linux-ext4 - Re: [PATCH 1/2] ext4: let mpage_submit_io works well when blocksize

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Message-ID: <4ED10448.1040204@tao.ma>
Date:	Sat, 26 Nov 2011 23:22:48 +0800
From:	Tao Ma <tm@....ma>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"achender@...ux.vnet.ibm.com" <achender@...ux.vnet.ibm.com>,
	"tytso@....edu" <tytso@....edu>
Subject: Re: [PATCH 1/2] ext4: let mpage_submit_io works well when blocksize
 < pagesize

On 11/26/2011 11:13 PM, Yongqiang Yang wrote:
> Hi,
> 
> It can be tested by xfstests 61 or 91, I did not remember which one.
ok, I will try it later. But I still wonder how it happens. See my
comments below.
> 
> It can be reproduced by reading on a fallocted block and write the block
> after the fallocted block.  Then the written block can not be written
> out by da_writepages.
Why? in read no bh will be created, and in write only the written bh
will be mapped and set unwritten. How could that happen? Sorry, but this
explanation doesn't convince me.
> 
> Yongqiang.
> 
> On Saturday, November 26, 2011, Tao Ma <tm@....ma <mailto:tm@....ma>> wrote:
>> Hi Yongqiang,
>> On 11/23/2011 05:15 PM, Yongqiang Yang wrote:
>>> If there is a unwritten but clean buffer in a page and there is a
> dirty buffer
>>> after the buffer, then mpage_submit_io does not write the dirty
> buffer out.
>>> As a result, da_writepages loops forever.
>> Did you ever meet with this bug or just find it to be a possible bug by
>> skimming the code? Actually, I can't find a proper way to get a buffer
>> which can satisfy the check.
>>
>> Thanks
>> Tao
>>>
>>> This patch fixes the problem by checking dirty flag.
>>>
>>> Signed-off-by: Yongqiang Yang <xiaoqiangnk@...il.com
> <mailto:xiaoqiangnk@...il.com>>
>>> ---
>>>  fs/ext4/inode.c |    7 +++++--
>>>  1 files changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>>> index 755f6c7..20a1d17 100644
>>> --- a/fs/ext4/inode.c
>>> +++ b/fs/ext4/inode.c
>>> @@ -1339,8 +1339,11 @@ static int mpage_da_submit_io(struct
> mpage_da_data *mpd,
>>>                                       clear_buffer_unwritten(bh);
>>>                               }
>>>
>>> -                             /* skip page if block allocation undone */
>>> -                             if (buffer_delay(bh) ||
> buffer_unwritten(bh))
>>> +                             /*
>>> +                              * skip page if block allocation undone and
>>> +                              * block is dirty
>>> +                              */
>>> +                             if (ext4_bh_delay_or_unwritten(NULL, bh))
>>>                                       skip_page = 1;
>>>                               bh = bh->b_this_page;
>>>                               block_start += bh->b_size;
>>
>>
>>
> 
> -- 
> Best Wishes
> Yongqiang Yang
> 

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