linux-kernel - RE: open(O_DIRECT) on a tmpfs?

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Date:	Thu, 4 Jan 2007 22:57:49 -0800
From:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
To:	"'Hugh Dickins'" <hugh@...itas.com>,
	"Hua Zhong" <hzhong@...il.com>,
	"'Nick Piggin'" <nickpiggin@...oo.com.au>
Cc:	"Christoph Hellwig" <hch@...radead.com>,
	"'Bill Davidsen'" <davidsen@....com>,
	"'Linux-kernel'" <linux-kernel@...r.kernel.org>
Subject: RE: open(O_DIRECT) on a tmpfs?

Hugh Dickins wrote on Thursday, January 04, 2007 11:14 AM
> On Thu, 4 Jan 2007, Hua Zhong wrote:
> > So I'd argue that it makes more sense to support O_DIRECT
> > on tmpfs as the memory IS the backing store.
> 
> A few more voices in favour and I'll be persuaded.  Perhaps I'm
> out of date: when O_DIRECT came in, just a few filesystems supported
> it, and it was perfectly normal for open O_DIRECT to be failed; but
> I wouldn't want tmpfs to stand out now as a lone obstacle.

Maybe a bit hackish, all we need is to have an empty .direct_IO method
in shmem_aops to make __dentry_open() to pass the O_DIRECT check.  The
following patch adds 40 bytes to kernel text on x86-64.  An even more
hackish but zero cost route is to make .direct_IO variable non-zero via
a cast of -1 or some sort (that is probably ugly as hell).


diff -Nurp linus-2.6.git/mm/shmem.c linus-2.6.git.ken/mm/shmem.c
--- linus-2.6.git/mm/shmem.c	2006-12-27 19:06:11.000000000 -0800
+++ linus-2.6.git.ken/mm/shmem.c	2007-01-04 21:03:14.000000000 -0800
@@ -2314,10 +2314,18 @@ static void destroy_inodecache(void)
 	kmem_cache_destroy(shmem_inode_cachep);
 }
 
+ssize_t shmem_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
+			loff_t offset, unsigned long nr_segs)
+{
+	/* dummy direct_IO function.  Not to be executed */
+	BUG();
+}
+
 static const struct address_space_operations shmem_aops = {
 	.writepage	= shmem_writepage,
 	.set_page_dirty	= __set_page_dirty_nobuffers,
 #ifdef CONFIG_TMPFS
+	.direct_IO	= shmem_direct_IO,
 	.prepare_write	= shmem_prepare_write,
 	.commit_write	= simple_commit_write,
 #endif
-
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