[Qemu-devel] [PATCH v3 0/2] qcow2: Patch for shrinking qcow2 disk image

[Qemu-devel] [PATCH v3 0/2] qcow2: Patch for shrinking qcow2 disk image

[Jun Li]

This is the third version for qcow2 shrinking. In this version, fixed host
cluster leak when shrinking a disk image. Such as:
Step 1,
# /opt/qemu-git-arm/bin/qemu-img info /home/lijun/Work/tmp/shrink.qcow2
image: /home/lijun/Work/tmp/shrink.qcow2
file format: qcow2
virtual size: 2.0G (2147483648 bytes)
disk size: 2.0G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    corrupt: false

Step 2,
# /opt/qemu-git-arm/bin/qemu-img resize /home/lijun/Work/tmp/shrink.qcow2 -100M
Image resized.

Step 3,
# /opt/qemu-git-arm/bin/qemu-img info /home/lijun/Work/tmp/shrink.qcow2
image: /home/lijun/Work/tmp/shrink.qcow2
file format: qcow2
virtual size: 1.9G (2042626048 bytes)
disk size: 1.9G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    corrupt: false

Step 4,
# /opt/qemu-git-arm/bin/qemu-img check /home/lijun/Work/tmp/shrink.qcow2
No errors were found on the image.
31005/31168 = 99.48% allocated, 0.79% fragmented, 0.00% compressed clusters
Image end offset: 2033844224

As above show, in step 1, "disk size" and "virtual size" are all 2.0G.
After Step 3, "disk size" and "virtual size" are all 1.9G.

BTW, above is also a simple testing. I will submit a test case for
qemu-iotests later. But I am not so familar with howto write qemu-iotests now.

In file block/qcow2.c, just using ftruncate to fix host cluster leak.

In file block/qcow2-cluster.c, just re-copy qcow2_grow_l1_table to
realize qcow2_shrink_l1_and_l2_table.

In file block/qcow2-refcount.c, also update the realization to handle self-describing
refcount blocks in function update_refcount.

Thanks.


Jun Li (2):
  qcow2: Add qcow2_shrink_l1_and_l2_table for qcow2 shrinking
  qcow2: add update refcount table realization for update_refcount

 block/qcow2-cluster.c  | 173 +++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2-refcount.c |  44 +++++++++++++
 block/qcow2.c          |  40 ++++++++++--
 block/qcow2.h          |   2 +
 4 files changed, 255 insertions(+), 4 deletions(-)

-- 
1.9.3

[Qemu-devel] [PATCH v3 1/2] qcow2: Add qcow2_shrink_l1_and_l2_table for qcow2 shrinking

[Jun Li]

This patch is the realization of new function qcow2_shrink_l1_and_l2_table.
This function will shrink/discard l1 and l2 table when do qcow2 shrinking.

Signed-off-by: Jun Li <junmuzi@gmail.com>
---
Compared to v2, v3 fixed host cluster leak.
---
 block/qcow2-cluster.c | 173 ++++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c         |  40 ++++++++++--
 block/qcow2.h         |   2 +
 3 files changed, 211 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index f7dd8c0..2ac3536 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -29,6 +29,9 @@
 #include "block/qcow2.h"
 #include "trace.h"
 
+static int l2_load(BlockDriverState *bs, uint64_t l2_offset,
+                   uint64_t **l2_table);
+
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
                         bool exact_size)
 {
@@ -135,6 +138,176 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
     return ret;
 }
 
+int qcow2_shrink_l1_and_l2_table(BlockDriverState *bs, uint64_t new_l1_size,
+                                 int new_l2_index, bool exact_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int new_l1_size2, ret, i;
+    uint64_t *new_l1_table;
+    int64_t new_l1_table_offset;
+    int64_t old_l1_table_offset, old_l1_size;
+    uint8_t data[12];
+
+    new_l1_size2 = sizeof(uint64_t) * new_l1_size;
+    new_l1_table = qemu_try_blockalign(bs->file,
+                                       align_offset(new_l1_size2, 512));
+    if (new_l1_table == NULL) {
+        return -ENOMEM;
+    }
+    memset(new_l1_table, 0, align_offset(new_l1_size2, 512));
+
+    /* shrinking l1 table */
+    memcpy(new_l1_table, s->l1_table, new_l1_size2);
+
+    /* write new table (align to cluster) */
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
+    new_l1_table_offset = qcow2_alloc_clusters(bs, new_l1_size2);
+    if (new_l1_table_offset < 0) {
+        qemu_vfree(new_l1_table);
+        return new_l1_table_offset;
+    }
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* the L1 position has not yet been updated, so these clusters must
+     * indeed be completely free */
+    ret = qcow2_pre_write_overlap_check(bs, 0, new_l1_table_offset,
+                                        new_l1_size2);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_WRITE_TABLE);
+
+    for (i = 0; i < new_l1_size; i++) {
+        new_l1_table[i] = cpu_to_be64(new_l1_table[i]);
+    }
+
+    ret = bdrv_pwrite_sync(bs->file, new_l1_table_offset,
+                           new_l1_table, new_l1_size2);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    for (i = 0; i < new_l1_size; i++) {
+        new_l1_table[i] = be64_to_cpu(new_l1_table[i]);
+    }
+
+    /* set new table */
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ACTIVATE_TABLE);
+    cpu_to_be32w((uint32_t *)data, new_l1_size);
+    stq_be_p(data + 4, new_l1_table_offset);
+    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, l1_size),
+                           data, sizeof(data));
+    if (ret < 0) {
+        goto fail;
+    }
+
+    old_l1_table_offset = s->l1_table_offset;
+    s->l1_table_offset = new_l1_table_offset;
+    uint64_t *old_l1_table = s->l1_table;
+    s->l1_table = new_l1_table;
+    old_l1_size = s->l1_size;
+    s->l1_size = new_l1_size;
+
+    int num = old_l1_size - s->l1_size;
+
+    while (num >= 0) {
+        uint64_t l2_offset;
+        int ret;
+        uint64_t *l2_table, l2_entry;
+        int last_free_cluster = 0;
+
+        l2_offset = old_l1_table[s->l1_size + num - 1] & L1E_OFFSET_MASK;
+        if (l2_offset == 0) {
+            goto retry;
+        }
+
+        if (num == 0) {
+            if (new_l2_index == 0) {
+                goto retry;
+            }
+            last_free_cluster = new_l2_index + 1;
+        }
+
+        /* load l2_table into cache */
+        ret = l2_load(bs, l2_offset, &l2_table);
+
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (i = s->l2_size; i > last_free_cluster; i--) {
+            l2_entry = be64_to_cpu(l2_table[i - 1]);
+
+            switch (qcow2_get_cluster_type(l2_entry)) {
+            case QCOW2_CLUSTER_UNALLOCATED:
+                if (!bs->backing_hd) {
+                    continue;
+                }
+                break;
+
+            case QCOW2_CLUSTER_ZERO:
+                continue;
+
+            case QCOW2_CLUSTER_NORMAL:
+            case QCOW2_CLUSTER_COMPRESSED:
+                break;
+
+            default:
+                abort();
+            }
+
+            qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
+            if (s->qcow_version >= 3) {
+                l2_table[i - 1] = cpu_to_be64(QCOW_OFLAG_ZERO);
+            } else {
+                l2_table[i - 1] = cpu_to_be64(0);
+            }
+
+            /* before discard the specify item of l2_entry,
+             * set this entry with 0.
+             */
+            uint64_t data64 = cpu_to_be64(0);
+            ret = bdrv_pwrite_sync(bs->file,
+                                   l2_offset + (i-1) * sizeof(uint64_t),
+                                   &data64, sizeof(data64));
+            if (ret < 0) {
+                goto fail;
+            }
+
+            /* Then decrease the refcount */
+            qcow2_free_any_clusters(bs, l2_entry, 1, QCOW2_DISCARD_MAX);
+        }
+
+        ret = qcow2_cache_put(bs, s->l2_table_cache, (void **) &l2_table);
+        if (ret < 0) {
+            goto fail;
+        }
+        if (last_free_cluster == 0) {
+            qemu_vfree(l2_table);
+            qcow2_free_clusters(bs, l2_offset, s->cluster_size - 1,
+                                QCOW2_DISCARD_OTHER);
+        }
+retry:
+        num--;
+    }
+
+    qemu_vfree(old_l1_table);
+
+    qcow2_free_clusters(bs, old_l1_table_offset, old_l1_size * sizeof(uint64_t),
+                        QCOW2_DISCARD_OTHER);
+    return 0;
+ fail:
+    qemu_vfree(new_l1_table);
+    qcow2_free_clusters(bs, new_l1_table_offset, new_l1_size2,
+                        QCOW2_DISCARD_OTHER);
+    return ret;
+}
+
 /*
  * l2_load
  *
diff --git a/block/qcow2.c b/block/qcow2.c
index fb28493..4dfc15c 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2097,7 +2097,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
     int64_t new_l1_size;
     int ret;
 
-    if (offset & 511) {
+    if (offset <= 0 || offset & 511) {
         error_report("The new size must be a multiple of 512");
         return -EINVAL;
     }
@@ -2108,10 +2108,42 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
         return -ENOTSUP;
     }
 
-    /* shrinking is currently not supported */
+    /* shrinking image */
     if (offset < bs->total_sectors * 512) {
-        error_report("qcow2 doesn't support shrinking images yet");
-        return -ENOTSUP;
+
+        /* As l1 table, l2 table, refcount table, refcount block table
+         * and file header of the qcow2 image need to use some clusters,
+         * so offset need to be reduced.
+         */
+        int64_t nb_l2 = offset / (s->l2_size << s->cluster_bits);
+        int64_t nb_l1 = (s->l1_size >> 3) >> s->cluster_bits;
+        int64_t nb_refcount_block_table = offset / ((s->cluster_size >>
+                                          REFCOUNT_SHIFT) << s->cluster_bits);
+        int64_t nb_refcount_table = (nb_refcount_block_table << 3) >>
+                                     s->cluster_bits;
+        int64_t tatol_nb = nb_l2 + nb_l1 + nb_refcount_block_table +
+                           nb_refcount_table + 1 + 1000;
+        int64_t offset_for_shrink = offset - (tatol_nb << s->cluster_bits);
+        int new_l2_index = offset_to_l2_index(s, offset_for_shrink);
+
+        new_l1_size = size_to_l1(s, offset_for_shrink);
+        ret = qcow2_shrink_l1_and_l2_table(bs, new_l1_size, new_l2_index, true);
+        if (ret < 0) {
+            return ret;
+        }
+
+        int64_t actual_size = bdrv_get_allocated_file_size(bs);
+
+        if (offset < actual_size) {
+            int fd = qemu_open(bs->filename, O_WRONLY,
+                               O_APPEND);
+            if (fd >= 0) {
+                if (ftruncate(fd, offset) < 0) {
+                    return -errno;
+                }
+                qemu_close(fd);
+            }
+        }
     }
 
     new_l1_size = size_to_l1(s, offset);
diff --git a/block/qcow2.h b/block/qcow2.h
index 7d61e61..a9c8acb 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -516,6 +516,8 @@ int qcow2_pre_write_overlap_check(BlockDriverState *bs, int ign, int64_t offset,
 /* qcow2-cluster.c functions */
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
                         bool exact_size);
+int qcow2_shrink_l1_and_l2_table(BlockDriverState *bs, uint64_t new_l1_size,
+                                 int new_l2_index, bool exact_size);
 int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index);
 void qcow2_l2_cache_reset(BlockDriverState *bs);
 int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset);
-- 
1.9.3

[Qemu-devel] [PATCH v3 2/2] qcow2: add update refcount table realization for update_refcount

[Jun Li]

When every item of refcount block is NULL, free refcount block and reset the
corresponding item of refcount table with NULL. At the same time, put this
cluster to s->free_cluster_index.

Signed-off-by: Jun Li <junmuzi@gmail.com>
---
This version adding handle self-describing refcount blocks.
---
 block/qcow2-refcount.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 2bcaaf9..8594ffd 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -28,6 +28,7 @@
 #include "qemu/range.h"
 
 static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size);
+static int write_reftable_entry(BlockDriverState *bs, int rt_index);
 static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
                             int64_t offset, int64_t length,
                             int addend, enum qcow2_discard_type type);
@@ -599,6 +600,49 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
         if (refcount == 0 && s->discard_passthrough[type]) {
             update_refcount_discard(bs, cluster_offset, s->cluster_size);
         }
+
+        unsigned int refcount_table_index;
+        refcount_table_index = cluster_index >> (s->cluster_bits -
+                               REFCOUNT_SHIFT);
+
+        uint64_t refcount_block_offset =
+            s->refcount_table[refcount_table_index] & REFT_OFFSET_MASK;
+
+        int64_t self_block_index = refcount_block_offset >> s->cluster_bits;
+        int self_block_index2 = (refcount_block_offset >> s->cluster_bits) &
+            ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+
+        /* When refcount block is NULL, update refcount table */
+        if (!be16_to_cpu(refcount_block[0]) || self_block_index2 == 0) {
+            int k;
+            int refcount_block_entries = s->cluster_size /
+                                         sizeof(refcount_block[0]);
+            for (k = 0; k < refcount_block_entries; k++) {
+                if (k == self_block_index2) {
+                    k++;
+                }
+
+                if (be16_to_cpu(refcount_block[k])) {
+                    break;
+                }
+            }
+
+            if (k == refcount_block_entries) {
+                if (self_block_index < s->free_cluster_index) {
+                    s->free_cluster_index = self_block_index;
+                }
+
+                /* update refcount table */
+                s->refcount_table[refcount_table_index] = 0;
+                ret = write_reftable_entry(bs, refcount_table_index);
+                if (ret < 0) {
+                    fprintf(stderr, "Could not update refcount table: %s\n",
+                            strerror(-ret));
+                    goto fail;
+                }
+
+            }
+        }
     }
 
     ret = 0;
-- 
1.9.3

Re: [Qemu-devel] [PATCH v3 0/2] qcow2: Patch for shrinking qcow2 disk image

[Jun Li]

Please ignore this patch, I have submit a new version.

On Mon, 10/13 13:04, Jun Li wrote:
> This is the third version for qcow2 shrinking. In this version, fixed host
> cluster leak when shrinking a disk image. Such as:
> Step 1,
> # /opt/qemu-git-arm/bin/qemu-img info /home/lijun/Work/tmp/shrink.qcow2
> image: /home/lijun/Work/tmp/shrink.qcow2
> file format: qcow2
> virtual size: 2.0G (2147483648 bytes)
> disk size: 2.0G
> cluster_size: 65536
> Format specific information:
>     compat: 1.1
>     lazy refcounts: false
>     corrupt: false
> 
> Step 2,
> # /opt/qemu-git-arm/bin/qemu-img resize /home/lijun/Work/tmp/shrink.qcow2 -100M
> Image resized.
> 
> Step 3,
> # /opt/qemu-git-arm/bin/qemu-img info /home/lijun/Work/tmp/shrink.qcow2
> image: /home/lijun/Work/tmp/shrink.qcow2
> file format: qcow2
> virtual size: 1.9G (2042626048 bytes)
> disk size: 1.9G
> cluster_size: 65536
> Format specific information:
>     compat: 1.1
>     lazy refcounts: false
>     corrupt: false
> 
> Step 4,
> # /opt/qemu-git-arm/bin/qemu-img check /home/lijun/Work/tmp/shrink.qcow2
> No errors were found on the image.
> 31005/31168 = 99.48% allocated, 0.79% fragmented, 0.00% compressed clusters
> Image end offset: 2033844224
> 
> As above show, in step 1, "disk size" and "virtual size" are all 2.0G.
> After Step 3, "disk size" and "virtual size" are all 1.9G.
> 
> BTW, above is also a simple testing. I will submit a test case for
> qemu-iotests later. But I am not so familar with howto write qemu-iotests now.
> 
> In file block/qcow2.c, just using ftruncate to fix host cluster leak.
> 
> In file block/qcow2-cluster.c, just re-copy qcow2_grow_l1_table to
> realize qcow2_shrink_l1_and_l2_table.
> 
> In file block/qcow2-refcount.c, also update the realization to handle self-describing
> refcount blocks in function update_refcount.
> 
> Thanks.
> 
> 
> Jun Li (2):
>   qcow2: Add qcow2_shrink_l1_and_l2_table for qcow2 shrinking
>   qcow2: add update refcount table realization for update_refcount
> 
>  block/qcow2-cluster.c  | 173 +++++++++++++++++++++++++++++++++++++++++++++++++
>  block/qcow2-refcount.c |  44 +++++++++++++
>  block/qcow2.c          |  40 ++++++++++--
>  block/qcow2.h          |   2 +
>  4 files changed, 255 insertions(+), 4 deletions(-)
> 
> -- 
> 1.9.3
> 

Re: [Qemu-devel] [PATCH v3 2/2] qcow2: add update refcount table realization for update_refcount

[Jun Li]

Please ignore this patch, I have submit v4. thx.

On Mon, 10/13 13:04, Jun Li wrote:
> When every item of refcount block is NULL, free refcount block and reset the
> corresponding item of refcount table with NULL. At the same time, put this
> cluster to s->free_cluster_index.
> 
> Signed-off-by: Jun Li <junmuzi@gmail.com>
> ---
> This version adding handle self-describing refcount blocks.
> ---
>  block/qcow2-refcount.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 2bcaaf9..8594ffd 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -28,6 +28,7 @@
>  #include "qemu/range.h"
>  
>  static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size);
> +static int write_reftable_entry(BlockDriverState *bs, int rt_index);
>  static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
>                              int64_t offset, int64_t length,
>                              int addend, enum qcow2_discard_type type);
> @@ -599,6 +600,49 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
>          if (refcount == 0 && s->discard_passthrough[type]) {
>              update_refcount_discard(bs, cluster_offset, s->cluster_size);
>          }
> +
> +        unsigned int refcount_table_index;
> +        refcount_table_index = cluster_index >> (s->cluster_bits -
> +                               REFCOUNT_SHIFT);
> +
> +        uint64_t refcount_block_offset =
> +            s->refcount_table[refcount_table_index] & REFT_OFFSET_MASK;
> +
> +        int64_t self_block_index = refcount_block_offset >> s->cluster_bits;
> +        int self_block_index2 = (refcount_block_offset >> s->cluster_bits) &
> +            ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
> +
> +        /* When refcount block is NULL, update refcount table */
> +        if (!be16_to_cpu(refcount_block[0]) || self_block_index2 == 0) {
> +            int k;
> +            int refcount_block_entries = s->cluster_size /
> +                                         sizeof(refcount_block[0]);
> +            for (k = 0; k < refcount_block_entries; k++) {
> +                if (k == self_block_index2) {
> +                    k++;
> +                }
> +
> +                if (be16_to_cpu(refcount_block[k])) {
> +                    break;
> +                }
> +            }
> +
> +            if (k == refcount_block_entries) {
> +                if (self_block_index < s->free_cluster_index) {
> +                    s->free_cluster_index = self_block_index;
> +                }
> +
> +                /* update refcount table */
> +                s->refcount_table[refcount_table_index] = 0;
> +                ret = write_reftable_entry(bs, refcount_table_index);
> +                if (ret < 0) {
> +                    fprintf(stderr, "Could not update refcount table: %s\n",
> +                            strerror(-ret));
> +                    goto fail;
> +                }
> +
> +            }
> +        }
>      }
>  
>      ret = 0;
> -- 
> 1.9.3
> 

Re: [Qemu-devel] [PATCH v3 1/2] qcow2: Add qcow2_shrink_l1_and_l2_table for qcow2 shrinking

[Jun Li]

Please ignore this patch, I have submit v4. Thx.

On Mon, 10/13 13:04, Jun Li wrote:
> This patch is the realization of new function qcow2_shrink_l1_and_l2_table.
> This function will shrink/discard l1 and l2 table when do qcow2 shrinking.
> 
> Signed-off-by: Jun Li <junmuzi@gmail.com>
> ---
> Compared to v2, v3 fixed host cluster leak.
> ---
>  block/qcow2-cluster.c | 173 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  block/qcow2.c         |  40 ++++++++++--
>  block/qcow2.h         |   2 +
>  3 files changed, 211 insertions(+), 4 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index f7dd8c0..2ac3536 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -29,6 +29,9 @@
>  #include "block/qcow2.h"
>  #include "trace.h"
>  
> +static int l2_load(BlockDriverState *bs, uint64_t l2_offset,
> +                   uint64_t **l2_table);
> +
>  int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
>                          bool exact_size)
>  {
> @@ -135,6 +138,176 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
>      return ret;
>  }
>  
> +int qcow2_shrink_l1_and_l2_table(BlockDriverState *bs, uint64_t new_l1_size,
> +                                 int new_l2_index, bool exact_size)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int new_l1_size2, ret, i;
> +    uint64_t *new_l1_table;
> +    int64_t new_l1_table_offset;
> +    int64_t old_l1_table_offset, old_l1_size;
> +    uint8_t data[12];
> +
> +    new_l1_size2 = sizeof(uint64_t) * new_l1_size;
> +    new_l1_table = qemu_try_blockalign(bs->file,
> +                                       align_offset(new_l1_size2, 512));
> +    if (new_l1_table == NULL) {
> +        return -ENOMEM;
> +    }
> +    memset(new_l1_table, 0, align_offset(new_l1_size2, 512));
> +
> +    /* shrinking l1 table */
> +    memcpy(new_l1_table, s->l1_table, new_l1_size2);
> +
> +    /* write new table (align to cluster) */
> +    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
> +    new_l1_table_offset = qcow2_alloc_clusters(bs, new_l1_size2);
> +    if (new_l1_table_offset < 0) {
> +        qemu_vfree(new_l1_table);
> +        return new_l1_table_offset;
> +    }
> +
> +    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
> +    if (ret < 0) {
> +        goto fail;
> +    }
> +
> +    /* the L1 position has not yet been updated, so these clusters must
> +     * indeed be completely free */
> +    ret = qcow2_pre_write_overlap_check(bs, 0, new_l1_table_offset,
> +                                        new_l1_size2);
> +    if (ret < 0) {
> +        goto fail;
> +    }
> +
> +    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_WRITE_TABLE);
> +
> +    for (i = 0; i < new_l1_size; i++) {
> +        new_l1_table[i] = cpu_to_be64(new_l1_table[i]);
> +    }
> +
> +    ret = bdrv_pwrite_sync(bs->file, new_l1_table_offset,
> +                           new_l1_table, new_l1_size2);
> +    if (ret < 0) {
> +        goto fail;
> +    }
> +
> +    for (i = 0; i < new_l1_size; i++) {
> +        new_l1_table[i] = be64_to_cpu(new_l1_table[i]);
> +    }
> +
> +    /* set new table */
> +    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ACTIVATE_TABLE);
> +    cpu_to_be32w((uint32_t *)data, new_l1_size);
> +    stq_be_p(data + 4, new_l1_table_offset);
> +    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, l1_size),
> +                           data, sizeof(data));
> +    if (ret < 0) {
> +        goto fail;
> +    }
> +
> +    old_l1_table_offset = s->l1_table_offset;
> +    s->l1_table_offset = new_l1_table_offset;
> +    uint64_t *old_l1_table = s->l1_table;
> +    s->l1_table = new_l1_table;
> +    old_l1_size = s->l1_size;
> +    s->l1_size = new_l1_size;
> +
> +    int num = old_l1_size - s->l1_size;
> +
> +    while (num >= 0) {
> +        uint64_t l2_offset;
> +        int ret;
> +        uint64_t *l2_table, l2_entry;
> +        int last_free_cluster = 0;
> +
> +        l2_offset = old_l1_table[s->l1_size + num - 1] & L1E_OFFSET_MASK;
> +        if (l2_offset == 0) {
> +            goto retry;
> +        }
> +
> +        if (num == 0) {
> +            if (new_l2_index == 0) {
> +                goto retry;
> +            }
> +            last_free_cluster = new_l2_index + 1;
> +        }
> +
> +        /* load l2_table into cache */
> +        ret = l2_load(bs, l2_offset, &l2_table);
> +
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +
> +        for (i = s->l2_size; i > last_free_cluster; i--) {
> +            l2_entry = be64_to_cpu(l2_table[i - 1]);
> +
> +            switch (qcow2_get_cluster_type(l2_entry)) {
> +            case QCOW2_CLUSTER_UNALLOCATED:
> +                if (!bs->backing_hd) {
> +                    continue;
> +                }
> +                break;
> +
> +            case QCOW2_CLUSTER_ZERO:
> +                continue;
> +
> +            case QCOW2_CLUSTER_NORMAL:
> +            case QCOW2_CLUSTER_COMPRESSED:
> +                break;
> +
> +            default:
> +                abort();
> +            }
> +
> +            qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
> +            if (s->qcow_version >= 3) {
> +                l2_table[i - 1] = cpu_to_be64(QCOW_OFLAG_ZERO);
> +            } else {
> +                l2_table[i - 1] = cpu_to_be64(0);
> +            }
> +
> +            /* before discard the specify item of l2_entry,
> +             * set this entry with 0.
> +             */
> +            uint64_t data64 = cpu_to_be64(0);
> +            ret = bdrv_pwrite_sync(bs->file,
> +                                   l2_offset + (i-1) * sizeof(uint64_t),
> +                                   &data64, sizeof(data64));
> +            if (ret < 0) {
> +                goto fail;
> +            }
> +
> +            /* Then decrease the refcount */
> +            qcow2_free_any_clusters(bs, l2_entry, 1, QCOW2_DISCARD_MAX);
> +        }
> +
> +        ret = qcow2_cache_put(bs, s->l2_table_cache, (void **) &l2_table);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +        if (last_free_cluster == 0) {
> +            qemu_vfree(l2_table);
> +            qcow2_free_clusters(bs, l2_offset, s->cluster_size - 1,
> +                                QCOW2_DISCARD_OTHER);
> +        }
> +retry:
> +        num--;
> +    }
> +
> +    qemu_vfree(old_l1_table);
> +
> +    qcow2_free_clusters(bs, old_l1_table_offset, old_l1_size * sizeof(uint64_t),
> +                        QCOW2_DISCARD_OTHER);
> +    return 0;
> + fail:
> +    qemu_vfree(new_l1_table);
> +    qcow2_free_clusters(bs, new_l1_table_offset, new_l1_size2,
> +                        QCOW2_DISCARD_OTHER);
> +    return ret;
> +}
> +
>  /*
>   * l2_load
>   *
> diff --git a/block/qcow2.c b/block/qcow2.c
> index fb28493..4dfc15c 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2097,7 +2097,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
>      int64_t new_l1_size;
>      int ret;
>  
> -    if (offset & 511) {
> +    if (offset <= 0 || offset & 511) {
>          error_report("The new size must be a multiple of 512");
>          return -EINVAL;
>      }
> @@ -2108,10 +2108,42 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
>          return -ENOTSUP;
>      }
>  
> -    /* shrinking is currently not supported */
> +    /* shrinking image */
>      if (offset < bs->total_sectors * 512) {
> -        error_report("qcow2 doesn't support shrinking images yet");
> -        return -ENOTSUP;
> +
> +        /* As l1 table, l2 table, refcount table, refcount block table
> +         * and file header of the qcow2 image need to use some clusters,
> +         * so offset need to be reduced.
> +         */
> +        int64_t nb_l2 = offset / (s->l2_size << s->cluster_bits);
> +        int64_t nb_l1 = (s->l1_size >> 3) >> s->cluster_bits;
> +        int64_t nb_refcount_block_table = offset / ((s->cluster_size >>
> +                                          REFCOUNT_SHIFT) << s->cluster_bits);
> +        int64_t nb_refcount_table = (nb_refcount_block_table << 3) >>
> +                                     s->cluster_bits;
> +        int64_t tatol_nb = nb_l2 + nb_l1 + nb_refcount_block_table +
> +                           nb_refcount_table + 1 + 1000;
> +        int64_t offset_for_shrink = offset - (tatol_nb << s->cluster_bits);
> +        int new_l2_index = offset_to_l2_index(s, offset_for_shrink);
> +
> +        new_l1_size = size_to_l1(s, offset_for_shrink);
> +        ret = qcow2_shrink_l1_and_l2_table(bs, new_l1_size, new_l2_index, true);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +
> +        int64_t actual_size = bdrv_get_allocated_file_size(bs);
> +
> +        if (offset < actual_size) {
> +            int fd = qemu_open(bs->filename, O_WRONLY,
> +                               O_APPEND);
> +            if (fd >= 0) {
> +                if (ftruncate(fd, offset) < 0) {
> +                    return -errno;
> +                }
> +                qemu_close(fd);
> +            }
> +        }
>      }
>  
>      new_l1_size = size_to_l1(s, offset);
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 7d61e61..a9c8acb 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -516,6 +516,8 @@ int qcow2_pre_write_overlap_check(BlockDriverState *bs, int ign, int64_t offset,
>  /* qcow2-cluster.c functions */
>  int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
>                          bool exact_size);
> +int qcow2_shrink_l1_and_l2_table(BlockDriverState *bs, uint64_t new_l1_size,
> +                                 int new_l2_index, bool exact_size);
>  int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index);
>  void qcow2_l2_cache_reset(BlockDriverState *bs);
>  int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset);
> -- 
> 1.9.3
>