[Qemu-devel] [PATCH v4 09/20] block: Add bitmap successors

[John Snow <jsnow@redhat.com>]

A bitmap successor is an anonymous BdrvDirtyBitmap that is intended to
be created just prior to a sensitive operation (e.g. Incremental Backup)
that can either succeed or fail, but during the course of which we still
want a bitmap tracking writes.

On creating a successor, we "freeze" the parent bitmap which prevents
its deletion, enabling, anonymization, or creating a bitmap with the
same name.

On success, the parent bitmap can "abdicate" responsibility to the
successor, which will inherit its name. The successor will have been
tracking writes during the course of the backup operation. The parent
will be safely deleted.

On failure, we can "reclaim" the successor from the parent, unifying
them such that the resulting bitmap describes all writes occurring since
the last successful backup, for instance. Reclamation will thaw the
parent, but not explicitly re-enable it.

BdrvDirtyBitmap operations that target a single bitmap are protected
by assertions that the bitmap is not frozen and/or disabled.

BdrvDirtyBitmap operations that target a group of bitmaps, such as
bdrv_{set,reset}_dirty will ignore frozen/disabled drives with a
conditional instead.

QMP transactions that enable/disable bitmaps have extra error checking
surrounding them that prevent modifying bitmaps that are frozen.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block.c               | 104 +++++++++++++++++++++++++++++++++++++++++++++++++-
 blockdev.c            |   7 ++++
 include/block/block.h |  10 +++++
 qapi/block-core.json  |   1 +
 4 files changed, 121 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 0bf5565..2fff00b 100644
--- a/block.c
+++ b/block.c
@@ -51,8 +51,17 @@
 #include <windows.h>
 #endif
 
+/**
+ * A BdrvDirtyBitmap can be in three possible states:
+ * (1) successor is false and disabled is false: full r/w mode
+ * (2) successor is false and disabled is true: read only mode ("disabled")
+ * (3) successor is set: frozen mode.
+ *     A frozen bitmap cannot be renamed, deleted, anonymized, cleared, set,
+ *     or enabled. A frozen bitmap can only abdicate() or reclaim().
+ */
 struct BdrvDirtyBitmap {
     HBitmap *bitmap;
+    BdrvDirtyBitmap *successor;
     char *name;
     bool disabled;
     QLIST_ENTRY(BdrvDirtyBitmap) list;
@@ -5419,6 +5428,7 @@ BdrvDirtyBitmap *bdrv_find_dirty_bitmap(BlockDriverState *bs, const char *name)
 
 void bdrv_dirty_bitmap_make_anon(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
 {
+    assert(!bdrv_dirty_bitmap_frozen(bitmap));
     g_free(bitmap->name);
     bitmap->name = NULL;
 }
@@ -5454,9 +5464,98 @@ BdrvDirtyBitmap *bdrv_create_dirty_bitmap(BlockDriverState *bs,
     return bitmap;
 }
 
+bool bdrv_dirty_bitmap_frozen(BdrvDirtyBitmap *bitmap)
+{
+    return bitmap->successor;
+}
+
 bool bdrv_dirty_bitmap_enabled(BdrvDirtyBitmap *bitmap)
 {
-    return !bitmap->disabled;
+    return !(bitmap->disabled || bitmap->successor);
+}
+
+/**
+ * Create a successor bitmap destined to replace this bitmap after an operation.
+ * Requires that the bitmap is not frozen and has no successor.
+ */
+int bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
+                                       BdrvDirtyBitmap *bitmap, Error **errp)
+{
+    uint64_t granularity;
+    BdrvDirtyBitmap *child;
+
+    if (bdrv_dirty_bitmap_frozen(bitmap)) {
+        error_setg(errp, "Cannot create a successor for a bitmap that is "
+                   "currently frozen");
+        return -1;
+    }
+    assert(!bitmap->successor);
+
+    /* Create an anonymous successor */
+    granularity = bdrv_dirty_bitmap_granularity(bitmap);
+    child = bdrv_create_dirty_bitmap(bs, granularity, NULL, errp);
+    if (!child) {
+        return -1;
+    }
+
+    /* Successor will be on or off based on our current state. */
+    child->disabled = bitmap->disabled;
+
+    /* Install the successor and freeze the parent */
+    bitmap->successor = child;
+    return 0;
+}
+
+/**
+ * For a bitmap with a successor, yield our name to the successor,
+ * Delete the old bitmap, and return a handle to the new bitmap.
+ */
+BdrvDirtyBitmap *bdrv_dirty_bitmap_abdicate(BlockDriverState *bs,
+                                            BdrvDirtyBitmap *bitmap,
+                                            Error **errp)
+{
+    char *name;
+    BdrvDirtyBitmap *successor = bitmap->successor;
+
+    if (successor == NULL) {
+        error_setg(errp, "Cannot relinquish control if "
+                   "there's no successor present");
+        return NULL;
+    }
+
+    name = bitmap->name;
+    bitmap->name = NULL;
+    successor->name = name;
+    bitmap->successor = NULL;
+    bdrv_release_dirty_bitmap(bs, bitmap);
+
+    return successor;
+}
+
+/**
+ * In cases of failure where we can no longer safely delete the parent,
+ * We may wish to re-join the parent and child/successor.
+ * The merged parent will be un-frozen, but not explicitly re-enabled.
+ */
+BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
+                                           BdrvDirtyBitmap *parent,
+                                           Error **errp)
+{
+    BdrvDirtyBitmap *successor = parent->successor;
+
+    if (!successor) {
+        error_setg(errp, "Cannot reclaim a successor when none is present");
+        return NULL;
+    }
+
+    if (!hbitmap_merge(parent->bitmap, successor->bitmap)) {
+        error_setg(errp, "Merging of parent and successor bitmap failed");
+        return NULL;
+    }
+    bdrv_release_dirty_bitmap(bs, successor);
+    parent->successor = NULL;
+
+    return parent;
 }
 
 void bdrv_release_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
@@ -5464,6 +5563,7 @@ void bdrv_release_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
     BdrvDirtyBitmap *bm, *next;
     QLIST_FOREACH_SAFE(bm, &bs->dirty_bitmaps, list, next) {
         if (bm == bitmap) {
+            assert(!bdrv_dirty_bitmap_frozen(bm));
             QLIST_REMOVE(bitmap, list);
             hbitmap_free(bitmap->bitmap);
             g_free(bitmap->name);
@@ -5475,11 +5575,13 @@ void bdrv_release_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
 
 void bdrv_disable_dirty_bitmap(BdrvDirtyBitmap *bitmap)
 {
+    assert(!bdrv_dirty_bitmap_frozen(bitmap));
     bitmap->disabled = true;
 }
 
 void bdrv_enable_dirty_bitmap(BdrvDirtyBitmap *bitmap)
 {
+    assert(!bdrv_dirty_bitmap_frozen(bitmap));
     bitmap->disabled = false;
 }
 
diff --git a/blockdev.c b/blockdev.c
index 7d71b81..373d19a 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2064,9 +2064,16 @@ void qmp_block_dirty_bitmap_remove(const char *node, const char *name,
         return;
     }
 
+    if (bdrv_dirty_bitmap_frozen(bitmap)) {
+        error_setg(errp,
+                   "Bitmap '%s' is currently frozen and cannot be removed",
+                   name);
+        goto out;
+    }
     bdrv_dirty_bitmap_make_anon(bs, bitmap);
     bdrv_release_dirty_bitmap(bs, bitmap);
 
+ out:
     aio_context_release(aio_context);
 }
 
diff --git a/include/block/block.h b/include/block/block.h
index 029a8a7..b02c670 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -453,6 +453,15 @@ BdrvDirtyBitmap *bdrv_create_dirty_bitmap(BlockDriverState *bs,
                                           uint32_t granularity,
                                           const char *name,
                                           Error **errp);
+int bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
+                                       BdrvDirtyBitmap *bitmap,
+                                       Error **errp);
+BdrvDirtyBitmap *bdrv_dirty_bitmap_abdicate(BlockDriverState *bs,
+                                            BdrvDirtyBitmap *bitmap,
+                                            Error **errp);
+BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
+                                           BdrvDirtyBitmap *bitmap,
+                                           Error **errp);
 BdrvDirtyBitmap *bdrv_find_dirty_bitmap(BlockDriverState *bs,
                                         const char *name);
 void bdrv_dirty_bitmap_make_anon(BlockDriverState *bs, BdrvDirtyBitmap *bitmap);
@@ -463,6 +472,7 @@ BlockDirtyInfoList *bdrv_query_dirty_bitmaps(BlockDriverState *bs);
 uint32_t bdrv_get_default_bitmap_granularity(BlockDriverState *bs);
 uint32_t bdrv_dirty_bitmap_granularity(BdrvDirtyBitmap *bitmap);
 bool bdrv_dirty_bitmap_enabled(BdrvDirtyBitmap *bitmap);
+bool bdrv_dirty_bitmap_frozen(BdrvDirtyBitmap *bitmap);
 int bdrv_get_dirty(BlockDriverState *bs, BdrvDirtyBitmap *bitmap, int64_t sector);
 void bdrv_set_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap,
                            int64_t cur_sector, int nr_sectors);
diff --git a/qapi/block-core.json b/qapi/block-core.json
index faadd10..725a2a3 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1008,6 +1008,7 @@
 # Returns: nothing on success
 #          If @node is not a valid block device or node, DeviceNotFound
 #          If @name is not found, GenericError with an explanation
+#          if @name is frozen by an operation, GenericError
 #
 # Since 2.4
 ##
-- 
2.1.0