linux-kernel - Re: [PATCH] Btrfs: add another missing end_page_writeback on submit_extent

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Message-ID: <b8fa6a68-72a8-b745-b71c-c25d89a7a911@sslab.ics.keio.ac.jp>
Date:   Sat, 4 Feb 2017 21:42:17 +0900
From:   takafumi-sslab <takafumi.kubota1012@...ab.ics.keio.ac.jp>
To:     bo.li.liu@...cle.com
Cc:     linux-btrfs@...r.kernel.org, clm@...com, jbacik@...com,
        dsterba@...e.com, linux-kernel@...r.kernel.org, fdmanana@...e.com,
        naota@...sp.net
Subject: Re: [PATCH] Btrfs: add another missing end_page_writeback on
 submit_extent_page failure


> (But it could be changed after subpagesize block patchset, and there is
> more work rather than just adding a end_page_writeback, e.g. writepage
> endio also needs to be updated).

Ok... the discussion become complicated.
So, let me make this clear.

you think
a) this is a bug;
we need to clear the writeback bit in the error handling if the bit remains.

b) however, the way of fixing this bug has some concerns.　( and now we 
discuss the best solution )

Is my understanding correct?

Sincerely,

-takafumi
>
> Thanks,
>
> -liubo
>> Sincerely,
>>
>> On 2017/01/31 5:09, Liu Bo wrote:
>>> On Fri, Jan 13, 2017 at 03:12:31PM +0900, takafumi-sslab wrote:
>>>> Thanks for your replying.
>>>>
>>>> I understand this bug is more complicated than I expected.
>>>> I classify error cases under submit_extent_page() below
>>>>
>>>> A: ENOMEM error at btrfs_bio_alloc() in submit_extent_page()
>>>> I first assumed this case and sent the mail.
>>>> When bio_ret is NULL, submit_extent_page() calls btrfs_bio_alloc().
>>>> Then, btrfs_bio_alloc() may fail and submit_extent_page() returns -ENOMEM.
>>>> In this case, bio_endio() is not called and the page's writeback bit
>>>> remains.
>>>> So, there is a need to call end_page_writeback() in the error handling.
>>>>
>>>> B: errors under submit_one_bio() of submit_extent_page()
>>>> Errors that occur under submit_one_bio() handles at bio_endio(), and
>>>> bio_endio() would call end_page_writeback().
>>>>
>>>> Therefore, as you mentioned in the last mail, simply adding
>>>> end_page_writeback() like my last email and commit 55e3bd2e0c2e1 can
>>>> conflict in the case of B.
>>>> To avoid such conflict, one easy solution is adding PageWriteback() check
>>>> too.
>>>>
>>>> How do you think of this solution?
>>> (sorry for the late reply.)
>>>
>>> I think its caller, "__extent_writepage", has covered the above case
>>> by setting page writeback again.
>>>
>>> Thanks,
>>>
>>> -liubo
>>>> Sincerely,
>>>>
>>>> On 2016/12/22 15:20, Liu Bo wrote:
>>>>> On Fri, Dec 16, 2016 at 03:41:50PM +0900, Takafumi Kubota wrote:
>>>>>> This is actually inspired by Filipe's patch(55e3bd2e0c2e1).
>>>>>>
>>>>>> When submit_extent_page() in __extent_writepage_io() fails,
>>>>>> Btrfs misses clearing a writeback bit of the failed page.
>>>>>> This causes the false under-writeback page.
>>>>>> Then, another sync task hangs in filemap_fdatawait_range(),
>>>>>> because it waits the false under-writeback page.
>>>>>>
>>>>>> CPU0                            CPU1
>>>>>>
>>>>>> __extent_writepage_io()
>>>>>>      ret = submit_extent_page() // fail
>>>>>>
>>>>>>      if (ret)
>>>>>>        SetPageError(page)
>>>>>>        // miss clearing the writeback bit
>>>>>>
>>>>>>                                    sync()
>>>>>>                                      ...
>>>>>>                                      filemap_fdatawait_range()
>>>>>>                                        wait_on_page_writeback(page);
>>>>>>                                        // wait the false under-writeback page
>>>>>>
>>>>>> Signed-off-by: Takafumi Kubota <takafumi.kubota1012@...ab.ics.keio.ac.jp>
>>>>>> ---
>>>>>>     fs/btrfs/extent_io.c | 4 +++-
>>>>>>     1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>>>>>> index 1e67723..ef9793b 100644
>>>>>> --- a/fs/btrfs/extent_io.c
>>>>>> +++ b/fs/btrfs/extent_io.c
>>>>>> @@ -3443,8 +3443,10 @@ static noinline_for_stack int __extent_writepage_io(struct inode *inode,
>>>>>>     					 bdev->bio, max_nr,
>>>>>>     					 end_bio_extent_writepage,
>>>>>>     					 0, 0, 0, false);
>>>>>> -		if (ret)
>>>>>> +		if (ret) {
>>>>>>     			SetPageError(page);
>>>>>> +			end_page_writeback(page);
>>>>>> +		}
>>>>> OK...this could be complex as we don't know which part in
>>>>> submit_extent_page gets the error, if the page has been added into bio
>>>>> and bio_end would call end_page_writepage(page) as well, so whichever
>>>>> comes later, the BUG() in end_page_writeback() would complain.
>>>>>
>>>>> Looks like commit 55e3bd2e0c2e1 also has the same problem although I
>>>>> gave it my reviewed-by.
>>>>>
>>>>> Thanks,
>>>>>
>>>>> -liubo
>>>>>
>>>>>>     		cur = cur + iosize;
>>>>>>     		pg_offset += iosize;
>>>>>> -- 
>>>>>> 1.9.3
>>>>>>
>>>>>> --
>>>>>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>>>>>> the body of a message to majordomo@...r.kernel.org
>>>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>> -- 
>>>> Keio University
>>>> System Software Laboratory
>>>> Takafumi Kubota
>>>> takafumi.kubota1012@...ab.ics.keio.jp
>>>>
>> -- 
>> Keio University
>> System Software Laboratory
>> Takafumi Kubota
>> takafumi.kubota1012@...ab.ics.keio.jp
>>

-- 
Keio University
System Software Laboratory
Takafumi Kubota
takafumi.kubota1012@...ab.ics.keio.jp