linux-ext4 - Re: [PATCH 1/2] ext4: fix journal ref count in move_extent_par

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Message-ID: <6149e97b0908030617q2dfe598bsbbac45969f1f2625@mail.gmail.com>
Date:	Mon, 3 Aug 2009 21:17:10 +0800
From:	Peng Tao <bergwolf@...il.com>
To:	Akira Fujita <a-fujita@...jp.nec.com>
Cc:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH 1/2] ext4: fix journal ref count in move_extent_par_page

Hi, Akira,

2009/8/3 Akira Fujita <a-fujita@...jp.nec.com>:
> Hi Peng,
>
> Peng Tao wrote:
>> move_extent_par_page calls a_ops->write_begin() to increase journal handler's
>> reference count. However, if either mext_replace_branches() or ext4_get_block
>> fails, the increased reference count isn't decreased. This will cause later
>> umounting of the fs forever hangs. The patch addresses the issue by calling
>> ext4_journal_stop() if page is not NULL (which means a_ops->write_end() isn't
>> invoked).
>
> In case mext_replaced_branches() or ext4_get_block failed,
> ext4_journal_stop() is called at out2 label(*)
> and then journal reference counter is decreased.
> Therefore I think this fix is not necessary.
well, the orginal code calls both ext4_journal_start and
a_ops->write_begin(). So the journal reference was increased twice but
only gets decreased once in case of failure. This can be simply
verified by returning -1 at the begining of mext_replaced_branches().
With such modification, the fs cannot be umounted because of the wrong
reference count.
>
> static int
> move_extent_par_page(struct file *o_filp, struct inode *donor_inode,
>                  pgoff_t orig_page_offset, int data_offset_in_page,
>                  int block_len_in_page, int uninit)
> <snip>
>
> out:
>        if (unlikely(page)) {
>                if (PageLocked(page))
>                        unlock_page(page);
>                page_cache_release(page);
>        }
> out2:
> *       ext4_journal_stop(handle);
>
>        return ret < 0 ? ret : 0;
> }
>
> Regards,
> Akira Fujita
>



-- 
Cheers,
Peng Tao
State Key Laboratory of Networking and Switching Technology
Beijing Univ. of Posts and Telecoms.
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