From 4e553943c8fe4924d194884b4719c5459210c686 Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Tue, 26 Jan 2021 17:21:03 -0500
Subject: [PATCH 8/9] file-posix: Allow byte-aligned O_DIRECT with NFS

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20210126172103.136060-3-kwolf@redhat.com>
Patchwork-id: 100785
O-Subject: [RHEL-8.4.0 qemu-kvm PATCH 2/2] file-posix: Allow byte-aligned O_DIRECT with NFS
Bugzilla: 1834281
RH-Acked-by: Markus Armbruster <armbru@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>

Since commit a6b257a08e3 ('file-posix: Handle undetectable alignment'),
we assume that if we open a file with O_DIRECT and alignment probing
returns 1, we just couldn't find out the real alignment requirement
because some filesystems make the requirement only for allocated blocks.
In this case, a safe default of 4k is used.

This is too strict for NFS, which does actually allow byte-aligned
requests even with O_DIRECT. Because we can't distinguish both cases
with generic code, let's just look at the file system magic and disable
s->needs_alignment for NFS. This way, O_DIRECT can still be used on NFS
for images that are not aligned to 4k.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200716142601.111237-3-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 5edc85571e7b7269dce408735eba7507f18ac666)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jon Maloy <jmaloy.redhat.com>
---
 block/file-posix.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index adafbfa1be..2d834fbdf6 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -61,10 +61,12 @@
 #include <sys/ioctl.h>
 #include <sys/param.h>
 #include <sys/syscall.h>
+#include <sys/vfs.h>
 #include <linux/cdrom.h>
 #include <linux/fd.h>
 #include <linux/fs.h>
 #include <linux/hdreg.h>
+#include <linux/magic.h>
 #include <scsi/sg.h>
 #ifdef __s390__
 #include <asm/dasd.h>
@@ -298,6 +300,28 @@ static int probe_physical_blocksize(int fd, unsigned int *blk_size)
 #endif
 }
 
+/*
+ * Returns true if no alignment restrictions are necessary even for files
+ * opened with O_DIRECT.
+ *
+ * raw_probe_alignment() probes the required alignment and assume that 1 means
+ * the probing failed, so it falls back to a safe default of 4k. This can be
+ * avoided if we know that byte alignment is okay for the file.
+ */
+static bool dio_byte_aligned(int fd)
+{
+#ifdef __linux__
+    struct statfs buf;
+    int ret;
+
+    ret = fstatfs(fd, &buf);
+    if (ret == 0 && buf.f_type == NFS_SUPER_MAGIC) {
+        return true;
+    }
+#endif
+    return false;
+}
+
 /* Check if read is allowed with given memory buffer and length.
  *
  * This function is used to check O_DIRECT memory buffer and request alignment.
@@ -602,7 +626,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
 
     s->has_discard = true;
     s->has_write_zeroes = true;
-    if ((bs->open_flags & BDRV_O_NOCACHE) != 0) {
+    if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) {
         s->needs_alignment = true;
     }
 
-- 
2.18.2

