On 07/13/2018 06:14 AM, Max Reitz wrote:
Past the end of the source backing file, we memset() buf_old to zero, so
it is clearly easy to use blk_pwrite_zeroes() instead of blk_pwrite()
then.
Signed-off-by: Max Reitz <mre...@redhat.com>
---
qemu-img.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/qemu-img.c b/qemu-img.c
index dd684d8bf0..2552e7dad6 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3403,6 +3403,8 @@ static int img_rebase(int argc, char **argv)
}
for (offset = 0; offset < size; offset += n) {
+ bool buf_old_is_zero = false;
+
/* How many bytes can we handle with the next read? */
n = MIN(IO_BUF_SIZE, size - offset);
@@ -3423,6 +3425,7 @@ static int img_rebase(int argc, char **argv)
*/
if (offset >= old_backing_size) {
memset(buf_old, 0, n);
+ buf_old_is_zero = true;
Do we still need to spend time on the memset(), or...
} else {
if (offset + n > old_backing_size) {
n = old_backing_size - offset;
@@ -3458,8 +3461,12 @@ static int img_rebase(int argc, char **argv)
if (compare_buffers(buf_old + written, buf_new + written,
n - written, &pnum))
{
- ret = blk_pwrite(blk, offset + written,
- buf_old + written, pnum, 0);
+ if (buf_old_is_zero) {
+ ret = blk_pwrite_zeroes(blk, offset + written, pnum,
0);
...are we able to guarantee that old_buf will not be used when
buf_old_is_zero?
+ } else {
+ ret = blk_pwrite(blk, offset + written,
+ buf_old + written, pnum, 0);
+ }
if (ret < 0) {
error_report("Error while writing to COW image: %s",
strerror(-ret));
The series seems reasonable, but is post-3.0 material, so I haven't
reviewed it any closer than this question.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
