linux-ext4 - Re: Add SEEK_DATA/SEEK

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Date:	Wed, 28 Sep 2011 16:40:44 +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 04:11 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?
If so, looks the filefrag -v result is ok by waiting for a few seconds.

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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