Re: [PATCH v4 2/3] block: improve block_dirty_bitmap_merge(): don't allocate extra bitmap

[Eric Blake]

On Tue, May 17, 2022 at 02:12:05PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> From: Vladimir Sementsov-Ogievskiy 
> 
> We don't need extra bitmap. All we need is to backup the original
> bitmap when we do first merge. So, drop extra temporary bitmap and work
> directly with target and backup.
> 
> Still to keep old semantics, that on failure target is unchanged and
> user don't need to restore, we need a local_backup variable and do
> restore ourselves on failure path.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy 
> ---
>  block/monitor/bitmap-qmp-cmds.c | 41 +
>  1 file changed, 21 insertions(+), 20 deletions(-)

Reviewed-by: Eric Blake 

> 
> diff --git a/block/monitor/bitmap-qmp-cmds.c b/block/monitor/bitmap-qmp-cmds.c
> index bd10468596..282363606f 100644
> --- a/block/monitor/bitmap-qmp-cmds.c
> +++ b/block/monitor/bitmap-qmp-cmds.c
> @@ -261,8 +261,9 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char 
> *node, const char *target,
>HBitmap **backup, Error **errp)
>  {
>  BlockDriverState *bs;
> -BdrvDirtyBitmap *dst, *src, *anon;
> +BdrvDirtyBitmap *dst, *src;
>  BlockDirtyBitmapOrStrList *lst;
> +HBitmap *local_backup = NULL;

Basically, instead of tracking the backup via a full-featured
BdrvDirtyBitmap, we are instead tracking it via a lightweight HBitmap.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

[PATCH v4 2/3] block: improve block_dirty_bitmap_merge(): don't allocate extra bitmap

[Vladimir Sementsov-Ogievskiy]

From: Vladimir Sementsov-Ogievskiy 

We don't need extra bitmap. All we need is to backup the original
bitmap when we do first merge. So, drop extra temporary bitmap and work
directly with target and backup.

Still to keep old semantics, that on failure target is unchanged and
user don't need to restore, we need a local_backup variable and do
restore ourselves on failure path.

Signed-off-by: Vladimir Sementsov-Ogievskiy 
---
 block/monitor/bitmap-qmp-cmds.c | 41 +
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/block/monitor/bitmap-qmp-cmds.c b/block/monitor/bitmap-qmp-cmds.c
index bd10468596..282363606f 100644
--- a/block/monitor/bitmap-qmp-cmds.c
+++ b/block/monitor/bitmap-qmp-cmds.c
@@ -261,8 +261,9 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char *node, 
const char *target,
   HBitmap **backup, Error **errp)
 {
 BlockDriverState *bs;
-BdrvDirtyBitmap *dst, *src, *anon;
+BdrvDirtyBitmap *dst, *src;
 BlockDirtyBitmapOrStrList *lst;
+HBitmap *local_backup = NULL;
 
 GLOBAL_STATE_CODE();
 
@@ -271,12 +272,6 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char 
*node, const char *target,
 return NULL;
 }
 
-anon = bdrv_create_dirty_bitmap(bs, bdrv_dirty_bitmap_granularity(dst),
-NULL, errp);
-if (!anon) {
-return NULL;
-}
-
 for (lst = bms; lst; lst = lst->next) {
 switch (lst->value->type) {
 const char *name, *node;
@@ -285,8 +280,7 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char *node, 
const char *target,
 src = bdrv_find_dirty_bitmap(bs, name);
 if (!src) {
 error_setg(errp, "Dirty bitmap '%s' not found", name);
-dst = NULL;
-goto out;
+goto fail;
 }
 break;
 case QTYPE_QDICT:
@@ -294,29 +288,36 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char 
*node, const char *target,
 name = lst->value->u.external.name;
 src = block_dirty_bitmap_lookup(node, name, NULL, errp);
 if (!src) {
-dst = NULL;
-goto out;
+goto fail;
 }
 break;
 default:
 abort();
 }
 
-if (!bdrv_merge_dirty_bitmap(anon, src, NULL, errp)) {
-dst = NULL;
-goto out;
+/* We do backup only for first merge operation */
+if (!bdrv_merge_dirty_bitmap(dst, src,
+ local_backup ? NULL : _backup,
+ errp))
+{
+goto fail;
 }
 }
 
-/* Merge into dst; dst is unchanged on failure. */
-if (!bdrv_merge_dirty_bitmap(dst, anon, backup, errp)) {
-dst = NULL;
-goto out;
+if (backup) {
+*backup = local_backup;
+} else {
+hbitmap_free(local_backup);
 }
 
- out:
-bdrv_release_dirty_bitmap(anon);
 return dst;
+
+fail:
+if (local_backup) {
+bdrv_restore_dirty_bitmap(dst, local_backup);
+}
+
+return NULL;
 }
 
 void qmp_block_dirty_bitmap_merge(const char *node, const char *target,
-- 
2.35.1