Re: [patch 6/7] fs: fsync optimisations

[Nick Piggin <npiggin@kernel.dk>]

On Mon, Nov 29, 2010 at 10:03:25AM -0500, Christoph Hellwig wrote:
> On Wed, Nov 24, 2010 at 01:06:16AM +1100, npiggin@kernel.dk wrote:
> > Optimise fsync by adding a datasync parameter to sync_inode_metadata to
> > DTRT with writing back the inode (->write_inode in theory should have a
> > datasync parameter too perhaps, but that's for another time).
> > 
> > Also, implement the metadata sync optimally rather than reusing the
> > normal data writeback path. This means less useless moving the inode around the
> > writeback lists, and less dropping and retaking of inode_lock, and avoiding
> > the data writeback call with nr_pages == 0.
> 
> This patch looks good to me, but a few minor comments below:

(BTW. it actually had a bug with writeback_end not being called in some
cases, in case you test it)

> > @@ -1315,13 +1315,49 @@ EXPORT_SYMBOL(sync_inode);
> >   *
> >   * Note: only writes the actual inode, no associated data or other metadata.
> >   */
> > -int sync_inode_metadata(struct inode *inode, int wait)
> > +int sync_inode_metadata(struct inode *inode, int datasync, int wait)
> >  {
> > +	struct address_space *mapping = inode->i_mapping;
> >  	struct writeback_control wbc = {
> >  		.sync_mode = wait ? WB_SYNC_ALL : WB_SYNC_NONE,
> >  		.nr_to_write = 0, /* metadata-only */
> >  	};
> > +	unsigned dirty, mask;
> > +	int ret = 0;
> >  
> > -	return sync_inode(inode, &wbc);
> > +	/*
> > +	 * This is a similar implementation to writeback_single_inode.
> > +	 * Keep them in sync.
> > +	 */
> 
> I'd move this comment to above the function.

OK

> 
> > +	/*
> > +	 * Generic write_inode doesn't distinguish between sync and datasync,
> > +	 * so even a datasync can clear the sync state. Filesystems which
> > +	 * distiguish these cases must only clear 'mask' in their metadata
> > +	 * sync code.
> > +	 */
> 
> I don't understand this comment.  Currenly filesystems never clear
> i_state bits by themselves.

No, but with the new inode writeback routines they could. Basically
they might be expected to copy this function, and put their own
improvements in.