Re: [PATCH v2 2/4] mm: add file_fdatawait_range and file_write_and_wait

[Jeff Layton <jlayton@redhat.com>]

On Mon, 2017-07-31 at 12:32 +0100, Steven Whitehouse wrote:
> Hi,
> 
> 
> On 31/07/17 12:27, Jeff Layton wrote:
> > On Thu, 2017-07-27 at 08:48 -0400, Jeff Layton wrote:
> > > On Thu, 2017-07-27 at 10:49 +0200, Jan Kara wrote:
> > > > On Wed 26-07-17 13:55:36, Jeff Layton wrote:
> > > > > +int file_write_and_wait(struct file *file)
> > > > > +{
> > > > > +	int err = 0, err2;
> > > > > +	struct address_space *mapping = file->f_mapping;
> > > > > +
> > > > > +	if ((!dax_mapping(mapping) && mapping->nrpages) ||
> > > > > +	    (dax_mapping(mapping) && mapping->nrexceptional)) {
> > > > > +		err = filemap_fdatawrite(mapping);
> > > > > +		/* See comment of filemap_write_and_wait() */
> > > > > +		if (err != -EIO) {
> > > > > +			loff_t i_size = i_size_read(mapping->host);
> > > > > +
> > > > > +			if (i_size != 0)
> > > > > +				__filemap_fdatawait_range(mapping, 0,
> > > > > +							  i_size - 1);
> > > > > +		}
> > > > > +	}
> > > > 
> > > > Err, what's the i_size check doing here? I'd just pass ~0 as the end of the
> > > > range and ignore i_size. It is much easier than trying to wrap your head
> > > > around possible races with file operations modifying i_size.
> > > > 
> > > > 								Honza
> > > 
> > > I'm basically emulating _exactly_ what filemap_write_and_wait does here,
> > > as I'm leery of making subtle behavior changes in the actual writeback
> > > behavior. For example:
> > > 
> > > -----------------8<----------------
> > > static inline int __filemap_fdatawrite(struct address_space *mapping,
> > >          int sync_mode)
> > > {
> > >          return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);
> > > }
> > > 
> > > int filemap_fdatawrite(struct address_space *mapping)
> > > {
> > >          return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
> > > }
> > > EXPORT_SYMBOL(filemap_fdatawrite);
> > > -----------------8<----------------
> > > 
> > > ...which then sets up the wbc with the right ranges and sync mode and
> > > kicks off writepages. But then, it does the i_size_read to figure out
> > > what range it should wait on (with the shortcut for the size == 0 case).
> > > 
> > > My assumption was that it was intentionally designed that way, but I'm
> > > guessing from your comments that it wasn't? If so, then we can turn
> > > file_write_and_wait a static inline wrapper around
> > > file_write_and_wait_range.
> > 
> > FWIW, I did a bit of archaeology in the linux-history tree and found
> > this patch from Marcelo in 2004. Is this optimization still helpful? If
> > not, then that does simplify the code a bit.
> > 
> > -------------------8<--------------------
> > 
> > [PATCH] small wait_on_page_writeback_range() optimization
> > 
> > filemap_fdatawait() calls wait_on_page_writeback_range() with -1 as "end"
> > parameter.  This is not needed since we know the EOF from the inode.  Use
> > that instead.
> > 
> > Signed-off-by: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
> > Signed-off-by: Andrew Morton <akpm@osdl.org>
> > Signed-off-by: Linus Torvalds <torvalds@osdl.org>
> > ---
> >   mm/filemap.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 78e18b7639b6..55fb7b4141e4 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -287,7 +287,13 @@ EXPORT_SYMBOL(sync_page_range);
> >    */
> >   int filemap_fdatawait(struct address_space *mapping)
> >   {
> > -	return wait_on_page_writeback_range(mapping, 0, -1);
> > +	loff_t i_size = i_size_read(mapping->host);
> > +
> > +	if (i_size == 0)
> > +		return 0;
> > +
> > +	return wait_on_page_writeback_range(mapping, 0,
> > +				(i_size - 1) >> PAGE_CACHE_SHIFT);
> >   }
> >   EXPORT_SYMBOL(filemap_fdatawait);
> > 
> 
> Does this ever get called in cases where we would not hold fs locks? In 
> that case we definitely don't want to be relying on i_size,
> 
> Steve.
> 

Yes. We can initiate and wait on writeback from any context where you
can sleep, really.

We're just waiting on whole file writeback here, so I don't think
there's anything wrong. As long as the i_size was valid at some point in
time prior to waiting then you're ok.

The question I have is more whether this optimization is still useful. 

What we do now is just walk the radix tree and wait_on_page_writeback
for each page. Do we gain anything by avoiding ranges beyond the current
EOF with the pagecache infrastructure of 2017?

-- 
Jeff Layton <jlayton@redhat.com>

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