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Subject: Re: [PATCH] ext4: properly check for dirty state in
 ext4_inode_datasync_dirty()
To:     harshad shirwadkar <harshadshirwadkar@gmail.com>,
        Andrea Righi <andrea.righi@canonical.com>
Cc:     "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        linux-kernel@vger.kernel.org
References: <20201024140115.GA35973@xps-13-7390>
 <CAD+ocby3hA0GCm5Rf6T3UF+2UWgWoUjrz7=VzbeUMjX6Qx8D5g@mail.gmail.com>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Message-ID: <da6697a0-4a23-ee68-fa2e-121b3d23c972@linux.ibm.com>
Date:   Wed, 28 Oct 2020 08:57:03 +0530
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On 10/27/20 3:58 AM, harshad shirwadkar wrote:
> Thanks Andrea for catching and sending out a fix for this.
> 
> On Sat, Oct 24, 2020 at 7:01 AM Andrea Righi <andrea.righi@canonical.com> wrote:
>>
>> ext4_inode_datasync_dirty() needs to return 'true' if the inode is
>> dirty, 'false' otherwise, but the logic seems to be incorrectly changed
>> by commit aa75f4d3daae ("ext4: main fast-commit commit path").
>>
>> This introduces a problem with swap files that are always failing to be
>> activated, showing this error in dmesg:
>>
>>   [   34.406479] swapon: file is not committed
>>

Well, I too noticed this yesterday while I was testing xfstests -g swap.
Those tests were returning _notrun, hence that could be the reason why
it didn't get notice in XFSTESTing from Ted.

- I did notice that this code was introduced in v10 only.
This wasn't there in v9 though.


>> Simple test case to reproduce the problem:
>>
>>    # fallocate -l 8G swapfile
>>    # chmod 0600 swapfile
>>    # mkswap swapfile
>>    # swapon swapfile
>>
>> Fix the logic to return the proper state of the inode.
>>
>> Link: https://lore.kernel.org/lkml/20201024131333.GA32124@xps-13-7390
>> Fixes: aa75f4d3daae ("ext4: main fast-commit commit path")
>> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
>> ---
>>   fs/ext4/inode.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 03c2253005f0..a890a17ab7e1 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -3308,8 +3308,8 @@ static bool ext4_inode_datasync_dirty(struct inode *inode)
>>          if (journal) {
>>                  if (jbd2_transaction_committed(journal,
>>                                          EXT4_I(inode)->i_datasync_tid))
>> -                       return true;
>> -               return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) >=
>> +                       return false;
>> +               return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) <
>>                          EXT4_I(inode)->i_fc_committed_subtid;
> In addition, the above condition should only be checked if fast
> commits are enabled. So, in effect this overall condition will look
> like this:
> 
> if (journal) {
>      if (jbd2_transaction_committed(journal, EXT4_I(inode)->i_datasync_tid))
>          return false;
>      if (test_opt2(sb, JOURNAL_FAST_COMMIT))
>          return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) <
> EXT4_I(inode)->i_fc_committed_subtid;
>      return true;
> }

Yup - I too had made a similar patch. But then I also noticed that below
condition will always remain false. Since we never update
"i_fc_committed_subtid" other than at these 2 places
(one during init where we set it to 0 and other during ext4_fc_commit()
where we set it to sbi->s_fc_subtid).

<condition>
atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid < 
EXT4_I(inode)->i_fc_committed_subtid


Maybe I need more reading around this.

-ritesh


> 
> Thanks,
> Harshad
> 
>>          }
>>
>> --
>> 2.27.0
>>