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

[Jeff Layton]

On Mon, 2017-07-31 at 14:07 +0200, Jan Kara wrote:
> On Mon 31-07-17 07:44:16, Jeff Layton wrote:
> > On Mon, 2017-07-31 at 12:32 +0100, Steven Whitehouse wrote:
> > > 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@xxxxxxxxxxxx>
> > > > Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
> > > > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>
> > > > ---
> > > >   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?
> 
> FWIW I'm not aware of any significant benefit of using i_size in
> filemap_fdatawait() - we iterate to the end of the radix tree node anyway
> since pagevec_lookup_tag() does not support range searches anyway (I'm
> working on fixing that however even after that the benefit would be still
> rather marginal).
> 
> What Marcello might have meant even back in 2004 was that if we are in the
> middle of truncate, i_size is already reduced but page cache not truncated
> yet, then filemap_fdatawait() does not have to wait for writeback of
> truncated pages. That might be a noticeable benefit even today if such race
> happens however I'm not sure it's worth optimizing for and surprises
> arising from randomly snapshotting i_size (which especially for clustered
> filesystems may be out of date) IMHO overweight the possible advantage.
> 
> 								Honza

Thanks for clarifying.

Given that file_write_and_wait is a new helper function anyway, I'll
just make it a wrapper around file_write_and_wait_range. Since it might
be racy, should remove this optimization from the "legacy"
filemap_fdatawait / filemap_fdatawait_keep_errors calls?

-- 
Jeff Layton <jlayton@xxxxxxxxxx>