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 out of date: when O_DIRECT came in, just a few filesystems supported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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