Message-ID: <20250908092253.52cd4df0@fedora>
Date: Mon, 8 Sep 2025 09:22:53 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Matthew Brost
 <matthew.brost@...el.com>, "Thomas Hellström"
 <thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Steven Price <steven.price@....com>,
 Daniel Almeida <daniel.almeida@...labora.com>, Liviu Dudau
 <liviu.dudau@....com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/gpuvm: add deferred vm_bo cleanup

On Sun, 7 Sep 2025 11:15:20 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Sat, Sep 06, 2025 at 12:47:36AM +0200, Danilo Krummrich wrote:
> > On Fri Sep 5, 2025 at 8:18 PM CEST, Alice Ryhl wrote:  
> > > On Fri, Sep 5, 2025 at 3:25 PM Boris Brezillon
> > > <boris.brezillon@...labora.com> wrote:  
> > >> On Fri, 05 Sep 2025 12:11:28 +0000
> > >> Alice Ryhl <aliceryhl@...gle.com> wrote:  
> > >> > +static bool
> > >> > +drm_gpuvm_bo_is_dead(struct drm_gpuvm_bo *vm_bo)
> > >> > +{
> > >> > +     return !kref_read(&vm_bo->kref);  
> > >>
> > >> I'm not too sure I like the idea of [ab]using vm_bo::kref to defer the
> > >> vm_bo release. I get why it's done like that, but I'm wondering why we
> > >> don't defer the release of drm_gpuva objects instead (which is really
> > >> what's being released in va_unlink()). I can imagine drivers wanting to
> > >> attach resources to the gpuva that can't be released in the
> > >> dma-signalling path in the future, and if we're doing that at the gpuva
> > >> level, we also get rid of this kref dance, since the va will hold a
> > >> vm_bo ref until it's destroyed.
> > >>
> > >> Any particular reason you went for vm_bo destruction deferral instead
> > >> of gpuva?  
> > >
> > > All of the things that were unsafe to release in the signalling path
> > > were tied to the vm_bo, so that is why I went for vm_bo cleanup.
> > > Another advantage is that it lets us use the same deferred logic for
> > > the vm_bo_put() call that drops the refcount from vm_bo_obtain().
> > >
> > > Of course if gpuvas might have resources that need deferred cleanup,
> > > that might change the situation somewhat.  
> > 
> > I think we want to track PT(E) allocations, or rather reference counts of page
> > table structures carried by the drm_gpuva, but we don't need to release them on
> > drm_gpuva_unlink(), which is where we drop the reference count of the vm_bo.
> > 
> > Deferring drm_gpuva_unlink() isn't really an option I think, the GEMs list of
> > VM_BOs and the VM_BOs list of VAs is usually used in ttm_device_funcs::move to
> > map or unmap all VAs associated with a GEM object.
> > 
> > I think PT(E) reference counts etc. should be rather released when the drm_gpuva
> > is freed, i.e. page table allocations can be bound to the lifetime of a
> > drm_gpuva. Given that, I think that eventually we'll need a cleanup list for
> > those as well, since once they're removed from the VM tree (in the fence
> > signalling critical path), we loose access otherwise.  
> 
> Hmm. Another more conceptual issue with deferring gpuva is that
> "immediate mode" is defined as having the GPUVM match the GPU's actual
> address space at all times, which deferred gpuva cleanup would go
> against.
> 
> Deferring vm_bo cleanup doesn't have this issue because even though the
> vm_bo isn't kfreed immediately, all GPUVM apis still treat it as-if it
> isn't there anymore.
> 
> > >> > +static void
> > >> > +drm_gpuvm_bo_defer_locked(struct kref *kref)
> > >> > +{
> > >> > +     struct drm_gpuvm_bo *vm_bo = container_of(kref, struct drm_gpuvm_bo,
> > >> > +                                               kref);
> > >> > +     struct drm_gpuvm *gpuvm = vm_bo->vm;
> > >> > +
> > >> > +     if (!drm_gpuvm_resv_protected(gpuvm)) {
> > >> > +             drm_gpuvm_bo_list_del(vm_bo, extobj, true);
> > >> > +             drm_gpuvm_bo_list_del(vm_bo, evict, true);
> > >> > +     }
> > >> > +
> > >> > +     list_del(&vm_bo->list.entry.gem);
> > >> > +     mutex_unlock(&vm_bo->obj->gpuva.lock);  
> > >>
> > >> I got tricked by this implicit unlock, and the conditional unlocks it
> > >> creates in drm_gpuva_unlink_defer(). Honestly, I'd rather see this
> > >> unlocked moved to drm_gpuva_unlink_defer() and a conditional unlock
> > >> added to drm_gpuvm_bo_put_deferred(), because it's easier to reason
> > >> about when the lock/unlock calls are in the same function
> > >> (kref_put_mutex() being the equivalent of a conditional lock).  
> > >
> > > Ok. I followed the docs of kref_put_mutex() that say to unlock it from
> > > the function.  
> > 
> > Yes, please keep it the way it is, I don't want to deviate from what is
> > documented and everyone else does. Besides that, I also think it's a little
> > less error prone.  
> 
> I gave it a try:
> 
> bool
> drm_gpuvm_bo_put_deferred(struct drm_gpuvm_bo *vm_bo)
> {
> 	drm_WARN_ON(vm_bo->vm->drm, !drm_gpuvm_immediate_mode(vm_bo->vm));
> 
> 	if (!vm_bo)
> 		return false;
> 
> 	if (kref_put_mutex(&vm_bo->kref, drm_gpuvm_bo_defer_locked,
> 			   &vm_bo->obj->gpuva.lock)) {
> 		/*
> 		 * It's important that the GEM stays alive for the duration in which
> 		 * drm_gpuvm_bo_defer_locked() holds the mutex, but the instant we add
> 		 * the vm_bo to bo_defer, another thread might call
> 		 * drm_gpuvm_bo_deferred_cleanup() and put the GEM. For this reason, we
> 		 * add the vm_bo to bo_defer *after* releasing the GEM's mutex.
> 		 */
> 		mutex_unlock(&vm_bo->obj->gpuva.lock);
> 		drm_gpuvm_bo_list_add(vm_bo, bo_defer, true);
> 		return true;
> 	}
> 
> 	return false;
> }
> 
> void
> drm_gpuva_unlink_defer(struct drm_gpuva *va)
> {
> 	struct drm_gem_object *obj = va->gem.obj;
> 	struct drm_gpuvm_bo *vm_bo = va->vm_bo;
> 	bool should_defer_bo;
> 
> 	if (unlikely(!obj))
> 		return;
> 
> 	drm_WARN_ON(vm_bo->vm->drm, !drm_gpuvm_immediate_mode(vm_bo->vm));
> 
> 	mutex_lock(&obj->gpuva.lock);
> 	list_del_init(&va->gem.entry);
> 
> 	/*
> 	 * This is drm_gpuvm_bo_put_deferred() slightly modified since we
> 	 * already hold the mutex. It's important that we add the vm_bo to
> 	 * bo_defer after releasing the mutex for the same reason as in
> 	 * drm_gpuvm_bo_put_deferred().
> 	 */
> 	should_defer_bo = kref_put(&vm_bo->kref, drm_gpuvm_bo_defer_locked);
> 	mutex_unlock(&obj->gpuva.lock);
> 	if (should_defer_bo)
> 		drm_gpuvm_bo_list_add(vm_bo, bo_defer, true);
> 
> 	va->vm_bo = NULL;
> }
> 
> I do think it looks relatively nice like this, particularly
> drm_gpuva_unlink_defer().

I agree.

> But that's also the one not using
> kref_put_mutex().

Yeah, but that's the thing. I guess if drm_gpuvm_bo_defer_locked() was
only called from kref_put_mutex() this would be okay (though I still
have a hard time with those functions taking locks that have to be
released by the caller, but at least that's a well-known/documented
pattern). But it's also currently called from drm_gpuva_unlink_defer()
where the lock is taken but not released. I guess if the function name
was reflecting that (drm_gpuvm_bo_defer_locked_and_unlock()?), and with
a comment explaining why the lock is conditionally released in the
caller that would be acceptable, but I still find this locking scheme
quite confusing...

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