linux-ext4 - Re: potential memory leak on transaction commit

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Message-ID: <alpine.OSX.2.00.1103082241230.58225@zhanghuan.mc371>
Date:	Tue, 8 Mar 2011 22:43:40 +0800 (CST)
From:	Zhang Huan <zhhuan@...il.com>
To:	Lukas Czerner <lczerner@...hat.com>
cc:	linux-ext4@...r.kernel.org
Subject: Re: potential memory leak on transaction commit


On Tue, 8 Mar 2011, Lukas Czerner wrote:

> On Tue, 8 Mar 2011, Zhang Huan wrote:
>
>> Hi all,
>>
>> There is potential memory leak of journal head in function
>> jbd2_journal_commit_transaction. The problem is that JBD2 will not
>> reclaim the journal head of commit record if error occurs or journal is
>> abotred.
>>
>> I use the following script to reproduce this issue, on a RHEL6 system. I
>> found it very easy to reproduce with async commit enabled.
>>
>> mount /dev/sdb /mnt -o journal_checksum,journal_async_commit
>> touch /mnt/xxx
>> echo offline > /sys/block/sdb/device/state
>> sync
>> umount /mnt
>> rmmod ext4
>> rmmod jbd2
>>
>> Removal of the jbd2 module will make slab complaining that
>> "cache `jbd2_journal_head': can't free all objects".
>>
>>
>> Here is my fix for this issue. The commit record should be reclaimed no
>> matter error occurs or not.
>>
>> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
>> index f3ad159..37a973a 100644
>> --- a/fs/jbd2/commit.c
>> +++ b/fs/jbd2/commit.c
>> @@ -105,6 +105,8 @@ static int journal_submit_commit_record(journal_t *journal,
>>  	int ret;
>>  	struct timespec now = current_kernel_time();
>>
>> +	*cbh = NULL;
>> +
>>  	if (is_journal_aborted(journal))
>>  		return 0;
>>
>> @@ -808,7 +810,7 @@ wait_for_iobuf:
>>  		if (err)
>>  			__jbd2_journal_abort_hard(journal);
>>  	}
>> -	if (!err && !is_journal_aborted(journal))
>> +	if (cbh)
>
> Hi,
>
> I wonder if we could do rather this:
>
> 	if (!err && !is_journal_aborted(journal))
> 		err = journal_wait_on_commit_record(journal, cbh);
> 	else if (cbh) {
> 		put_bh(cbh);
> 		jbd2_journal_put_journal_head(bh2jh(cbh));
> 	}
>
> I think this is more readable...

Hi,

I don't think it is a good idea. Anyway, you need to wait for buffer to 
complete, and then release journal head on it. That is exactly what 
journal_wait_on_commit_record does.

>
> Thanks!
> -Lukas
>
>>  		err = journal_wait_on_commit_record(journal, cbh);
>>  	if (JBD2_HAS_INCOMPAT_FEATURE(journal,
>>  				      JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT) &&
>>
>>
>>
>> PS: Just out of curiosity, why would journal_submit_commit_record return a
>> value of 1 instead of an error number if get descriptor buffer is failed.
>>
>>
>> Zhang Huan
>> --
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>>
>
> -- 
>

Zhang Huan
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