linux-ext4 - Re: question about punch hole

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Date:	Sat, 27 Aug 2011 17:04:51 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: question about punch hole

On Sat, Aug 27, 2011 at 6:35 AM, Allison Henderson
<achender@...ux.vnet.ibm.com> wrote:
> On 08/25/2011 07:53 PM, Yongqiang Yang wrote:
>>
>> Hi Allison,
>>
>> Currently, punch hole flushes all pages to disk and releases pages in
>> page cache, and then calls ext4_ext_map_blocks.
>>
>> Assume that if a new page in the punching's range is mapped after
>> releasing pages and before down_write i_data_sem,
>> then ext4_ext_map_blocks will release map info of the page in extent
>> tree.  However, up layers does not know this, and they think the page
>> is mapped.
>>
>> I can not find how punch hole handle the situation above.  Could you
>> shed a light on it?
>>
>>
> Hi Yongqiang
>
> This is a really good question and at the moment Im still looking into it.
>  :)  The calling sequence in punch hole was modeled after truncate, which
> also only locks i_data_sem when modifying the extent tree.
> ext4_ext_map_blocks when called with the punch hole flag, only releases
> blocks in the extent tree, using the same routines truncate does, but it
> does not modify the state of the pages. Though that still does not prevent
> the race condition you describe, so I am still investigating it.
> I've found that I can catch a lot of race conditions by simply running the
> stress test over night, and so far I havnt had anything like this come up,
> but that certainly doesnt mean its not there.  I will let you know what I
> find.  Thx!

Hi Allison,

I had a look at truncate code, truncates and writes are serialized by
inode->i_mutex in vfs layer,  but fallocate does not take i_mutex, so
we need to take i_mutex in punching hole as well, I think.  Fallocate
behaves differently with punching hole, so it is safe without taking
i_mutex.


What's your opinion?

Yongqiang.
>
> Allison Henderson
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-- 
Best Wishes
Yongqiang Yang
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