linux-ext4 - Re: question about punch hole

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Date:	Sat, 27 Aug 2011 17:33:50 +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 5:04 PM, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
> 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.
It seems that race exists between reads and punching hole as well.  If
a read comes after releasing pages and before down_write(i_data_sem),
then a page will be mapped, if the page is written later, it will
introduce an error. truncate avoids this situation by set file size
before truncating pages.

Yongqiang.

>
>
> What's your opinion?
>
> Yongqiang.
>>
>> Allison Henderson
>> --
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>>
>
>
>
> --
> Best Wishes
> Yongqiang Yang
>



-- 
Best Wishes
Yongqiang Yang
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