On 11/10/23 09:50, Christian König wrote:
Am 09.11.23 um 19:34 schrieb Danilo Krummrich:
On 11/9/23 17:03, Christian König wrote:
Am 09.11.23 um 16:50 schrieb Thomas Hellström:
[SNIP]
Did we get any resolution on this?
FWIW, my take on this is that it would be possible to get GPUVM to
work both with and without internal refcounting; If with, the
driver needs a vm close to resolve cyclic references, if without
that's not necessary. If GPUVM is allowed to refcount in mappings
and vm_bos, that comes with a slight performance drop but as Danilo
pointed out, the VM lifetime problem iterating over a vm_bo's
mapping becomes much easier and the code thus becomes easier to
maintain moving forward. That convinced me it's a good thing.
I strongly believe you guys stumbled over one of the core problems
with the VM here and I think that reference counting is the right
answer to solving this.
The big question is that what is reference counted and in which
direction does the dependency points, e.g. we have here VM, BO,
BO_VM and Mapping objects.
Those patches here suggest a counted Mapping -> VM reference and I'm
pretty sure that this isn't a good idea. What we should rather
really have is a BO -> VM or BO_VM ->VM reference. In other words
that each BO which is part of the VM keeps a reference to the VM.
We have both. Please see the subsequent patch introducing VM_BO
structures for that.
As I explained, mappings (struct drm_gpuva) keep a pointer to their
VM they're mapped
in and besides that it doesn't make sense to free a VM that still
contains mappings,
the reference count ensures that. This simply ensures memory safety.
BTW: At least in amdgpu we can have BOs which (temporary) doesn't
have any mappings, but are still considered part of the VM.
That should be possible.
Another issue Christian brought up is that something intended to be
embeddable (a base class) shouldn't really have its own refcount. I
think that's a valid point. If you at some point need to derive
from multiple such structs each having its own refcount, things
will start to get weird. One way to resolve that would be to have
the driver's subclass provide get() and put() ops, and export a
destructor for the base-class, rather than to have the base-class
provide the refcount and a destructor ops.
GPUVM simply follows the same pattern we have with drm_gem_objects.
And I think it makes
sense. Why would we want to embed two struct drm_gpuvm in a single
driver structure?
Because you need one drm_gpuvm structure for each application using
the driver? Or am I missing something?
As far as I can see a driver would want to embed that into your fpriv
structure which is allocated during drm_driver.open callback.
I was thinking more of the general design of a base-class that needs to
be refcounted. Say a driver vm that inherits from gpu-vm, gem_object and
yet another base-class that supplies its own refcount. What's the
best-practice way to do refcounting? All base-classes supplying a
refcount of its own, or the subclass supplying a refcount and the
base-classes supply destroy helpers.
But to be clear this is nothing I see needing urgent attention.
Well, I have never seen stuff like that in the kernel. Might be that
this works, but I would rather not try if avoidable.
That would also make it possible for the driver to decide the
context for the put() call: If the driver needs to be able to call
put() from irq / atomic context but the base-class'es destructor
doesn't allow atomic context, the driver can push freeing out to a
work item if needed.
Finally, the refcount overflow Christian pointed out. Limiting the
number of mapping sounds like a reasonable remedy to me.
Well that depends, I would rather avoid having a dependency for
mappings.
Taking the CPU VM handling as example as far as I know
vm_area_structs doesn't grab a reference to their mm_struct either.
Instead they get automatically destroyed when the mm_struct is
destroyed.
Certainly, that would be possible. However, thinking about it, this
might call for
huge trouble.
First of all, we'd still need to reference count a GPUVM and take a
reference for each
VM_BO, as we do already. Now instead of simply increasing the
reference count for each
mapping as well, we'd need a *mandatory* driver callback that is
called when the GPUVM
reference count drops to zero. Maybe something like vm_destroy().
The reason is that GPUVM can't just remove all mappings from the tree
nor can it free them
by itself, since drivers might use them for tracking their allocated
page tables and/or
other stuff.
Now, let's think about the scope this callback might be called from.
When a VM_BO is destroyed
the driver might hold a couple of locks (for Xe it would be the VM's
shared dma-resv lock and
potentially the corresponding object's dma-resv lock if they're not
the same already). If
destroying this VM_BO leads to the VM being destroyed, the drivers
vm_destroy() callback would
be called with those locks being held as well.
I feel like doing this finally opens the doors of the locking hell
entirely. I think we should
really avoid that.
I don't think we need to worry much about this particular locking hell
because if we hold, for example a vm and bo resv when putting the vm_bo,
we need to keep additional strong references for the bo / vm pointer we
use for unlocking. Hence putting the vm_bo under those locks can never
lead to the vm getting destroyed.
Also, don't we already sort of have a mandatory vm_destroy callback?
+ if (drm_WARN_ON(gpuvm->drm, !gpuvm->ops->vm_free))
+ return;
That's a really good point, but I fear exactly that's the use case.
I would expect that VM_BO structures are added in the
drm_gem_object_funcs.open callback and freed in
drm_gem_object_funcs.close.
Since it is perfectly legal for userspace to close a BO while there
are still mappings (can trivial be that the app is killed) I would
expect that the drm_gem_object_funcs.close handling is something like
asking drm_gpuvm destroying the VM_BO and getting the mappings which
should be cleared in the page table in return.
In amdgpu we even go a step further and the VM structure keeps track
of all the mappings of deleted VM_BOs so that higher level can query
those and clear them later on.
Background is that the drm_gem_object_funcs.close can't fail, but it
can perfectly be that the app is killed because of an OOM situation
and we can't do page tables updates in that moment because of this.
Which makes sense in that case because when the mm_struct is gone
the vm_area_struct doesn't make sense any more either.
What we clearly need is a reference to prevent the VM or at least
the shared resv to go away to early.
Yeah, that was a good hint and we've covered that.
Regards,
Christian.
But I think all of this is fixable as follow-ups if needed, unless
I'm missing something crucial.
Fully agree, I think at this point we should go ahead and land this
series.
+1.
/Thomas
Yeah, agree this is not UAPI so not nailed in stone. Feel free to add
my acked-by as well if you want.
Only keep in mind that when you give drivers some functionality in a
common component they usually expect to keep that functionality.
For example changing the dma_resv object to make sure that drivers
can't cause use after free errors any more was an extremely annoying
experience since every user of those interface had to change at once.
Regards,
Christian.
Just my 2 cents.
/Thomas
