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Re: [Qemu-block] [Qemu-devel] [PATCH 05/11] block: add delayed bitmap su

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 05/11] block: add delayed bitmap successor cleanup
Date: Tue, 17 Mar 2015 15:12:31 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 03/17/2015 02:44 PM, Max Reitz wrote:
On 2015-03-04 at 23:15, John Snow wrote:
Allow bitmap successors to carry reference counts.

We can in a later patch use this ability to clean up the dirty bitmap
according to both the individual job's success and the success of all
jobs in the transaction group.

The code for cleaning up a bitmap is also moved from backup_run to

Signed-off-by: John Snow <address@hidden>
  block.c               | 79
  block/backup.c        | 23 ++++++---------
  include/block/block.h | 11 ++++---
  3 files changed, 87 insertions(+), 26 deletions(-)

diff --git a/block.c b/block.c
index 5eaa874..a0036af 100644
--- a/block.c
+++ b/block.c
@@ -51,6 +51,12 @@
  #include <windows.h>
+typedef enum BitmapSuccessorAction {
+} BitmapSuccessorAction;

Maybe making this a QAPI enum can come in handy later. I don't know in
which QMP commands one would use it, but if I could predict the future,
I'd make trillions (or even billions) at the stock market.

(It's just that it already looks so much alike to a QAPI enum that I
can't help but to think of making it one)

I don't see a reason to expose it, so I won't. We can always add it in later.

   * A BdrvDirtyBitmap can be in three possible states:
   * (1) successor is false and disabled is false: full r/w mode
@@ -65,6 +71,8 @@ struct BdrvDirtyBitmap {
      char *name;                 /* Optional non-empty unique ID */
      int64_t size;               /* Size of the bitmap (Number of
sectors) */
      bool disabled;              /* Bitmap is read-only */
+    int successor_refcount;     /* Number of active handles to the
successor */
+    BitmapSuccessorAction act;  /* Action to take on successor upon
release */
      QLIST_ENTRY(BdrvDirtyBitmap) list;
@@ -5508,6 +5516,7 @@ int
bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
      /* Install the successor and freeze the parent */
      bitmap->successor = child;
+    bitmap->successor_refcount = 1;
      return 0;
@@ -5515,9 +5524,9 @@ int
bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
   * 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)
+static BdrvDirtyBitmap *bdrv_dirty_bitmap_abdicate(BlockDriverState *bs,
+                                                   BdrvDirtyBitmap
+                                                   Error **errp)
      char *name;
      BdrvDirtyBitmap *successor = bitmap->successor;
@@ -5542,9 +5551,9 @@ BdrvDirtyBitmap
*bdrv_dirty_bitmap_abdicate(BlockDriverState *bs,
   * 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)
+static BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
+                                                  BdrvDirtyBitmap
+                                                  Error **errp)
      BdrvDirtyBitmap *successor = parent->successor;
@@ -5563,6 +5572,64 @@ BdrvDirtyBitmap
*bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
      return parent;
+static BdrvDirtyBitmap *bdrv_free_bitmap_successor(BlockDriverState *bs,
+                                                   BdrvDirtyBitmap
+                                                   Error **errp)
+    if (parent->successor_refcount) {

I don't know whether you're intending to call this function from
anywhere outside of bdrv_dirty_bitmap_decref(), but if you don't, please
just make this an assert(!parent->successor_refcount).


+        error_setg(errp, "Cannot free the successor for bitmap '%s', "
+                   "because the refcount is non-zero.", parent->name);

Hm, again with the full stops? *g*

Can we make checkpatch check for this? I am serious. I can't stop myself from doing this.\n

+        return NULL;
+    }
+    switch (parent->act) {
+        error_setg(errp, "Cannot free the successor for bitmap '%s', "
+                   "because the successor action has not yet been set.",

Indeed, again with the full stops. ;-)

+                   parent->name);
+        return NULL;

So you're (for now) only calling this function from
bdrv_dirty_bitmap_decref(), and that function always makes sure that
parent->act is set to SUCCESSOR_ACTION_{RECLAIM,ABDICATE}. Why not add
an assert(parent->act != SUCCESSOR_ACTION_UNDEFINED) before the switch?
(If that causes problems in regards to compiler warnings or something
(not having all enum values covered in the switch), just add an
assert(0); under "case SUCCESSOR_ACTION_UNDEFINED:".)

OK, sure.

+        return bdrv_reclaim_dirty_bitmap(bs, parent, errp);
+        return bdrv_dirty_bitmap_abdicate(bs, parent, errp);
+    default:
+        error_setg(errp,
+                   "Unrecognized successor action (%d), "
+                   "cannot free successor for bitmap '%s'",
+                   parent->act, parent->name);
+        return NULL;

This can be made an assert(0), a g_assert_not_reached(), or simply an
abort() (with the latter probably being the preferred way).

So I think that all the error_setg() calls are actually cases that
should never happen. Better make them abort() and drop the Error
parameter (because *_decref() and *_free() functions normally (for good
reason) don't have Error parameters...).

Uhh, yeah. I'll harden these functions to be meaner about failing.

+    }
+BdrvDirtyBitmap *bdrv_dirty_bitmap_decref(BlockDriverState *bs,

I don't know whether I'm that content with the name chosen, because
you're actually decrementing the refcount of the successor; but since
the successor is basically a clone of the original bitmap (and I mean in
the Star Trek sense, that it's a teleported clone and the original is
intended to be destroyed so the successor can replace it), decrementing
the refcount of the successor basically is equal to decrementing the
refcount of the bitmap itself (as long as there is a successor, which
you are asserting; maybe you want to add a comment about that to
include/block/block.h, that one can only use this on frozen bitmaps?).

Actual struggle I had when naming it myself. Went with the simpler answer that was less accurate. I might just augment with a comment explaining what exactly is being decremented.

+                                          BdrvDirtyBitmap *parent,
+                                          int ret,
+                                          Error **errp)
+    assert(bdrv_dirty_bitmap_frozen(parent));
+    assert(parent->successor);
+    if (ret) {
+        parent->act = SUCCESSOR_ACTION_RECLAIM;
+    } else if (parent->act != SUCCESSOR_ACTION_RECLAIM) {
+        parent->act = SUCCESSOR_ACTION_ABDICATE;
+    }
+    parent->successor_refcount--;
+    if (parent->successor_refcount == 0) {
+        return bdrv_free_bitmap_successor(bs, parent, errp);

If you drop the Error parameter from bdrv_free_bitmap_successor(), you
can drop it from this function, too.


+    }
+    return parent;
+void bdrv_dirty_bitmap_incref(BdrvDirtyBitmap *parent)
+    assert(bdrv_dirty_bitmap_frozen(parent));
+    assert(parent->successor);
+    parent->successor_refcount++;
  static void dirty_bitmap_truncate(BdrvDirtyBitmap *bitmap, uint64_t
      /* Should only be frozen during a block backup job, which should
diff --git a/block/backup.c b/block/backup.c
index 41bd9af..4332df4 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -240,6 +240,12 @@ static void backup_complete(BlockJob *job, void
+    if (s->sync_bitmap) {
+        BdrvDirtyBitmap *bm;
+        bm = bdrv_dirty_bitmap_decref(job->bs, s->sync_bitmap,
data->ret, NULL);
+        assert(bm);

You can use &error_abort as the Error object and drop the assert(); or,
if you are dropping the Error parameter, there is no need to check the
return value at all, because it will always be non-NULL (there won't be
any code path in the function returning NULL at all). Maybe you can even
drop the return value, too.

I just looked through the series: Actually, you're never using the Error
parameter for bdrv_dirty_bitmap_decref() at all. Seems to me like you
really should drop it (and maybe the return value along with it).

Return code is there for future usages, I suppose. I wanted to carry it around as a "generic" thing. In this one usage, we don't wind up needing it, but I wanted to make an allowance for it in this API.

I'll trim down the soft errors.

+    }
      block_job_completed(job, data->ret);
@@ -419,19 +425,6 @@ leave:
-    if (job->sync_bitmap) {
-        BdrvDirtyBitmap *bm;
-        if (ret < 0) {
-            /* Merge the successor back into the parent, delete
nothing. */
-            bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
-            assert(bm);
-            bdrv_enable_dirty_bitmap(job->sync_bitmap);

Hm, what is that function call doing here? It feels like it shouldn't
have been part of your transactionless series (because other than this,
there is no caller of bdrv_{en,dis}able_dirty_bitmap() at all).

Originally it was to re-enable a bitmap after it was frozen for the operation, but that concept has long since been ditched, so it's just a NOP right now, basically.

I'll try to pull it out of the original series.

(You're silently removing it here; removing it is fine, but I guess it
shouldn't have been there in the first place)

-        } else {
-            /* Everything is fine, delete this bitmap and install the
backup. */
-            bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
-            assert(bm);
-        }
-    }
@@ -535,6 +528,8 @@ void backup_start(BlockDriverState *bs,
BlockDriverState *target,
      if (sync_bitmap) {
-        bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
+        BdrvDirtyBitmap *ret;
+        ret = bdrv_dirty_bitmap_decref(bs, sync_bitmap, -1, NULL);
+        assert(ret);
diff --git a/include/block/block.h b/include/block/block.h
index 3a85690..d7859a7 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -458,12 +458,11 @@ void bdrv_dirty_bitmap_truncate(BlockDriverState
  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_dirty_bitmap_decref(BlockDriverState *bs,
+                                          BdrvDirtyBitmap *parent,
+                                          int ret,
+                                          Error **errp);
+void bdrv_dirty_bitmap_incref(BdrvDirtyBitmap *parent);
  BdrvDirtyBitmap *bdrv_find_dirty_bitmap(BlockDriverState *bs,
                                          const char *name);
  void bdrv_dirty_bitmap_make_anon(BdrvDirtyBitmap *bitmap);

I'm happy with this patch if you drop the Error parameter from
bdrv_dirty_bitmap_decref() (possibly dropping the return value, too) and
turn all the error cases into assertions.




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