Re: ext2 + -osync: not as easy as it seems

["Fernando Luis Vázquez Cao" <fernando@oss.ntt.co.jp>]

On Fri, 2009-01-16 at 00:45 +0100, Jan Kara wrote:
> On Thu 15-01-09 21:06:51, Fernando Luis Vázquez Cao wrote:
> > On Wed, 2009-01-14 at 11:59 -0500, Theodore Tso wrote: 
> > > On Wed, Jan 14, 2009 at 03:37:56PM +0100, Jan Kara wrote:
> > > > > Um, we have that already; the sync_inode() followed by
> > > > > blkdev_issue_flush() is the path taken by fdatasync(), I do believe.
> > > >
> > > >   Maybe ext4-patch-queue changes that area but in Linus's tree I see:
> > > > 
> > > >   if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
> > > >          goto out;
> > > > 
> > > >   So if we just overwrite some data, we send them to disk via fdatawrite()
> > > > and then we quickly bail out from ext4_sync_file() without doing
> > > > blkdev_issue_flush().
> > > 
> > > So you're thinking about fdatawrite() being called by some code path
> > > other than ext4_sync_file() before we call fsync()?  Yeah, that could
> > > happen....  I think that will only happen if the file is opened
> > > O_SYNC, but that raises another issue, which is that we're not forcing
> > > a flush for writes when the file is opened O_SYNC.
> > 
> > Hi Jan, Ted
> > 
> > Is something like the patch below what you had in mind?
> > 
> > --
> > 
> > From: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
> > Subject: ext3: call blkdev_issue_flush on fsync
> > 
> > To ensure that bits are truly on-disk after an fsync or fdatasync, we
> > should call blkdev_issue_flush if barriers are supported.
> > 
> > Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
> > ---
> > 
> > --- linux-2.6.29-rc1-orig/fs/ext4/fsync.c	2008-12-25 08:26:37.000000000 +0900
> > +++ linux-2.6.29-rc1/fs/ext4/fsync.c	2009-01-15 21:03:19.000000000 +0900
> > @@ -48,6 +48,7 @@ int ext4_sync_file(struct file *file, st
> >  {
> >  	struct inode *inode = dentry->d_inode;
> >  	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
> > +	unsigned long i_state = inode->i_state;
> >  	int ret = 0;
> >  
> >  	J_ASSERT(ext4_journal_current_handle() == NULL);
> > @@ -79,22 +80,35 @@ int ext4_sync_file(struct file *file, st
> >  		goto out;
> >  	}
> >  
> > -	if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
> > -		goto out;
> > +	if (datasync && !(i_state & I_DIRTY_DATASYNC))
> > +		goto flush_blkdev;
> >  
> >  	/*
> >  	 * The VFS has written the file data.  If the inode is unaltered
> >  	 * then we need not start a commit.
> >  	 */
> > -	if (inode->i_state & (I_DIRTY_SYNC|I_DIRTY_DATASYNC)) {
> > +	if (i_state & (I_DIRTY_SYNC|I_DIRTY_DATASYNC)) {
> >  		struct writeback_control wbc = {
> >  			.sync_mode = WB_SYNC_ALL,
> >  			.nr_to_write = 0, /* sys_fsync did this */
> >  		};
> >  		ret = sync_inode(inode, &wbc);
> > -		if (journal && (journal->j_flags & JBD2_BARRIER))
> > -			blkdev_issue_flush(inode->i_sb->s_bdev, NULL);
> > +		/*
> > +		 * When there are no blocks attached to the journal transaction
> > +		 * some optimizations are possible, but if there were dirty
> > +		 * pages sync_inode() should have ensured that all data gets
> > +		 * actually written to disk. Thus, we can skip
> > +		 * blkdev_issue_flush() below.
> > +		 */
> > +		if (!(i_state & I_DIRTY_PAGES))
> > +			goto flush_blkdev;
>   Uh. Here I don't get it. When we did sync_inode(), blkdev_issue_flush()
> is needed only if the journal does not do barriers. So I'd expect here:
> 	if (!(journal->j_flags & JBD2_BARRIER))
> 		goto flush_blkdev;
> 	goto out;

Ups, you are right. I somehow managed to mangle the logic that I
intended to put here and under flush_blkdev. By the way, I think that
the same check may be needed for the data==journal case too.

Thank you for the feedback, Jan.

I'll be replying to this email with new patches for ext2/ext3.

- Fernando