linux-ext4 - Re: Add SEEK_DATA/SEEK

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Date:	Wed, 28 Sep 2011 17:06:30 +0800
From:	Jeff Liu <jeff.liu@...cle.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	linux-ext4@...r.kernel.org,
	Sunil Mushran <sunil.mushran@...cle.com>
Subject: Re: Add SEEK_DATA/SEEK_HOLE support V2

On 09/28/2011 04:53 PM, Yongqiang Yang wrote:

> On Wed, Sep 28, 2011 at 4:40 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
>> On 09/28/2011 04:11 PM, Yongqiang Yang wrote:
>>
>>> On Wed, Sep 28, 2011 at 4:02 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
>>>> On 09/28/2011 03:21 PM, Yongqiang Yang wrote:
>>>>
>>>>> On Wed, Sep 28, 2011 at 2:32 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
>>>>>> Hello all,
>>>>>>
>>>>>> This is the V2 for add SEEK_DATA/SEEK_HOLE support to ext4.
>>>>>> In V1, I copied all the codes regarding fetching delayed-extents out to
>>>>>> my seek callback function, it's stupid to duplicate the code blocks.
>>>>>>
>>>>>> Now the idea is, since the delayed-extents is the main concern to search
>>>>>> extents for both fiemap and seek, so IMHO, it's better to consolidate
>>>>>> this procedure into a new function(get_delayed_extent() in my patch),
>>>>>> and call it in fiemap and seek call-back functions if necessary.  In
>>>>>> this way, when yongqiang's delayed-extents tree is ready, we only need
>>>>>> to modify the code block at get_delayed_extent() accordingly.
>>>>>>
>>>>>> In addition, yongqiang has pointed out there might be a deadlock, but
>>>>>> per my understood, as the target inode already locked via i_mutex in
>>>>>> ext4_llseek() before, this lock can prevent any modification to the
>>>>>> file, and we call find_get_pages_tag() when trying to fetch the dirty
>>>>>> pages, this function using rcu read lock, it also safe to read.  So I
>>>>>> have no idea what's wrong, could anyone please kindly point me out in
>>>>>> this case?  I'm definitely a newbie to this list, sorry for my ignorant.
>>>>> Ok.  There is no deadlock, fiemap releases i_data_sem before calling
>>>>> get_delayed_extent().
>>>>>
>>>>> BTW: It seems that there is a bug in fiemap according to code, I am
>>>>> not sure which commit introduced it, delayed extents beyond last
>>>>> allocated block are ignored.
>>>>
>>>> Hmm... could you show me a bit more detailed info with the test scenario
>>>> if convenient? I can help testing, at least 3.1.0-rc2+ is works to me.
>>> Could you test a file with map info like AAAADDDDDHHHHDDDD,
>>> A-allocated block, D-deayed block, H-hole.  then filefrag -v and have
>>> a look if extent map info is right.
>>
>> Do you means, open a file, write something, fsync(), then write
>> something, seek to some position and write something again?
> Yep.
>> If so, looks the filefrag -v result is ok by waiting for a few seconds.
> What do you mean waiting for a few seconds?

If run filefrag -v immediately against a just created file, the delayed
extents will not shown, we have to wait a little while until they are
really allocated IMHO.

Thanks,
-Jeff

> 
> 
> Yongqiang.
>>
>> Thanks,
>> -Jeff
>>
>>>
>>> Thanks,
>>> Yongqiang.
>>>>
>>>> Also, upstream coreutils-cp(1) has been well tested for ext4, it could
>>>> be used for fiemap tests based on the extent_scan module:
>>>> http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/extent-scan.c?h=next&id=b56b53bd70b1f8fa2b5a95d4569bb72a2419b5cd
>>>>
>>>> Thanks,
>>>> -Jeff
>>>>
>>>>>
>>>>> Yongqiang.
>>>>>>
>>>>>> Anyway, I'd like to post the revised for your guys review first.
>>>>>> In have split the modification to two patches this time, since I hope to
>>>>>> make it looks more convenient for the review purpose.
>>>>>>
>>>>>> The 1st patch, remove the delayed-extents fetching procedure to a
>>>>>> function named as get_delayed_extent(), and modify the
>>>>>> ext4_ext_fiemap_cb() accordingly.
>>>>>>
>>>>>> The 2nd path add the ext4_ext_seek_cb() to implement the
>>>>>> SEEK_HOLE/SEEK_DATA stuff based on previous code change.
>>>>>>
>>>>>> Testing:
>>>>>> ========
>>>>>> In addition to the test cases mentioned in V1, I also done another tests
>>>>>> through sunil's seek test program, please refer to:
>>>>>> http://oss.oracle.com/~smushran/seek_data/seek_test.c
>>>>>>
>>>>>> To ensure the 1st patch does not cause regression issue on fiemap, I
>>>>>> have done some tests via cp(1) with fiemap copy support, the sparse file
>>>>>> used for this test is produced by:
>>>>>> python -c "f=open('./sptest', 'w'); [(f.seek(x) or f.write(str(x))) for
>>>>>> x in range(1, 1000000000, 99999)]; f.close()
>>>>>>
>>>>>>
>>>>>> Any feedback are appreciated!
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> -Jeff
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>>
> 
> 
> 


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