On Sun, 7 Sep 2025 11:15:20 +0000 Alice Ryhl <[email protected]> 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 > > > <[email protected]> wrote: > > >> On Fri, 05 Sep 2025 12:11:28 +0000 > > >> Alice Ryhl <[email protected]> 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...
