Some BDS drivers have a cap on how much block status they can supply
in one query (for example, NBD talking to an older server cannot
inspect more than 4G per query; and qcow2 tends to cap its answers
rather than cross a cluster boundary of an L1 table). Although the
existing callers of bdrv_co_is_zero_fast are not passing in that large
of a 'bytes' parameter, an upcoming caller wants to query the entire
image at once, and will thus benefit from being able to treat adjacent
zero regions in a coalesced manner, rather than claiming the region is
non-zero merely because pnum was truncated and didn't match the
incoming bytes.
While refactoring this into a loop, note that there is no need to
assign pnum prior to calling bdrv_co_common_block_status_above() (it
is guaranteed to be assigned deeper in the callstack).
Signed-off-by: Eric Blake <ebl...@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
Message-ID: <20250509204341.3553601-18-ebl...@redhat.com>
---
block/io.c | 29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)
diff --git a/block/io.c b/block/io.c
index b5b143cd1b0..50dc0e193f0 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2751,28 +2751,31 @@ int coroutine_fn bdrv_co_block_status(BlockDriverState
*bs, int64_t offset,
* by @offset and @bytes is known to read as zeroes.
* Return 1 if that is the case, 0 otherwise and -errno on error.
* This test is meant to be fast rather than accurate so returning 0
- * does not guarantee non-zero data.
+ * does not guarantee non-zero data; but a return of 1 is reliable.
*/
int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset,
int64_t bytes)
{
int ret;
- int64_t pnum = bytes;
+ int64_t pnum;
IO_CODE();
- if (!bytes) {
- return 1;
+ while (bytes) {
+ ret = bdrv_co_common_block_status_above(bs, NULL, false,
+ BDRV_WANT_ZERO, offset, bytes,
+ &pnum, NULL, NULL, NULL);
+
+ if (ret < 0) {
+ return ret;
+ }
+ if (!(ret & BDRV_BLOCK_ZERO)) {
+ return 0;
+ }
+ offset += pnum;
+ bytes -= pnum;
}
- ret = bdrv_co_common_block_status_above(bs, NULL, false, BDRV_WANT_ZERO,
- offset, bytes, &pnum, NULL, NULL,
- NULL);
-
- if (ret < 0) {
- return ret;
- }
-
- return (pnum == bytes) && (ret & BDRV_BLOCK_ZERO);
+ return 1;
}
int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset,
--
2.49.0
