linux-kernel - Re: [PATCH v2 2/4] mm: add file_fdatawait_range and file_write_and

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Date:   Thu, 27 Jul 2017 08:48:30 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Jan Kara <jack@...e.cz>, Jeff Layton <jlayton@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Matthew Wilcox <willy@...radead.org>,
        Bob Peterson <rpeterso@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        cluster-devel@...hat.com
Subject: Re: [PATCH v2 2/4] mm: add file_fdatawait_range and
 file_write_and_wait

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.
-- 
Jeff Layton <jlayton@...hat.com>