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Date:	Mon, 22 Aug 2011 10:30:02 -0700
From:	Aditya Kali <adityakali@...gle.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.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

On Mon, Aug 22, 2011 at 6:54 AM, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
> 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.
>
The BH_Da_Mapped is set only during page writeback when the page is
locked (in write_cache_pages_da). So, the setting of BH_Da_Mapped
should be safe, no ?

> 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?
>
I agree that having a delayed extent list will greatly simply this
solution in this case.

>
> --
> Best Wishes
> Yongqiang Yang
>
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