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

David Howells
2021-11-04T14:32:46Z

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