[PATCH] Fix bad data from non-direct-io read after direct-io write

[Karl Schendel <kschendel@datallegro.com>]

This patch fixes a race between direct IO writes and non-direct IO
reads on the same file.  The symptom is a stale file page seen by
any non-direct-IO reader, which persists until the page is invalidated
somehow (e.g. page rewritten again, or memory pressure, or reboot).

An improper return test caused direct-IO's after-write page invalidations
to be skipped.  If we're writing page N, and the reader is reading
page N-x for small x, and the read code decides to readahead, it's
not too hard to cause a race that leaves an old, stale copy of the
page in the page cache.  Retval is usually +nonzero after the
mapping->a_ops->direct_IO call!

Signed-off-by: Karl Schendel <kschendel@datallegro.com>

---

By the way, I agree that the userland situation is stupid, and I'm
addressing that in the application (happens to be the Ingres DBMS).
However, the kernel shouldn't compound the stupidity.

I'll try to watch for replies, but it would be very useful to
cc me at kschendel@datallegro.com if any discussion is needed;
I'm not subscribed to lkml.


--- linux-2.6.23.1-base/mm/filemap.c	2007-10-12 12:43:44.000000000 -0400
+++ linux-2.6.23.1/mm/filemap.c	2007-10-26 16:12:08.000000000 -0400
@@ -2194,7 +2194,7 @@ generic_file_direct_IO(int rw, struct ki
 	}

 	retval = mapping->a_ops->direct_IO(rw, iocb, iov, offset, nr_segs);
-	if (retval)
+	if (retval < 0)
 		goto out;

 	/*