[PATCH] invalidate_inode_pages2_range() debug

A failure in invalidate_inode_pages2_range() can result in unpleasant things
happening in NFS (at least).  Stick a WARN_ON_ONCE() in there so we can find
out if it happens, and maybe why.

(akpm: might be a -mm-only patch, we'll see..)

Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Steve Dickson <SteveD@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/truncate.c

diff --git a/mm/truncate.c b/mm/truncate.c
index f4edbc1..fca2883 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -396,6 +396,7 @@
 		pagevec_release(&pvec);
 		cond_resched();
 	}
+	WARN_ON_ONCE(ret);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(invalidate_inode_pages2_range);