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

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Date:   Thu, 04 Nov 2021 14:32:18 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, Jeff Layton <jlayton@...nel.org>,
        Marc Dionne <marc.dionne@...istor.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        v9fs-developer@...ts.sourceforge.net,
        linux-afs@...ts.infradead.org, ceph-devel@...r.kernel.org,
        linux-cachefs@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        devel@...ts.orangefs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/6] netfs, 9p, afs, ceph: Use folios

Matthew Wilcox <willy@...radead.org> wrote:

> On Wed, Nov 03, 2021 at 02:58:12PM +0000, David Howells wrote:
> > Matthew Wilcox <willy@...radead.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