On 11/10/23 11:42, Christian König wrote:
Am 10.11.23 um 10:39 schrieb Thomas Hellström:
[SNIP]
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.
From my experience the most common design pattern in the Linux kernel
is that you either have reference counted objects which contain a
private pointer (like struct file, struct inode etc..) or the lifetime
is defined by the user of the object instead of reference counting and
in this case you can embed it into your own object.
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
I have to agree with Danilo here. Especially you have cases where you
usually lock BO->VM (for example eviction) as well as cases where you
need to lock VM->BO (command submission).
Because of this in amdgpu we used (or abused?) the dma_resv of the
root BO as lock for the VM. Since this is a ww_mutex locking it in
both VM, BO as well as BO, VM order works.
Yes, gpuvm is doing the same. (although not necessarily using the
page-table root bo, but any bo of the driver's choice). But I read it as
Danilo feared the case where the VM destructor was called with a VM resv
(or possibly bo resv) held. I meant the driver can easily ensure that's
not happening, and in some cases it can't happen.
Thanks,
Thomas
Regards,
Christian.
, 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
