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 <kw...@redhat.com>
Reviewed-by: Eric Blake <ebl...@redhat.com>
Message-Id: <20200716142601.111237-3-kw...@redhat.com>
Reviewed-by: Max Reitz <mre...@redhat.com>
Signed-off-by: Kevin Wolf <kw...@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 8067e238cb..ae8190edab 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -62,10 +62,12 @@
#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>
@@ -300,6 +302,28 @@ 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.
@@ -629,7 +653,7 @@ 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;
}
--
2.25.4
