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Re: [Qemu-devel] [PATCH v3 1/5] block: Allow replacement of a BDS by its
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v3 1/5] block: Allow replacement of a BDS by its overlay |
Date: |
Mon, 13 Jun 2016 17:13:40 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 12.06.2016 um 05:15 hat Fam Zheng geschrieben:
> On Fri, 06/10 20:57, Max Reitz wrote:
> > change_parent_backing_link() asserts that the BDS to be replaced is not
> > used as a backing file. However, we may want to replace a BDS by its
> > overlay in which case that very link should not be redirected.
> >
> > For instance, when doing a sync=none drive-mirror operation, we may have
> > the following BDS/BB forest before block job completion:
> >
> > target
> >
> > base <- source <- BlockBackend
> >
> > During job completion, we want to establish the source BDS as the
> > target's backing node:
> >
> > target
> > |
> > v
> > base <- source <- BlockBackend
> >
> > This makes the target a valid replacement for the source:
> >
> > target <- BlockBackend
> > |
> > v
> > base <- source
> >
> > Without this modification to change_parent_backing_link() we have to
> > inject the target into the graph before the source is its backing node,
> > thus temporarily creating a wrong graph:
> >
> > target <- BlockBackend
> >
> > base <- source
> >
> > Signed-off-by: Max Reitz <address@hidden>
> > ---
> > block.c | 16 +++++++++++++++-
> > 1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/block.c b/block.c
> > index f54bc25..dc76c159 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -2224,9 +2224,23 @@ void bdrv_close_all(void)
> > static void change_parent_backing_link(BlockDriverState *from,
> > BlockDriverState *to)
> > {
> > - BdrvChild *c, *next;
> > + BdrvChild *c, *next, *to_c;
> >
> > QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
> > + if (c->role == &child_backing) {
> > + /* @from is generally not allowed to be a backing file, except
> > for
> > + * when @to is the overlay. In that case, @from may not be
> > replaced
> > + * by @to as @to's backing node. */
> > + QLIST_FOREACH(to_c, &to->children, next) {
> > + if (to_c == c) {
> > + break;
> > + }
> > + }
>
> Why not just test "c->bs == to" here, why is it necessary to iterate through
> to->children list?
Because c->bs == from. We want to check whether 'to' is the parent, not
whether it is the child.
Kevin
- [Qemu-devel] [PATCH v3 0/5] block/mirror: Fix (?) target backing BDS, Max Reitz, 2016/06/10
- [Qemu-devel] [PATCH v3 1/5] block: Allow replacement of a BDS by its overlay, Max Reitz, 2016/06/10
- [Qemu-devel] [PATCH v3 4/5] iotests: Add test for post-mirror backing chains, Max Reitz, 2016/06/10
- [Qemu-devel] [PATCH v3 3/5] block/null: Implement bdrv_refresh_filename(), Max Reitz, 2016/06/10
- [Qemu-devel] [PATCH v3 5/5] iotests: Add test for oVirt-like storage migration, Max Reitz, 2016/06/10
- [Qemu-devel] [PATCH v3 2/5] block/mirror: Fix target backing BDS, Max Reitz, 2016/06/10
- Re: [Qemu-devel] [PATCH v3 0/5] block/mirror: Fix (?) target backing BDS, Fam Zheng, 2016/06/12
- Re: [Qemu-devel] [PATCH v3 0/5] block/mirror: Fix (?) target backing BDS, Kevin Wolf, 2016/06/13