[Qemu-devel] [PATCH v3 2/5] qcow2: simplify logic in qcow2_co_write_zeroes

[Eric Blake <eblake@redhat.com>]

From: "Denis V. Lunev" <den@openvz.org>

Unaligned requests will occupy only one cluster. This is true since the
previous commit. Simplify the code taking this consideration into
account.

In other words, the caller is now buggy if it ever passes us an unaligned
request that crosses cluster boundaries (the only requests that can cross
boundaries will be aligned).

There are no other changes so far.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1463476543-3087-3-git-send-email-den@openvz.org>
---
 block/qcow2.c | 23 +++++------------------
 1 file changed, 5 insertions(+), 18 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index c9306a7..2f73201 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2438,33 +2438,20 @@ static coroutine_fn int qcow2_co_write_zeroes(BlockDriverState *bs,
     int tail = (sector_num + nb_sectors) % s->cluster_sectors;

     if (head != 0 || tail != 0) {
-        int64_t cl_end = -1;
+        int64_t cl_start = sector_num - head;

-        sector_num -= head;
-        nb_sectors += head;
+        assert(cl_start + s->cluster_sectors >= sector_num + nb_sectors);

-        if (tail != 0) {
-            nb_sectors += s->cluster_sectors - tail;
-        }
+        sector_num = cl_start;
+        nb_sectors = s->cluster_sectors;

         if (!is_zero_cluster(bs, sector_num)) {
             return -ENOTSUP;
         }

-        if (nb_sectors > s->cluster_sectors) {
-            /* Technically the request can cover 2 clusters, f.e. 4k write
-               at s->cluster_sectors - 2k offset. One of these cluster can
-               be zeroed, one unallocated */
-            cl_end = sector_num + nb_sectors - s->cluster_sectors;
-            if (!is_zero_cluster(bs, cl_end)) {
-                return -ENOTSUP;
-            }
-        }
-
         qemu_co_mutex_lock(&s->lock);
         /* We can have new write after previous check */
-        if (!is_zero_cluster_top_locked(bs, sector_num) ||
-                (cl_end > 0 && !is_zero_cluster_top_locked(bs, cl_end))) {
+        if (!is_zero_cluster_top_locked(bs, sector_num)) {
             qemu_co_mutex_unlock(&s->lock);
             return -ENOTSUP;
         }
-- 
2.5.5