[Qemu-devel] [PATCH 2/2] osdep: warn if opening a file O_DIRECT on tmpfs fails

[Stefan Hajnoczi <stefanha@redhat.com>]

Print a warning when opening a file O_DIRECT on tmpfs fails.  This saves
users a lot of time trying to figure out the EINVAL error.

Daniel P. Berrange <berrange@redhat.com> suggested opening the file
without O_DIRECT as a portable way to check whether the file system
supports O_DIRECT.  That gets messy when flags contains O_CREAT since
we'd create a file but return an error - or a race condition if we try
to unlink the file.  It's simpler to check the file system type.

Reported-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 util/osdep.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/util/osdep.c b/util/osdep.c
index 685c8ae..446a1dc 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -30,6 +30,11 @@
 #include <unistd.h>
 #include <fcntl.h>
 
+#ifdef __linux__
+#include <sys/vfs.h>
+#include <linux/magic.h>
+#endif
+
 /* Needed early for CONFIG_BSD etc. */
 #include "config-host.h"
 
@@ -207,6 +212,20 @@ int qemu_open(const char *name, int flags, ...)
     }
 #endif
 
+#ifdef __linux__
+    /* It is not possible to open files O_DIRECT on tmpfs.  Provide a hint that
+     * this may be the case (of course it could change in future kernel
+     * versions).
+     */
+    if (ret == -1 && errno == EINVAL && (flags & O_DIRECT)) {
+        struct statfs st;
+        if (statfs(name, &st) == 0 && st.f_type == TMPFS_MAGIC) {
+            error_report("tmpfs file systems may not support O_DIRECT");
+        }
+        errno = EINVAL; /* in case it was clobbered */
+    }
+#endif /* __linux__ */
+
     return ret;
 }
 
-- 
1.8.3.1