Re: [Qemu-devel] [PATCH V3 1/4] block: drop bs_snapshots global variable

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, May 29, 2013 at 03:54:46PM +0800, Wenchao Xia wrote:
> 于 2013-5-28 15:46, Stefan Hajnoczi 写道:
> >On Mon, May 27, 2013 at 05:25:16PM +0200, Kevin Wolf wrote:
> >>Am 25.05.2013 um 05:09 hat Wenchao Xia geschrieben:
> >>>From: Stefan Hajnoczi <stefanha@redhat.com>
> >>>
> >>>The bs_snapshots global variable points to the BlockDriverState which
> >>>will be used to save vmstate.  This is really a savevm.c concept but was
> >>>moved into block.c:bdrv_snapshots() when it became clear that hotplug
> >>>could result in a dangling pointer.
> >>>
> >>>While auditing the block layer's global state I came upon bs_snapshots
> >>>and realized that a variable is not necessary here.  Simply find the
> >>>first BlockDriverState capable of internal snapshots each time this is
> >>>needed.
> >>>
> >>>The behavior of bdrv_snapshots() is preserved across hotplug because new
> >>>drives are always appended to the bdrv_states list.  This means that
> >>>calling the new find_vmstate_bs() function is idempotent - it returns
> >>>the same BlockDriverState unless it was hot-unplugged.
> >>>
> >>>Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> >>>Reviewed-by: Eric Blake <eblake@redhat.com>
> >>>Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> >>>Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> >>>---
> >>>  block.c               |   28 ----------------------------
> >>>  include/block/block.h |    1 -
> >>>  savevm.c              |   19 +++++++++++++++----
> >>>  3 files changed, 15 insertions(+), 33 deletions(-)
> >>>
> >>>diff --git a/block.c b/block.c
> >>>index 3f87489..478a3b2 100644
> >>>--- a/block.c
> >>>+++ b/block.c
> >>>@@ -99,9 +99,6 @@ static QTAILQ_HEAD(, BlockDriverState) bdrv_states =
> >>>  static QLIST_HEAD(, BlockDriver) bdrv_drivers =
> >>>      QLIST_HEAD_INITIALIZER(bdrv_drivers);
> >>>
> >>>-/* The device to use for VM snapshots */
> >>>-static BlockDriverState *bs_snapshots;
> >>>-
> >>>  /* If non-zero, use only whitelisted block drivers */
> >>>  static int use_bdrv_whitelist;
> >>>
> >>>@@ -1357,9 +1354,6 @@ void bdrv_close(BlockDriverState *bs)
> >>>      notifier_list_notify(&bs->close_notifiers, bs);
> >>>
> >>>      if (bs->drv) {
> >>>-        if (bs == bs_snapshots) {
> >>>-            bs_snapshots = NULL;
> >>>-        }
> >>>          if (bs->backing_hd) {
> >>>              bdrv_delete(bs->backing_hd);
> >>>              bs->backing_hd = NULL;
> >>>@@ -1591,7 +1585,6 @@ void bdrv_delete(BlockDriverState *bs)
> >>>
> >>>      bdrv_close(bs);
> >>>
> >>>-    assert(bs != bs_snapshots);
> >>>      g_free(bs);
> >>>  }
> >>>
> >>>@@ -1635,9 +1628,6 @@ void bdrv_set_dev_ops(BlockDriverState *bs, const BlockDevOps *ops,
> >>>  {
> >>>      bs->dev_ops = ops;
> >>>      bs->dev_opaque = opaque;
> >>>-    if (bdrv_dev_has_removable_media(bs) && bs == bs_snapshots) {
> >>>-        bs_snapshots = NULL;
> >>>-    }
> >>
> >>This hunk isn't replaced by any other code. If I understand correctly
> >>what it's doing, it prevented you from saving the VM state to a
> >>removable device, which would be allowed after this patch.
> >>
> >>Is this a change we want to make? Why isn't it described in the commit
> >>message?
> >
> >My understanding of this change is different.  Markus is on CC so maybe
> >he can confirm.
> >
> >The point of bs_snapshots = NULL is not to prevent you from saving
> >snapshots.  It's simply to reset the pointer to the next snapshottable
> >device (used by bdrv_snapshots()).
> >
> >See the bdrv_close() hunk above which does the same thing, as well as
> >bdrv_snapshots() which iterates bdrv_states and updates bs_snapshots.
> >
> >So what this hunk does is to reset the bdrv_snapshots() iterator when a
> >removable device is hooked up to an emulated storage controller.  It's
> >no longer necessary since this patch drops the global state
> >(bs_snapshots) and users will always iterate from scratch.
> >
> >The whole stateful approach was not necessary.
> >
> >Stefan
> >
>   Reading the code, original logic actually forbidded saving vmstate
> into a removable device, now it is possible since find_vmstate_bs()
> doesn't check it. How about forbid again in find_vmstate_bs()?

I don't follow.

The hunk that Kevin quoted ensures that we restart bs_snapshots
iteration - this prevents us from choosing a device that has no medium
inserted (also see bdrv_can_snapshot() which checks for an inserted
medium).

This behavior is preserved in this patch because we now always restart
iteration and it's no longer necessary to reset global iterator state.

But I don't see any code that forbids/skips inserted, read-write
removable devices in the orginal code.  Please point out the specific
piece of code you think has been dropped.

Stefan