linux-ext4 - Re: question about punch hole

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Date:	Fri, 26 Aug 2011 15:35:31 -0700
From:	Allison Henderson <achender@...ux.vnet.ibm.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: question about punch hole

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!

Allison Henderson
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