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

[Jeff Layton <jlayton@redhat.com>]

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);