batched write

["Vladimir V. Saveliev" <vs@namesys.com>]

Hello

On Thu, 2006-06-08 at 13:10 +0100, Christoph Hellwig wrote:
> On Thu, Jun 08, 2006 at 03:26:40PM +0400, Vladimir V. Saveliev wrote:
> > > > It may go to the kernel as a 64MB write, but VFS sends it to the FS as
> > > > 64MB/4k separate 4k writes.
> > > 
> > > Nonsense, 
> > 
> > Hans refers to generic_file_write which does
> > prepare_write
> > copy_from_user
> > commit_write
> > for each page.
> 
> That's not really the vfs but the generic pagecache routines.  For some
> filesystems (e.g. XFS) only reservations for delayed allocations are
> performed in this path so it doesn't really matter.  For not so advanced
> filesystems batching these calls would definitly be very helpful.  Patches
> to get there are very welcome.
> 

The core of generic_file_buffered_write is 
do {
	grab_cache_page();
	a_ops->prepare_write();
	copy_from_user();
	a_ops->commit_write();
	
	filemap_set_next_iovec();
	balance_dirty_pages_ratelimited();
} while (count);


Would it make sence to rework this code with adding new address_space
operation - fill_pages so that looks like:

do {
	a_ops->fill_pages();
	filemap_set_next_iovec();
	balance_dirty_pages_ratelimited();
} while (count);

generic implementation of fill_pages would look like:

generic_fill_pages()
{
	grab_cache_page();
	a_ops->prepare_write();
	copy_from_user();
	a_ops->commit_write();
}

I believe that filesystem developers will want to exploit that
operation. 

Any opinion on this plan is welcomed. I would try to code whatever we
will have developed (I hope) in result of this discussion.