[v4,15/23] qemu-img: Add find_nonzero()

Message ID 20170913160333.23622-16-eblake@redhat.com
State New
Headers show
Series
  • make bdrv_get_block_status byte-based
Related show

Commit Message

Eric Blake Sept. 13, 2017, 4:03 p.m.
During 'qemu-img compare', when we are checking that an allocated
portion of one file is all zeros, we don't need to waste time
computing how many additional sectors after the first non-zero
byte are also non-zero.  Create a new helper find_nonzero() to do
the check for a first non-zero sector, and rebase
check_empty_sectors() to use it.

The new interface intentionally uses bytes in its interface, even
though it still crawls the buffer a sector at a time; it is robust
to a partial sector at the end of the buffer.

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v3: new patch
---
 qemu-img.c | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

Comments

John Snow Sept. 27, 2017, 9:16 p.m. | #1
On 09/13/2017 12:03 PM, Eric Blake wrote:
> During 'qemu-img compare', when we are checking that an allocated
> portion of one file is all zeros, we don't need to waste time
> computing how many additional sectors after the first non-zero
> byte are also non-zero.  Create a new helper find_nonzero() to do
> the check for a first non-zero sector, and rebase
> check_empty_sectors() to use it.
> 
> The new interface intentionally uses bytes in its interface, even
> though it still crawls the buffer a sector at a time; it is robust
> to a partial sector at the end of the buffer.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

Reviewed-by: John Snow <jsnow@redhat.com>

Patch

diff --git a/qemu-img.c b/qemu-img.c
index f5ab29d176..dfccebe6bc 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1064,6 +1064,28 @@  done:
 }

 /*
+ * Returns -1 if 'buf' contains only zeroes, otherwise the byte index
+ * of the first sector boundary within buf where the sector contains a
+ * non-zero byte.  This function is robust to a buffer that is not
+ * sector-aligned.
+ */
+static int64_t find_nonzero(const uint8_t *buf, int64_t n)
+{
+    int64_t i;
+    int64_t end = QEMU_ALIGN_DOWN(n, BDRV_SECTOR_SIZE);
+
+    for (i = 0; i < end; i += BDRV_SECTOR_SIZE) {
+        if (!buffer_is_zero(buf + i, BDRV_SECTOR_SIZE)) {
+            return i;
+        }
+    }
+    if (i < n && !buffer_is_zero(buf + i, n - end)) {
+        return i;
+    }
+    return -1;
+}
+
+/*
  * Returns true iff the first sector pointed to by 'buf' contains at least
  * a non-NUL byte.
  *
@@ -1188,7 +1210,9 @@  static int check_empty_sectors(BlockBackend *blk, int64_t sect_num,
                                int sect_count, const char *filename,
                                uint8_t *buffer, bool quiet)
 {
-    int pnum, ret = 0;
+    int ret = 0;
+    int64_t idx;
+
     ret = blk_pread(blk, sect_num << BDRV_SECTOR_BITS, buffer,
                     sect_count << BDRV_SECTOR_BITS);
     if (ret < 0) {
@@ -1196,10 +1220,10 @@  static int check_empty_sectors(BlockBackend *blk, int64_t sect_num,
                      sectors_to_bytes(sect_num), filename, strerror(-ret));
         return ret;
     }
-    ret = is_allocated_sectors(buffer, sect_count, &pnum);
-    if (ret || pnum != sect_count) {
+    idx = find_nonzero(buffer, sect_count * BDRV_SECTOR_SIZE);
+    if (idx >= 0) {
         qprintf(quiet, "Content mismatch at offset %" PRId64 "!\n",
-                sectors_to_bytes(ret ? sect_num : sect_num + pnum));
+                sectors_to_bytes(sect_num) + idx);
         return 1;
     }