Re: [PATCH v3 5/6] netfs, 9p, afs, ceph: Use folios

[David Howells <dhowells@redhat.com>]

Matthew Wilcox <willy@infradead.org> wrote:

> On Wed, Nov 03, 2021 at 02:58:12PM +0000, David Howells wrote:
> > Matthew Wilcox <willy@infradead.org> wrote:
> > 
> > > > +	len = (size >= start + gran) ? gran : size - start;
> > > 
> > > This seems like the most complicated way to write this ... how about:
> > > 
> > >         size_t len = min_t(loff_t, isize - start, folio_size(folio));
> > 
> > I was trying to hedge against isize-start going negative.  Can this code race
> > against truncate?  truncate_setsize() changes i_size *before* invalidating the
> > pages.
> 
> We should check for isize < start separately, and skip the writeback
> entirely.

So, something like the following

	static int v9fs_vfs_write_folio_locked(struct folio *folio)
	{
		struct inode *inode = folio_inode(folio);
		struct v9fs_inode *v9inode = V9FS_I(inode);
		loff_t start = folio_pos(folio);
		loff_t i_size = i_size_read(inode);
		struct iov_iter from;
		size_t len = folio_size(folio);
		int err;

		if (start >= i_size)
			return 0; /* Simultaneous truncation occurred */

		len = min_t(loff_t, i_size - start, len);

		iov_iter_xarray(&from, ..., start, len);
		...
	}

David