[PATCH V2] libxfs: use FALLOC_FL_ZERO_RANGE in libxfs_device_zero

[Eric Sandeen <sandeen@sandeen.net>]

I had a request from someone who cared about mkfs speed(!)
over a slower network block device to look into using faster
zeroing methods, particularly for the log, during mkfs.

Using FALLOC_FL_ZERO_RANGE is faster in this case than writing
a bunch of zeros across a wire.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

V2: Clean up all the nasty stuff I'd flung out there as a wild first
cut, thanks Dave.

diff --git a/include/builddefs.in b/include/builddefs.in
index 4700b527..1dd27f76 100644
--- a/include/builddefs.in
+++ b/include/builddefs.in
@@ -144,6 +144,9 @@ endif
 ifeq ($(HAVE_GETFSMAP),yes)
 PCFLAGS+= -DHAVE_GETFSMAP
 endif
+ifeq ($(HAVE_FALLOCATE),yes)
+PCFLAGS += -DHAVE_FALLOCATE
+endif
 
 LIBICU_LIBS = @libicu_LIBS@
 LIBICU_CFLAGS = @libicu_CFLAGS@
diff --git a/include/linux.h b/include/linux.h
index 8f3c32b0..8d5c4584 100644
--- a/include/linux.h
+++ b/include/linux.h
@@ -20,6 +20,10 @@
 #include <stdio.h>
 #include <asm/types.h>
 #include <mntent.h>
+#include <fcntl.h>
+#if defined(HAVE_FALLOCATE)
+#include <linux/falloc.h>
+#endif
 #ifdef OVERRIDE_SYSTEM_FSXATTR
 # define fsxattr sys_fsxattr
 #endif
@@ -164,6 +168,24 @@ static inline void platform_mntent_close(struct mntent_cursor * cursor)
 	endmntent(cursor->mtabp);
 }
 
+#if defined(FALLOC_FL_ZERO_RANGE)
+static inline int
+platform_zero_range(
+	int		fd,
+	xfs_off_t	start,
+	size_t		len)
+{
+	int ret;
+
+	ret = fallocate(fd, FALLOC_FL_ZERO_RANGE, start, len);
+	if (!ret)
+		return 0;
+	return -errno;
+}
+#else
+#define platform_zero_range(fd, s, l)	(-EOPNOTSUPP)
+#endif
+
 /*
  * Check whether we have to define FS_IOC_FS[GS]ETXATTR ourselves. These
  * are a copy of the definitions moved to linux/uapi/fs.h in the 4.5 kernel,
diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
index 0d9d7202..2f6a3eb3 100644
--- a/libxfs/rdwr.c
+++ b/libxfs/rdwr.c
@@ -60,9 +60,19 @@ int
 libxfs_device_zero(struct xfs_buftarg *btp, xfs_daddr_t start, uint len)
 {
 	xfs_off_t	start_offset, end_offset, offset;
-	ssize_t		zsize, bytes;
+	ssize_t		zsize, bytes, len_bytes;
 	char		*z;
-	int		fd;
+	int		error, fd;
+
+	fd = libxfs_device_to_fd(btp->dev);
+	start_offset = LIBXFS_BBTOOFF64(start);
+	end_offset = LIBXFS_BBTOOFF64(start + len) - start_offset;
+
+	/* try to use special zeroing methods, fall back to writes if needed */
+	len_bytes = LIBXFS_BBTOOFF64(len);
+	error = platform_zero_range(fd, start_offset, len_bytes);
+	if (!error)
+		return 0;
 
 	zsize = min(BDSTRAT_SIZE, BBTOB(len));
 	if ((z = memalign(libxfs_device_alignment(), zsize)) == NULL) {
@@ -73,9 +83,6 @@ libxfs_device_zero(struct xfs_buftarg *btp, xfs_daddr_t start, uint len)
 	}
 	memset(z, 0, zsize);
 
-	fd = libxfs_device_to_fd(btp->dev);
-	start_offset = LIBXFS_BBTOOFF64(start);
-
 	if ((lseek(fd, start_offset, SEEK_SET)) < 0) {
 		fprintf(stderr, _("%s: %s seek to offset %llu failed: %s\n"),
 			progname, __FUNCTION__,
@@ -83,7 +90,6 @@ libxfs_device_zero(struct xfs_buftarg *btp, xfs_daddr_t start, uint len)
 		exit(1);
 	}
 
-	end_offset = LIBXFS_BBTOOFF64(start + len) - start_offset;
 	for (offset = 0; offset < end_offset; ) {
 		bytes = min((ssize_t)(end_offset - offset), zsize);
 		if ((bytes = write(fd, z, bytes)) < 0) {