Nouveau - Nov 2023 - [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count drm_gpuvm structures

On 11/10/23 09:50, Christian K?nig wrote:
> Am 09.11.23 um 19:34 schrieb Danilo Krummrich:
>> On 11/9/23 17:03, Christian K?nig wrote:
>>> Am 09.11.23 um 16:50 schrieb Thomas Hellstr?m:
>>>> [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
>>>>
>>>>
>>>
>>
>

Am 10.11.23 um 10:39 schrieb Thomas Hellstr?m:
>
> [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.

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
>>>>>
>>>>>
>>>>
>>>
>>

On 11/10/23 10:39, Thomas Hellstr?m wrote:
> 
> On 11/10/23 09:50, Christian K?nig wrote:
>> Am 09.11.23 um 19:34 schrieb Danilo Krummrich:
>>> On 11/9/23 17:03, Christian K?nig wrote:
>>>> Am 09.11.23 um 16:50 schrieb Thomas Hellstr?m:
>>>>> [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?
Sure, I just wanted to say that we'd then have a mandatory callback where
drivers *must* ensure
to remove *all* mappings before returning from this callback. I could imagine
that there could
be some pitfalls with that.

So I'm not worried about the callback itself being mandatory, but about
enforcing this semantics
on it. Maybe I didn't phrase this very well.
> 
> +??? 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
>>>>>
>>>>>
>>>>
>>>
>>
>