linux-ext4 - Re: [PATCH] ext4: check missed return value ext4_sync

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Date:	Fri, 12 Mar 2010 20:20:04 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] ext4: check missed return value ext4_sync_file

Dmitry Monakhov <dmonakhov@...nvz.org> writes:

> Jan Kara <jack@...e.cz> writes:
>
>>> We have to submit barrier before we start journal commit process.
>>> otherwise transaction may be committed before data flushed to disk.
>>> There is no difference from performance of view, but definitely
>>> fsync becomes more correct.
> Unfortunately this change does affect performance because latency
> will be increased since we have to wait barrier before we start
> journal commit. 
>>> 
>>> If jbd2_log_start_commit return 0 then it means that transaction
>>> was already committed. So we don't have to issue barrier for
>>> ordered mode, because it was already done during commit.
>>   Umm, we have to - when a file has just been rewritten (i.e. no block
>> allocation), then i_datasync_tid is not updated and thus we won't commit
>> any transaction as a part of fdatasync (and that is correct because there
>> are no metadata that need to be written for that fdatasync). But we still
>> have to flush disk caches with data submitted by filemap_fdatawrite_and_wait.
> Yepp. I've missed that. i thought that transaction id updated
> even in that case.
> The most unpleasant part in ext4_sync_file implementation is that 
> barrier is issued on each fsync() call.  So some bad user may perform:
> while(1) fsync(fd);
> which result in bad system performance. And since barrier request is 
> empty it is hard to detect the reason of troubles.
> Off course we may solve it by introducing some sort of dirty flag
> which is set in write_page, and clear in fsync. But it looks as
> ugly workaround.
>>
>>> By unknown reason we ignored ret val from jbd2_log_wait_commit()
>>> so even in case of EIO fsync will succeed.
>>   I just forgot jbd2_log_wait_commit can return a failure...
> In respect to previous comments the patch reduced to simple missed
> error check fix.
It is fun but I've found what journalled mode is still broken in ext4
in case of external journal. We forget to issue io-barrier to j_fs_dev 
if transaction has only metadata and has no data blocks :)
This affect all data modes.
It is easy to reproduce on classic test-case with data=journall
for(i=0; i < 3; i++) {
  memset(buf, 'a'+i);
  pwrite(fd, buf, 1024*1024, 0)
  fsync(fd);
}
/* At this time transaction was committed so journal is empty */
<<POWER_OFF
Later i've found old data('b' chars) at the end of the file.
So i've prepared another patch  which supersede previous one.
> BTW: While investigating similar code in ext3 i've found what
> fsync is broken in case of external journal. JBD itself does not
> send barrier to j_fs_dev. So if fsync goes via
> log_start_commit/log_wait_commit path data loss is still possible.
> I'm able to reproduce this via simple write test
> wile (1) {
>  write(fd, buf, 1024*1024)
>  fsync(fd);
> }
> and then reboot in the middle of operation.
> Later file content check spotted data inconsistency.
> Will send a fix ASAP.
>
> From 1f7382ea4a8b8e3880e1938d161f924ea572a1e1 Mon Sep 17 00:00:00 2001
> From: Dmitry Monakhov <dmonakhov@...nvz.org>
> Date: Thu, 11 Mar 2010 20:14:13 +0300
> Subject: [PATCH] ext4: check missed return value ext4_sync_file
>
>
> Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> ---
>  fs/ext4/fsync.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
> index 0d0c323..42bd94a 100644
> --- a/fs/ext4/fsync.c
> +++ b/fs/ext4/fsync.c
> @@ -101,7 +101,7 @@ int ext4_sync_file(struct file *file, struct dentry *dentry, int datasync)
>  		    (journal->j_fs_dev != journal->j_dev) &&
>  		    (journal->j_flags & JBD2_BARRIER))
>  			blkdev_issue_flush(inode->i_sb->s_bdev, NULL);
> -		jbd2_log_wait_commit(journal, commit_tid);
> +		ret = jbd2_log_wait_commit(journal, commit_tid);
>  	} else if (journal->j_flags & JBD2_BARRIER)
>  		blkdev_issue_flush(inode->i_sb->s_bdev, NULL);
>  	return ret;
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