[Qemu-block] [PATCH v2 3/4] qcow2: add shrink image support

[Pavel Butsykin]

This patch add shrinking of the image file for qcow2. As a result, this allows
us to reduce the virtual image size and free up space on the disk without
copying the image. Image can be fragmented and shrink is done by punching holes
in the image file.

Signed-off-by: Pavel Butsykin <address@hidden>
---
 block/qcow2-cluster.c  | 42 ++++++++++++++++++++++++++++++++
 block/qcow2-refcount.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c          | 40 +++++++++++++++++++++++--------
 block/qcow2.h          |  2 ++
 qapi/block-core.json   |  3 ++-
 5 files changed, 141 insertions(+), 11 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index d779ea19cf..a84b7e607e 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -32,6 +32,48 @@
 #include "qemu/bswap.h"
 #include "trace.h"
 
+int qcow2_shrink_l1_table(BlockDriverState *bs, uint64_t max_size)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int new_l1_size, i, ret;
+
+    if (max_size >= s->l1_size) {
+        return 0;
+    }
+
+    new_l1_size = max_size;
+
+#ifdef DEBUG_ALLOC2
+    fprintf(stderr, "shrink l1_table from %d to %" PRId64 "\n",
+            s->l1_size, new_l1_size);
+#endif
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_WRITE_TABLE);
+    ret = bdrv_pwrite_zeroes(bs->file, s->l1_table_offset +
+                                       sizeof(uint64_t) * new_l1_size,
+                             (s->l1_size - new_l1_size) * sizeof(uint64_t), 0);
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = bdrv_flush(bs->file->bs);
+    if (ret < 0) {
+        return ret;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_FREE_L2_CLUSTERS);
+    for (i = s->l1_size - 1; i > new_l1_size - 1; i--) {
+        if ((s->l1_table[i] & L1E_OFFSET_MASK) == 0) {
+            continue;
+        }
+        qcow2_free_clusters(bs, s->l1_table[i] & L1E_OFFSET_MASK,
+                            s->l2_size * sizeof(uint64_t),
+                            QCOW2_DISCARD_ALWAYS);
+        s->l1_table[i] = 0;
+    }
+    return 0;
+}
+
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
                         bool exact_size)
 {
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 576ab551d6..e98306acd8 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -29,6 +29,7 @@
 #include "block/qcow2.h"
 #include "qemu/range.h"
 #include "qemu/bswap.h"
+#include "qemu/cutils.h"
 
 static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size);
 static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
@@ -2936,3 +2937,67 @@ done:
     qemu_vfree(new_refblock);
     return ret;
 }
+
+int qcow2_shrink_reftable(BlockDriverState *bs)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t *reftable_tmp =
+        g_try_malloc(sizeof(uint64_t) * s->refcount_table_size);
+    int i, ret;
+
+    if (s->refcount_table_size && reftable_tmp == NULL) {
+        return -ENOMEM;
+    }
+
+    for (i = 0; i < s->refcount_table_size; i++) {
+        int64_t refblock_offs = s->refcount_table[i] & REFT_OFFSET_MASK;
+        void *refblock;
+        bool unused_block;
+
+        if (refblock_offs == 0) {
+            reftable_tmp[i] = 0;
+            continue;
+        }
+        ret = qcow2_cache_get(bs, s->refcount_block_cache, refblock_offs,
+                              &refblock);
+        if (ret < 0) {
+            goto out;
+        }
+
+        /* the refblock has own reference */
+        if (i == refblock_offs >> (s->refcount_block_bits + s->cluster_bits)) {
+            uint64_t blk_index = (refblock_offs >> s->cluster_bits) &
+                                 (s->refcount_block_size - 1);
+            uint64_t refcount = s->get_refcount(refblock, blk_index);
+
+            s->set_refcount(refblock, blk_index, 0);
+
+            unused_block = buffer_is_zero(refblock, s->refcount_block_size);
+
+            s->set_refcount(refblock, blk_index, refcount);
+        } else {
+            unused_block = buffer_is_zero(refblock, s->refcount_block_size);
+        }
+        qcow2_cache_put(bs, s->refcount_block_cache, &refblock);
+
+        reftable_tmp[i] = unused_block ? 0 : cpu_to_be64(s->refcount_table[i]);
+    }
+
+    ret = bdrv_pwrite_sync(bs->file, s->refcount_table_offset, reftable_tmp,
+                           sizeof(uint64_t) * s->refcount_table_size);
+    if (ret < 0) {
+        goto out;
+    }
+
+    for (i = 0; i < s->refcount_table_size; i++) {
+        if (s->refcount_table[i] && !reftable_tmp[i]) {
+            qcow2_free_clusters(bs, s->refcount_table[i] & REFT_OFFSET_MASK,
+                                s->cluster_size, QCOW2_DISCARD_ALWAYS);
+            s->refcount_table[i] = 0;
+        }
+    }
+
+out:
+    g_free(reftable_tmp);
+    return ret;
+}
diff --git a/block/qcow2.c b/block/qcow2.c
index b3ba5daa93..0ad46d2776 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2545,6 +2545,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t 
offset, Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
     int64_t new_l1_size;
+    uint64_t total_size;
     int ret;
 
     if (offset & 511) {
@@ -2558,17 +2559,36 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t 
offset, Error **errp)
         return -ENOTSUP;
     }
 
-    /* shrinking is currently not supported */
-    if (offset < bs->total_sectors * 512) {
-        error_setg(errp, "qcow2 doesn't support shrinking images yet");
-        return -ENOTSUP;
-    }
-
     new_l1_size = size_to_l1(s, offset);
-    ret = qcow2_grow_l1_table(bs, new_l1_size, true);
-    if (ret < 0) {
-        error_setg_errno(errp, -ret, "Failed to grow the L1 table");
-        return ret;
+    total_size = bs->total_sectors << BDRV_SECTOR_BITS;
+
+    if (offset < total_size) {
+        ret = qcow2_cluster_discard(bs, ROUND_UP(offset, s->cluster_size),
+                                    total_size - ROUND_UP(offset,
+                                                          s->cluster_size),
+                                    QCOW2_DISCARD_ALWAYS, true);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to discard reduced clasters");
+            return ret;
+        }
+
+        ret = qcow2_shrink_l1_table(bs, new_l1_size);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to reduce the L1 table");
+            return ret;
+        }
+
+        ret = qcow2_shrink_reftable(bs);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to shrink the refcount 
table");
+            return ret;
+        }
+    } else {
+        ret = qcow2_grow_l1_table(bs, new_l1_size, true);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to grow the L1 table");
+            return ret;
+        }
     }
 
     /* write updated header.size */
diff --git a/block/qcow2.h b/block/qcow2.h
index 07faa6dc78..600463bf8e 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -531,10 +531,12 @@ int qcow2_pre_write_overlap_check(BlockDriverState *bs, 
int ign, int64_t offset,
 int qcow2_change_refcount_order(BlockDriverState *bs, int refcount_order,
                                 BlockDriverAmendStatusCB *status_cb,
                                 void *cb_opaque, Error **errp);
+int qcow2_shrink_reftable(BlockDriverState *bs);
 
 /* qcow2-cluster.c functions */
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
                         bool exact_size);
+int qcow2_shrink_l1_table(BlockDriverState *bs, uint64_t max_size);
 int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index);
 int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset);
 int qcow2_encrypt_sectors(BDRVQcow2State *s, int64_t sector_num,
diff --git a/qapi/block-core.json b/qapi/block-core.json
index f85c2235c7..bcbffa3339 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2372,7 +2372,8 @@
             'cluster_alloc_bytes', 'cluster_free', 'flush_to_os',
             'flush_to_disk', 'pwritev_rmw_head', 'pwritev_rmw_after_head',
             'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
-            'pwritev_zero', 'pwritev_done', 'empty_image_prepare' ] }
+            'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
+            'l1_shrink_write_table', 'l1_shrink_free_l2_clusters' ] }
 
 ##
 # @BlkdebugInjectErrorOptions:
-- 
2.13.0