[Qemu-devel] [PATCH] qcow2: Discard/zero clusters by byte count

[Eric Blake]

Passing a byte offset, but sector count, when we ultimately
want to operate on cluster granularity, is madness.  Clean up
the interfaces to take byte offset and count.  Rename
qcow2_discard_clusters() and qcow2_zero_clusters() to the
shorter qcow2_discard() and qcow2_zero() to make sure backports
don't get confused by changed units.

Signed-off-by: Eric Blake <address@hidden>
---

2.9 material, depends on 'Don't strand clusters near 2G intervals
during commit'

 block/qcow2.h          |  8 ++++----
 block/qcow2-cluster.c  | 20 +++++++++++---------
 block/qcow2-snapshot.c |  7 +++----
 block/qcow2.c          | 22 ++++++++++------------
 4 files changed, 28 insertions(+), 29 deletions(-)

diff --git a/block/qcow2.h b/block/qcow2.h
index 1823414..a0d169b 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -543,10 +543,10 @@ uint64_t 
qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
                                          int compressed_size);

 int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m);
-int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
-    int nb_sectors, enum qcow2_discard_type type, bool full_discard);
-int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors,
-                        int flags);
+int qcow2_discard(BlockDriverState *bs, uint64_t offset, uint64_t count,
+                  enum qcow2_discard_type type, bool full_discard);
+int qcow2_zero(BlockDriverState *bs, uint64_t offset, uint64_t count,
+               int flags);

 int qcow2_expand_zero_clusters(BlockDriverState *bs,
                                BlockDriverAmendStatusCB *status_cb,
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 928c1e2..3ee0815 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1511,19 +1511,17 @@ static int discard_single_l2(BlockDriverState *bs, 
uint64_t offset,
     return nb_clusters;
 }

-int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
-    int nb_sectors, enum qcow2_discard_type type, bool full_discard)
+int qcow2_discard(BlockDriverState *bs, uint64_t offset, uint64_t count,
+                  enum qcow2_discard_type type, bool full_discard)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t end_offset;
     uint64_t nb_clusters;
     int ret;

-    end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);
-
-    /* Round start up and end down */
+    /* Round start up and end down to cluster boundary */
+    end_offset = start_of_cluster(s, offset + count);
     offset = align_offset(offset, s->cluster_size);
-    end_offset = start_of_cluster(s, end_offset);

     if (offset > end_offset) {
         return 0;
@@ -1595,20 +1593,24 @@ static int zero_single_l2(BlockDriverState *bs, 
uint64_t offset,
     return nb_clusters;
 }

-int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors,
-                        int flags)
+int qcow2_zero(BlockDriverState *bs, uint64_t offset, uint64_t count,
+               int flags)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t nb_clusters;
     int ret;

+    /* Block layer guarantees cluster alignment */
+    assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
+    assert(QEMU_IS_ALIGNED(count, s->cluster_size));
+
     /* The zero flag is only supported by version 3 and newer */
     if (s->qcow_version < 3) {
         return -ENOTSUP;
     }

     /* Each L2 table is handled by its own loop iteration */
-    nb_clusters = size_to_clusters(s, nb_sectors << BDRV_SECTOR_BITS);
+    nb_clusters = size_to_clusters(s, count);

     s->cache_discards = true;

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 0324243..fd088e5 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -440,10 +440,9 @@ int qcow2_snapshot_create(BlockDriverState *bs, 
QEMUSnapshotInfo *sn_info)

     /* The VM state isn't needed any more in the active L1 table; in fact, it
      * hurts by causing expensive COW for the next snapshot. */
-    qcow2_discard_clusters(bs, qcow2_vm_state_offset(s),
-                           align_offset(sn->vm_state_size, s->cluster_size)
-                                >> BDRV_SECTOR_BITS,
-                           QCOW2_DISCARD_NEVER, false);
+    qcow2_discard(bs, qcow2_vm_state_offset(s),
+                  align_offset(sn->vm_state_size, s->cluster_size),
+                  QCOW2_DISCARD_NEVER, false);

 #ifdef DEBUG_ALLOC
     {
diff --git a/block/qcow2.c b/block/qcow2.c
index 369b542..601831f 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2487,7 +2487,7 @@ static coroutine_fn int 
qcow2_co_pwrite_zeroes(BlockDriverState *bs,
     trace_qcow2_pwrite_zeroes(qemu_coroutine_self(), offset, count);

     /* Whatever is left can use real zero clusters */
-    ret = qcow2_zero_clusters(bs, offset, count >> BDRV_SECTOR_BITS, flags);
+    ret = qcow2_zero(bs, offset, count, flags);
     qemu_co_mutex_unlock(&s->lock);

     return ret;
@@ -2505,8 +2505,7 @@ static coroutine_fn int 
qcow2_co_pdiscard(BlockDriverState *bs,
     }

     qemu_co_mutex_lock(&s->lock);
-    ret = qcow2_discard_clusters(bs, offset, count >> BDRV_SECTOR_BITS,
-                                 QCOW2_DISCARD_REQUEST, false);
+    ret = qcow2_discard(bs, offset, count, QCOW2_DISCARD_REQUEST, false);
     qemu_co_mutex_unlock(&s->lock);
     return ret;
 }
@@ -2807,9 +2806,8 @@ fail:
 static int qcow2_make_empty(BlockDriverState *bs)
 {
     BDRVQcow2State *s = bs->opaque;
-    uint64_t start_sector;
-    int sector_step = QEMU_ALIGN_DOWN(INT_MAX / BDRV_SECTOR_SIZE,
-                                      s->cluster_size);
+    uint64_t offset;
+    int step = QEMU_ALIGN_DOWN(INT_MAX, s->cluster_size);
     int l1_clusters, ret = 0;

     l1_clusters = DIV_ROUND_UP(s->l1_size, s->cluster_size / sizeof(uint64_t));
@@ -2826,18 +2824,18 @@ static int qcow2_make_empty(BlockDriverState *bs)

     /* This fallback code simply discards every active cluster; this is slow,
      * but works in all cases */
-    for (start_sector = 0; start_sector < bs->total_sectors;
-         start_sector += sector_step)
+    for (offset = 0; offset < bs->total_sectors * BDRV_SECTOR_SIZE;
+         offset += step)
     {
         /* As this function is generally used after committing an external
          * snapshot, QCOW2_DISCARD_SNAPSHOT seems appropriate. Also, the
          * default action for this kind of discard is to pass the discard,
          * which will ideally result in an actually smaller image file, as
          * is probably desired. */
-        ret = qcow2_discard_clusters(bs, start_sector * BDRV_SECTOR_SIZE,
-                                     MIN(sector_step,
-                                         bs->total_sectors - start_sector),
-                                     QCOW2_DISCARD_SNAPSHOT, true);
+        ret = qcow2_discard(bs, offset,
+                            MIN(step,
+                                bs->total_sectors * BDRV_SECTOR_SIZE - offset),
+                            QCOW2_DISCARD_SNAPSHOT, true);
         if (ret < 0) {
             break;
         }
-- 
2.9.3