Message-ID: <7eba1e4d-b594-4b88-9f8a-694fc6663f85@redhat.com>
Date:   Thu, 2 Nov 2023 18:32:43 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>, airlied@...il.com,
        daniel@...ll.ch, matthew.brost@...el.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, boris.brezillon@...labora.com,
        christian.koenig@....com, faith@...strand.net
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count
 drm_gpuvm structures

Hi Thomas,

thanks for your timely response on that!

On 11/2/23 18:09, Thomas Hellström wrote:
> On Thu, 2023-11-02 at 00:31 +0100, Danilo Krummrich wrote:
>> Implement reference counting for struct drm_gpuvm.
>>
>> Signed-off-by: Danilo Krummrich <dakr@...hat.com>
>> ---
>>   drivers/gpu/drm/drm_gpuvm.c            | 44 +++++++++++++++++++-----
>> --
>>   drivers/gpu/drm/nouveau/nouveau_uvmm.c | 20 +++++++++---
>>   include/drm/drm_gpuvm.h                | 31 +++++++++++++++++-
>>   3 files changed, 78 insertions(+), 17 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gpuvm.c
>> b/drivers/gpu/drm/drm_gpuvm.c
>> index 53e2c406fb04..6a88eafc5229 100644
>> --- a/drivers/gpu/drm/drm_gpuvm.c
>> +++ b/drivers/gpu/drm/drm_gpuvm.c
>> @@ -746,6 +746,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const
>> char *name,
>>          gpuvm->rb.tree = RB_ROOT_CACHED;
>>          INIT_LIST_HEAD(&gpuvm->rb.list);
>>   
>> +       kref_init(&gpuvm->kref);
>> +
>>          gpuvm->name = name ? name : "unknown";
>>          gpuvm->flags = flags;
>>          gpuvm->ops = ops;
>> @@ -770,15 +772,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const
>> char *name,
>>   }
>>   EXPORT_SYMBOL_GPL(drm_gpuvm_init);
>>   
>> -/**
>> - * drm_gpuvm_destroy() - cleanup a &drm_gpuvm
>> - * @gpuvm: pointer to the &drm_gpuvm to clean up
>> - *
>> - * Note that it is a bug to call this function on a manager that
>> still
>> - * holds GPU VA mappings.
>> - */
>> -void
>> -drm_gpuvm_destroy(struct drm_gpuvm *gpuvm)
>> +static void
>> +drm_gpuvm_fini(struct drm_gpuvm *gpuvm)
>>   {
>>          gpuvm->name = NULL;
>>   
>> @@ -790,7 +785,33 @@ drm_gpuvm_destroy(struct drm_gpuvm *gpuvm)
>>   
>>          drm_gem_object_put(gpuvm->r_obj);
>>   }
>> -EXPORT_SYMBOL_GPL(drm_gpuvm_destroy);
>> +
>> +static void
>> +drm_gpuvm_free(struct kref *kref)
>> +{
>> +       struct drm_gpuvm *gpuvm = container_of(kref, struct
>> drm_gpuvm, kref);
>> +
>> +       if (drm_WARN_ON(gpuvm->drm, !gpuvm->ops->vm_free))
>> +               return;
>> +
>> +       drm_gpuvm_fini(gpuvm);
>> +
>> +       gpuvm->ops->vm_free(gpuvm);
>> +}
>> +
>> +/**
>> + * drm_gpuvm_bo_put() - drop a struct drm_gpuvm reference
> copy-paste error in function name.
> 
> Also it appears like xe might put a vm from irq context so we should
> document the context where this function call is allowable, and if
> applicable add a might_sleep().

 From GPUVM PoV I don't see why we can't call this from an IRQ context.
It depends on the driver callbacks of GPUVM (->vm_free) and the resv GEM's
free callback. Both are controlled by the driver. Hence, I don't see the
need for a restriction here.

> 
> If this function needs to sleep we can work around that in Xe by
> keeping an xe-private refcount for the xe vm container, but I'd like to
> avoid that if possible and piggy-back on the refcount introduced here.
> 
>> + * @gpuvm: the &drm_gpuvm to release the reference of
>> + *
>> + * This releases a reference to @gpuvm.
>> + */
>> +void
>> +drm_gpuvm_put(struct drm_gpuvm *gpuvm)
>> +{
>> +       if (gpuvm)
>> +               kref_put(&gpuvm->kref, drm_gpuvm_free);
>> +}
>> +EXPORT_SYMBOL_GPL(drm_gpuvm_put);
>>   
>>   static int
>>   __drm_gpuva_insert(struct drm_gpuvm *gpuvm,
>> @@ -843,7 +864,7 @@ drm_gpuva_insert(struct drm_gpuvm *gpuvm,
>>          if (unlikely(!drm_gpuvm_range_valid(gpuvm, addr, range)))
>>                  return -EINVAL;
>>   
>> -       return __drm_gpuva_insert(gpuvm, va);
>> +       return __drm_gpuva_insert(drm_gpuvm_get(gpuvm), va);
> 
> Here we leak a reference if __drm_gpuva_insert() fails, and IMO the
> reference should be taken where the pointer holding the reference is
> assigned (in this case in __drm_gpuva_insert()), or document the
> reference transfer from the argument close to the assignment.

Ah, good catch. I had it in __drm_gpuva_insert() originally, but that
doesn't work, because __drm_gpuva_insert() is used to insert the
kernel_alloc_node. And we need to __drm_gpuva_remove() the kernel_alloc_node
from drm_gpuvm_fini(), which is called when the reference count is at zero
already. In fact, the __* variants are only there to handle the
kernel_alloc_node and this one clearly doesn't need reference counting.

> 
> But since a va itself is not refcounted it clearly can't outlive the
> vm, so is a reference really needed here?

Well, technically, it can. It just doesn't make any sense and would be
considered to be a bug. The reference count comes in handy to prevent
that in the first place.

I'd like to keep the reference count and just fix up the code.

> 
> I'd suggest using an accessor that instead of using va->vm uses va-
>> vm_bo->vm, to avoid needing to worry about the vm->vm refcount
> altoghether.

No, I want to keep that optional. Drivers should be able to use GPUVM to
track mappings without being required to implement everything else.

I think PowerVR, for instance, currently uses GPUVM only to track mappings
without everything else.

- Danilo

> 
> Thanks,
> Thomas
> 

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