Re: ext3: call blkdev_issue_flush on fsync

[Fernando Luis Vázquez Cao]

On Fri, 2009-01-16 at 17:30 +0100, Jan Kara wrote:
> On Fri 16-01-09 22:55:01, Fernando Luis VÃzquez Cao wrote:
> > To ensure that bits are truly on-disk after an fsync or fdatasync, we
> > should force a disk flush explicitly when there is dirty data/metadata
> > and the journal didn't emit a write barrier (either because metadata is
> > not being synched or barriers are disabled).
> > 
> > Signed-off-by: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
> > ---
>   Only two minor nits:
> 
> > --- linux-2.6.29-rc1-orig/fs/ext3/fsync.c	2008-12-25 08:26:37.000000000 +0900
> > +++ linux-2.6.29-rc1/fs/ext3/fsync.c	2009-01-16 22:18:53.000000000 +0900
> > @@ -27,6 +27,7 @@
> >  #include <linux/sched.h>
> >  #include <linux/writeback.h>
> >  #include <linux/jbd.h>
> > +#include <linux/blkdev.h>
> >  #include <linux/ext3_fs.h>
> >  #include <linux/ext3_jbd.h>
> >  
> > @@ -45,6 +46,8 @@
> >  int ext3_sync_file(struct file * file, struct dentry *dentry, int datasync)
> >  {
> >  	struct inode *inode = dentry->d_inode;
> > +	journal_t *journal = EXT3_SB(inode->i_sb)->s_journal;
> > +	unsigned long i_state = inode->i_state;
> >  	int ret = 0;
> >  
> >  	J_ASSERT(ext3_journal_current_handle() == NULL);
> > @@ -69,23 +72,33 @@ int ext3_sync_file(struct file * file, s
> >  	 */
> >  	if (ext3_should_journal_data(inode)) {
> >  		ret = ext3_force_commit(inode->i_sb);
> > +		if (!(journal->j_flags & JFS_BARRIER))
> > +			goto no_journal_barrier;
> >  		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 & JFS_BARRIER))
> > +			goto no_journal_barrier;
>   I cannot imagine "journal" will be NULL here.

I'll try to check whether that is always so just in case.

>   And we can also optimize here a bit and do "goto out" because here
> we know the barrier has been issued.

Yep, I was considering the same optimization. By the way, I was
wondering if we should honor ext3 and ext4's "barrier" mount option for
sys_fsync()/sys_fdatasync() and do not force a flush when "barrier=1".
What are your thoughts on this?

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