail

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Date:	Tue, 3 May 2011 15:46:36 +0200
From:	Jan Kara <jack@...e.cz>
To:	Surbhi Palande <surbhi.palande@...onical.com>
Cc:	jack@...e.cz, toshi.okajima@...fujitsu.com, tytso@....edu,
	m.mizuma@...fujitsu.com, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: your mail

On Tue 03-05-11 16:08:36, Surbhi Palande wrote:
> On munmap() zap_pte_range() is called which dirties the PTE dirty pages as
> Toshiyuki pointed out.
> 
> zap_pte_range()
>   mapping->a_ops->set_page_dirty (= ext4_journalled_set_page_dirty)  
> 
> So, I think that it is here that we should do the checking for a ext4 F.S
> frozen state and also prevent a parallel ext4 F.S freeze from happening.
> 
> Attaching a patch for initial review. Please do let me know your thoughts! 
  This is definitely the wrong place. ->set_page_dirty() callbacks are
called with various locks held and the page need not be locked (thus
dereferencing page->mapping is oopsable). Moreover this particular callback
is called only in data=journal mode.

Believe me, the right place is page_mkwrite() - you have to catch the
read-only => read-write page transition. Once the page is mapped
read-write, you've already lost the race.

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