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

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Date:	Tue, 23 Aug 2011 09:12:10 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Aditya Kali <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

On Tue, Aug 23, 2011 at 1:30 AM, Aditya Kali <adityakali@...gle.com> wrote:
> 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 ?
Sorry for noise and thank you for your explanation, you are right.
IMHO, we can clear BH_Delayed and set BH_Mapped instead.

Yongqiang.
>
>> 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
>>
>



-- 
Best Wishes
Yongqiang Yang
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