Re: [PATCH v4 6/7] memory: Do not create circular reference with subregion

[Akihiko Odaki]

On 2024/08/27 4:42, Peter Xu wrote:
> On Mon, Aug 26, 2024 at 06:10:25PM +0100, Peter Maydell wrote:
> > On Mon, 26 Aug 2024 at 16:22, Peter Xu <peterx@redhat.com> wrote:
> > > On Fri, Aug 23, 2024 at 03:13:11PM +0900, Akihiko Odaki wrote:
> > > > memory_region_update_container_subregions() used to call
> > > > memory_region_ref(), which creates a reference to the owner of the
> > > > subregion, on behalf of the owner of the container. This results in a
> > > > circular reference if the subregion and container have the same owner.
> > > >
> > > > memory_region_ref() creates a reference to the owner instead of the
> > > > memory region to match the lifetime of the owner and memory region. We
> > > > do not need such a hack if the subregion and container have the same
> > > > owner because the owner will be alive as long as the container is.
> > > > Therefore, create a reference to the subregion itself instead ot its
> > > > owner in such a case; the reference to the subregion is still necessary
> > > > to ensure that the subregion gets finalized after the container.
> > > >
> > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > > ---
> > > >   system/memory.c | 8 ++++++--
> > > >   1 file changed, 6 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/system/memory.c b/system/memory.c
> > > > index 5e6eb459d5de..e4d3e9d1f427 100644
> > > > --- a/system/memory.c
> > > > +++ b/system/memory.c
> > > > @@ -2612,7 +2612,9 @@ static void 
> > > > memory_region_update_container_subregions(MemoryRegion *subregion)
> > > >
> > > >       memory_region_transaction_begin();
> > > >
> > > > -    memory_region_ref(subregion);
> > > > +    object_ref(mr->owner == subregion->owner ?
> > > > +               OBJECT(subregion) : subregion->owner);
> > >
> > > The only place that mr->refcount is used so far is the owner with the
> > > object property attached to the mr, am I right (ignoring name-less MRs)?
> > >
> > > I worry this will further complicate refcounting, now we're actively using
> > > two refcounts for MRs..

The actor of object_ref() is the owner of the memory region also in this 
case. We are calling object_ref() on behalf of mr->owner so we use 
mr->refcount iff mr->owner == subregion->owner. In this sense there is 
only one user of mr->refcount even after this change.

> > > Continue discussion there:
> > >
> > > https://lore.kernel.org/r/067b17a4-cdfc-4f7e-b7e4-28c38e1c10f0@daynix.com
> > >
> > > What I don't see is how mr->subregions differs from mr->container, so we
> > > allow subregions to be attached but not the container when finalize()
> > > (which is, afaict, the other way round).
> > >
> > > It seems easier to me that we allow both container and subregions to exist
> > > as long as within the owner itself, rather than start heavier use of
> > > mr->refcount.
> >
> > I don't think just "same owner" necessarily will be workable --
> > you can have a setup like:
> >    * device A has a container C_A
> >    * device A has a child-device B
> >    * device B has a memory region R_B
> >    * device A's realize method puts R_B into C_A
> >
> > R_B's owner is B, and the container's owner is A,
> > but we still want to be able to get rid of A (in the process
> > getting rid of B because it gets unparented and unreffed,
> > and R_B and C_A also).
>
> For cross-device references, should we rely on an explicit call to
> memory_region_del_subregion(), so as to detach the link between C_A and
> R_B?

Yes, I agree.

> My understanding so far: logically when MR finalize() it should guarantee
> both (1) mr->container==NULL, and (2) mr->subregions empty.  That's before
> commit 2e2b8eb70fdb7dfb and could be the ideal world (though at the very
> beginning we don't assert on ->container==NULL yet).  It requires all
> device emulations to do proper unrealize() to unlink all the MRs.
>
> However what I'm guessing is QEMU probably used to have lots of devices
> that are not following the rules and leaking these links.  Hence we have
> had 2e2b8eb70fdb7dfb, allowing that to happen as long as it's safe, and
> it's justified by comment in 2e2b8eb70fdb7dfb on why it's safe.
>
> What I was thinking is this comment seems to apply too to mr->container, so
> that it should be safe too to unlink ->container the same way as its own
> subregions. >
> IIUC that means for device-internal MR links we should be fine leaving
> whatever link between MRs owned by such device; the device->refcount
> guarantees none of them will be visible in any AS.  But then we need to
> always properly unlink the MRs when the link is across >1 device owners,
> otherwise it's prone to leak.

There is one principle we must satisfy in general: keep a reference to a 
memory region if it is visible to the guest.

It is safe to call memory_region_del_subregion() and to trigger the 
finalization of subregions when the container is not referenced because 
they are no longer visible. This is not true for the other way around; 
even when subregions are not referenced by anyone else, they are still 
visible to the guest as long as the container is visible to the guest. 
It is not safe to unref and finalize them in such a case.

A memory region and its owner will leak if a memory region kept visible 
for a too long period whether the chain of reference contains a 
container/subregion relationship or not.

Regards,
Akihiko Odaki