linux-ext4 - Re: attempt to fix race in bigalloc code path

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Message-ID: <CAGBYx2ZTzrLR2yRRnCu3pvFWvofYzJp7+WsdQt2=Y7yWGEzyTg@mail.gmail.com>
Date:	Mon, 22 Aug 2011 21:54:06 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	adityakali@...gle.com
Cc:	Jan Kara <jack@...e.cz>, "Ted Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: attempt to fix race in bigalloc code path

Hi,

I can not find the patch - attempt to fix race in bigalloc code path -
in the mailing list.

I don't think this patch can work.  set_buffers_da_mapped() is called
under i_data_sem, so it can not lock page.  If the page is not locked,
calling page_has_buffers() is not secure as Jan pointed out
previously.  If a page is partially mapped(block size < page size),
setting BH_Da_Mapped without locking page is not safe too.

BTW:  find_delalloc_range() does not lock page as ext4_fiemap_cb().
As Jan said, it can crash system.  BUT we can not lock page in both
scenarios, because i_data_sem is hold. So delayed extent list is
necessary.  I have finished the code implementing delayed extent list,
which needs some further testing.

What are your opinions?

-- 
Best Wishes
Yongqiang Yang
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