On 2024/08/28 1:11, Peter Xu wrote:
On Tue, Aug 27, 2024 at 01:14:51PM +0900, Akihiko Odaki wrote:
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 <pet...@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.od...@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.
Yes it's still one user, but it's not that straightforward to see, also
it's still an extension to how we use mr->refcount right now. Currently
it's about "true / false" just to describe, now it's a real counter.
I wished that counter doesn't even exist if we'd like to stick with device
/ owner's counter. Adding this can definitely also make further effort
harder if we want to remove mr->refcount.
I don't think it will make removing mr->refcount harder. With this
change, mr->refcount will count the parent and container. If we remove
mr->refcount, we need to trigger object_finalize() in a way other than
checking mr->refcount, which can be achieved by simply evaluating
OBJECT(mr)->parent && mr->container.
Continue discussion there:
https://lore.kernel.org/r/067b17a4-cdfc-4f7e-b7e4-28c38e1c1...@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.
Could you elaborate why it's still visible to the guest if
owner->refcount==0 && mr->container!=NULL?
Firstly, mr->container != NULL means the MR has an user indeed. It's the
matter of who's using it. If that came from outside this device, it should
require memory_region_ref(mr) before hand when adding the subregion, and
that will hold one reference on the owner->refcount.
Here owner->refcount==0 means there's no such reference, so it seems to me
it's guaranteed to not be visible to anything outside of this device / owner.
Then from that POV it's safe to unlink when the owner is finalizing just
like what we do with mr->subregions, no?
An object is alive during instance_finalize even though its refcount ==
0. We can't assume all memory regions are dead even if owner->refcount
== 0 because of that. In particular, docs/devel/memory.rst says you can
call object_unparent() in the instance_finalize of the owner. This
assumes a memory region will not vanish during the execution of the
function unless object_unparent() is already called for the memory region.
Regards,
Akihiko Odaki
