[Qemu-devel] [PATCH 1/2] block: use fallocate(FALLOC_FL_ZERO_RANGE) in handle_aiocb_write_zeroes

["Denis V. Lunev" <den@openvz.org>]

this efficiently writes zeroes in the middle of the file on Linux
systems if the kernel is capable enough.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/raw-posix.c | 11 +++++++++++
 configure         | 19 +++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index e51293a..9e66cb7 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -919,6 +919,17 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb)
             return xfs_write_zeroes(s, aiocb->aio_offset, aiocb->aio_nbytes);
         }
 #endif
+
+#ifdef CONFIG_FALLOCATE_ZERO_RANGE
+        do {
+            if (fallocate(s->fd, CONFIG_FALLOCATE_ZERO_RANGE,
+                          aiocb->aio_offset, aiocb->aio_nbytes) == 0) {
+                return 0;
+            }
+        } while (errno == EINTR);
+
+        ret = -errno;
+#endif
     }
 
     if (ret == -ENODEV || ret == -ENOSYS || ret == -EOPNOTSUPP ||
diff --git a/configure b/configure
index cae588c..dfcf7b3 100755
--- a/configure
+++ b/configure
@@ -3309,6 +3309,22 @@ if compile_prog "" "" ; then
   fallocate_punch_hole=yes
 fi
 
+# check that fallocate supports range zeroing inside the file
+fallocate_zero_range=no
+cat > $TMPC << EOF
+#include <fcntl.h>
+#include <linux/falloc.h>
+
+int main(void)
+{
+    fallocate(0, FALLOC_FL_ZERO_RANGE, 0, 0);
+    return 0;
+}
+EOF
+if compile_prog "" "" ; then
+  fallocate_zero_range=yes
+fi
+
 # check for posix_fallocate
 posix_fallocate=no
 cat > $TMPC << EOF
@@ -4538,6 +4554,9 @@ fi
 if test "$fallocate_punch_hole" = "yes" ; then
   echo "CONFIG_FALLOCATE_PUNCH_HOLE=y" >> $config_host_mak
 fi
+if test "$fallocate_zero_range" = "yes" ; then
+  echo "CONFIG_FALLOCATE_ZERO_RANGE=y" >> $config_host_mak
+fi
 if test "$posix_fallocate" = "yes" ; then
   echo "CONFIG_POSIX_FALLOCATE=y" >> $config_host_mak
 fi
-- 
1.9.1