Nouveau - Sep 2022 - [PATCH 6/7] nouveau/dmem: Evict device private memory during release

On 2022-09-26 17:35, Lyude Paul wrote:
> On Mon, 2022-09-26 at 16:03 +1000, Alistair Popple wrote:
>> When the module is unloaded or a GPU is unbound from the module it is
>> possible for device private pages to be left mapped in currently
running
>> processes. This leads to a kernel crash when the pages are either freed
>> or accessed from the CPU because the GPU and associated data structures
>> and callbacks have all been freed.
>>
>> Fix this by migrating any mappings back to normal CPU memory prior to
>> freeing the GPU memory chunks and associated device private pages.
>>
>> Signed-off-by: Alistair Popple <apopple at nvidia.com>
>>
>> ---
>>
>> I assume the AMD driver might have a similar issue. However I can't
see
>> where device private (or coherent) pages actually get unmapped/freed
>> during teardown as I couldn't find any relevant calls to
>> devm_memunmap(), memunmap(), devm_release_mem_region() or
>> release_mem_region(). So it appears that ZONE_DEVICE pages are not
being
>> properly freed during module unload, unless I'm missing something?
> I've got no idea, will poke Ben to see if they know the answer to this
I guess we're relying on devm to release the region. Isn't the whole 
point of using devm_request_free_mem_region that we don't have to 
remember to explicitly release it when the device gets destroyed? I 
believe we had an explicit free call at some point by mistake, and that 
caused a double-free during module unload. See this commit for reference:

commit 22f4f4faf337d5fb2d2750aff13215726814273e
Author: Philip Yang <Philip.Yang at amd.com>
Date:   Mon Sep 20 17:25:52 2021 -0400

     drm/amdkfd: fix svm_migrate_fini warning
     
     Device manager releases device-specific resources when a driver
     disconnects from a device, devm_memunmap_pages and
     devm_release_mem_region calls in svm_migrate_fini are redundant.
     
     It causes below warning trace after patch "drm/amdgpu: Split
     amdgpu_device_fini into early and late", so remove function
     svm_migrate_fini.
     
     BUG: https://gitlab.freedesktop.org/drm/amd/-/issues/1718
     
     WARNING: CPU: 1 PID: 3646 at drivers/base/devres.c:795
     devm_release_action+0x51/0x60
     Call Trace:
         ? memunmap_pages+0x360/0x360
         svm_migrate_fini+0x2d/0x60 [amdgpu]
         kgd2kfd_device_exit+0x23/0xa0 [amdgpu]
         amdgpu_amdkfd_device_fini_sw+0x1d/0x30 [amdgpu]
         amdgpu_device_fini_sw+0x45/0x290 [amdgpu]
         amdgpu_driver_release_kms+0x12/0x30 [amdgpu]
         drm_dev_release+0x20/0x40 [drm]
         release_nodes+0x196/0x1e0
         device_release_driver_internal+0x104/0x1d0
         driver_detach+0x47/0x90
         bus_remove_driver+0x7a/0xd0
         pci_unregister_driver+0x3d/0x90
         amdgpu_exit+0x11/0x20 [amdgpu]
     
     Signed-off-by: Philip Yang <Philip.Yang at amd.com>
     Reviewed-by: Felix Kuehling <Felix.Kuehling at amd.com>
     Signed-off-by: Alex Deucher <alexander.deucher at amd.com>

Furthermore, I guess we are assuming that nobody is using the GPU when 
the module is unloaded. As long as any processes have /dev/kfd open, you 
won't be able to unload the module (except by force-unload). I suppose 
with ZONE_DEVICE memory, we can have references to device memory pages 
even when user mode has closed /dev/kfd. We do have a cleanup handler 
that runs in an MMU-free-notifier. In theory that should run after all 
the pages in the mm_struct have been freed. It releases all sorts of 
other device resources and needs the driver to still be there. I'm not 
sure if there is anything preventing a module unload before the 
free-notifier runs. I'll look into that.

Regards,
 ? Felix

>
>> ---
>>   drivers/gpu/drm/nouveau/nouveau_dmem.c | 48
+++++++++++++++++++++++++++-
>>   1 file changed, 48 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c
b/drivers/gpu/drm/nouveau/nouveau_dmem.c
>> index 66ebbd4..3b247b8 100644
>> --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
>> +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
>> @@ -369,6 +369,52 @@ nouveau_dmem_suspend(struct nouveau_drm *drm)
>>   	mutex_unlock(&drm->dmem->mutex);
>>   }
>>   
>> +/*
>> + * Evict all pages mapping a chunk.
>> + */
>> +void
>> +nouveau_dmem_evict_chunk(struct nouveau_dmem_chunk *chunk)
>> +{
>> +	unsigned long i, npages = range_len(&chunk->pagemap.range)
>> PAGE_SHIFT;
>> +	unsigned long *src_pfns, *dst_pfns;
>> +	dma_addr_t *dma_addrs;
>> +	struct nouveau_fence *fence;
>> +
>> +	src_pfns = kcalloc(npages, sizeof(*src_pfns), GFP_KERNEL);
>> +	dst_pfns = kcalloc(npages, sizeof(*dst_pfns), GFP_KERNEL);
>> +	dma_addrs = kcalloc(npages, sizeof(*dma_addrs), GFP_KERNEL);
>> +
>> +	migrate_device_range(src_pfns, chunk->pagemap.range.start >>
PAGE_SHIFT,
>> +			npages);
>> +
>> +	for (i = 0; i < npages; i++) {
>> +		if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
>> +			struct page *dpage;
>> +
>> +			/*
>> +			 * _GFP_NOFAIL because the GPU is going away and there
>> +			 * is nothing sensible we can do if we can't copy the
>> +			 * data back.
>> +			 */
> You'll have to excuse me for a moment since this area of nouveau
isn't one of
> my strongpoints, but are we sure about this? IIRC __GFP_NOFAIL means
infinite
> retry, in the case of a GPU hotplug event I would assume we would rather
just
> stop trying to migrate things to the GPU and just drop the data instead of
> hanging on infinite retries.
>
>> +			dpage = alloc_page(GFP_HIGHUSER | __GFP_NOFAIL);
>> +			dst_pfns[i] = migrate_pfn(page_to_pfn(dpage));
>> +			nouveau_dmem_copy_one(chunk->drm,
>> +					migrate_pfn_to_page(src_pfns[i]), dpage,
>> +					&dma_addrs[i]);
>> +		}
>> +	}
>> +
>> +	nouveau_fence_new(chunk->drm->dmem->migrate.chan, false,
&fence);
>> +	migrate_device_pages(src_pfns, dst_pfns, npages);
>> +	nouveau_dmem_fence_done(&fence);
>> +	migrate_device_finalize(src_pfns, dst_pfns, npages);
>> +	kfree(src_pfns);
>> +	kfree(dst_pfns);
>> +	for (i = 0; i < npages; i++)
>> +		dma_unmap_page(chunk->drm->dev->dev, dma_addrs[i],
PAGE_SIZE, DMA_BIDIRECTIONAL);
>> +	kfree(dma_addrs);
>> +}
>> +
>>   void
>>   nouveau_dmem_fini(struct nouveau_drm *drm)
>>   {
>> @@ -380,8 +426,10 @@ nouveau_dmem_fini(struct nouveau_drm *drm)
>>   	mutex_lock(&drm->dmem->mutex);
>>   
>>   	list_for_each_entry_safe(chunk, tmp, &drm->dmem->chunks,
list) {
>> +		nouveau_dmem_evict_chunk(chunk);
>>   		nouveau_bo_unpin(chunk->bo);
>>   		nouveau_bo_ref(NULL, &chunk->bo);
>> +		WARN_ON(chunk->callocated);
>>   		list_del(&chunk->list);
>>   		memunmap_pages(&chunk->pagemap);
>>   		release_mem_region(chunk->pagemap.range.start,

Felix Kuehling <felix.kuehling at amd.com> writes:
> On 2022-09-26 17:35, Lyude Paul wrote:
>> On Mon, 2022-09-26 at 16:03 +1000, Alistair Popple wrote:
>>> When the module is unloaded or a GPU is unbound from the module it
is
>>> possible for device private pages to be left mapped in currently
running
>>> processes. This leads to a kernel crash when the pages are either
freed
>>> or accessed from the CPU because the GPU and associated data
structures
>>> and callbacks have all been freed.
>>>
>>> Fix this by migrating any mappings back to normal CPU memory prior
to
>>> freeing the GPU memory chunks and associated device private pages.
>>>
>>> Signed-off-by: Alistair Popple <apopple at nvidia.com>
>>>
>>> ---
>>>
>>> I assume the AMD driver might have a similar issue. However I
can't see
>>> where device private (or coherent) pages actually get
unmapped/freed
>>> during teardown as I couldn't find any relevant calls to
>>> devm_memunmap(), memunmap(), devm_release_mem_region() or
>>> release_mem_region(). So it appears that ZONE_DEVICE pages are not
being
>>> properly freed during module unload, unless I'm missing
something?
>> I've got no idea, will poke Ben to see if they know the answer to
this
>
> I guess we're relying on devm to release the region. Isn't the
whole point of
> using devm_request_free_mem_region that we don't have to remember to
explicitly
> release it when the device gets destroyed? I believe we had an explicit
free
> call at some point by mistake, and that caused a double-free during module
> unload. See this commit for reference:
Argh, thanks for that pointer. I was not so familiar with
devm_request_free_mem_region()/devm_memremap_pages() as currently
Nouveau explicitly manages that itself.
> commit 22f4f4faf337d5fb2d2750aff13215726814273e
> Author: Philip Yang <Philip.Yang at amd.com>
> Date:   Mon Sep 20 17:25:52 2021 -0400
>
>     drm/amdkfd: fix svm_migrate_fini warning
>          Device manager releases device-specific resources when a driver
>     disconnects from a device, devm_memunmap_pages and
>     devm_release_mem_region calls in svm_migrate_fini are redundant.
>          It causes below warning trace after patch "drm/amdgpu: Split
>     amdgpu_device_fini into early and late", so remove function
>     svm_migrate_fini.
>          BUG: https://gitlab.freedesktop.org/drm/amd/-/issues/1718
>          WARNING: CPU: 1 PID: 3646 at drivers/base/devres.c:795
>     devm_release_action+0x51/0x60
>     Call Trace:
>         ? memunmap_pages+0x360/0x360
>         svm_migrate_fini+0x2d/0x60 [amdgpu]
>         kgd2kfd_device_exit+0x23/0xa0 [amdgpu]
>         amdgpu_amdkfd_device_fini_sw+0x1d/0x30 [amdgpu]
>         amdgpu_device_fini_sw+0x45/0x290 [amdgpu]
>         amdgpu_driver_release_kms+0x12/0x30 [amdgpu]
>         drm_dev_release+0x20/0x40 [drm]
>         release_nodes+0x196/0x1e0
>         device_release_driver_internal+0x104/0x1d0
>         driver_detach+0x47/0x90
>         bus_remove_driver+0x7a/0xd0
>         pci_unregister_driver+0x3d/0x90
>         amdgpu_exit+0x11/0x20 [amdgpu]
>          Signed-off-by: Philip Yang <Philip.Yang at amd.com>
>     Reviewed-by: Felix Kuehling <Felix.Kuehling at amd.com>
>     Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
>
> Furthermore, I guess we are assuming that nobody is using the GPU when the
> module is unloaded. As long as any processes have /dev/kfd open, you
won't be
> able to unload the module (except by force-unload). I suppose with
ZONE_DEVICE
> memory, we can have references to device memory pages even when user mode
has
> closed /dev/kfd. We do have a cleanup handler that runs in an
MMU-free-notifier.
> In theory that should run after all the pages in the mm_struct have been
freed.
> It releases all sorts of other device resources and needs the driver to
still be
> there. I'm not sure if there is anything preventing a module unload
before the
> free-notifier runs. I'll look into that.
Right - module unload (or device unbind) is one of the other ways we can
hit this issue in Nouveau at least. You can end up with ZONE_DEVICE
pages mapped in a running process after the module has unloaded.
Although now you mention it that seems a bit wrong - the pgmap refcount
should provide some protection against that. Will have to look into
that too.
> Regards,
> ? Felix
>
>
>>
>>> ---
>>>   drivers/gpu/drm/nouveau/nouveau_dmem.c | 48
+++++++++++++++++++++++++++-
>>>   1 file changed, 48 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c
b/drivers/gpu/drm/nouveau/nouveau_dmem.c
>>> index 66ebbd4..3b247b8 100644
>>> --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
>>> +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
>>> @@ -369,6 +369,52 @@ nouveau_dmem_suspend(struct nouveau_drm *drm)
>>>   	mutex_unlock(&drm->dmem->mutex);
>>>   }
>>>   +/*
>>> + * Evict all pages mapping a chunk.
>>> + */
>>> +void
>>> +nouveau_dmem_evict_chunk(struct nouveau_dmem_chunk *chunk)
>>> +{
>>> +	unsigned long i, npages = range_len(&chunk->pagemap.range)
>> PAGE_SHIFT;
>>> +	unsigned long *src_pfns, *dst_pfns;
>>> +	dma_addr_t *dma_addrs;
>>> +	struct nouveau_fence *fence;
>>> +
>>> +	src_pfns = kcalloc(npages, sizeof(*src_pfns), GFP_KERNEL);
>>> +	dst_pfns = kcalloc(npages, sizeof(*dst_pfns), GFP_KERNEL);
>>> +	dma_addrs = kcalloc(npages, sizeof(*dma_addrs), GFP_KERNEL);
>>> +
>>> +	migrate_device_range(src_pfns, chunk->pagemap.range.start
>> PAGE_SHIFT,
>>> +			npages);
>>> +
>>> +	for (i = 0; i < npages; i++) {
>>> +		if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
>>> +			struct page *dpage;
>>> +
>>> +			/*
>>> +			 * _GFP_NOFAIL because the GPU is going away and there
>>> +			 * is nothing sensible we can do if we can't copy the
>>> +			 * data back.
>>> +			 */
>> You'll have to excuse me for a moment since this area of nouveau
isn't one of
>> my strongpoints, but are we sure about this? IIRC __GFP_NOFAIL means
infinite
>> retry, in the case of a GPU hotplug event I would assume we would
rather just
>> stop trying to migrate things to the GPU and just drop the data instead
of
>> hanging on infinite retries.
>>
>>> +			dpage = alloc_page(GFP_HIGHUSER | __GFP_NOFAIL);
>>> +			dst_pfns[i] = migrate_pfn(page_to_pfn(dpage));
>>> +			nouveau_dmem_copy_one(chunk->drm,
>>> +					migrate_pfn_to_page(src_pfns[i]), dpage,
>>> +					&dma_addrs[i]);
>>> +		}
>>> +	}
>>> +
>>> +	nouveau_fence_new(chunk->drm->dmem->migrate.chan, false,
&fence);
>>> +	migrate_device_pages(src_pfns, dst_pfns, npages);
>>> +	nouveau_dmem_fence_done(&fence);
>>> +	migrate_device_finalize(src_pfns, dst_pfns, npages);
>>> +	kfree(src_pfns);
>>> +	kfree(dst_pfns);
>>> +	for (i = 0; i < npages; i++)
>>> +		dma_unmap_page(chunk->drm->dev->dev, dma_addrs[i],
PAGE_SIZE, DMA_BIDIRECTIONAL);
>>> +	kfree(dma_addrs);
>>> +}
>>> +
>>>   void
>>>   nouveau_dmem_fini(struct nouveau_drm *drm)
>>>   {
>>> @@ -380,8 +426,10 @@ nouveau_dmem_fini(struct nouveau_drm *drm)
>>>   	mutex_lock(&drm->dmem->mutex);
>>>     	list_for_each_entry_safe(chunk, tmp,
&drm->dmem->chunks, list) {
>>> +		nouveau_dmem_evict_chunk(chunk);
>>>   		nouveau_bo_unpin(chunk->bo);
>>>   		nouveau_bo_ref(NULL, &chunk->bo);
>>> +		WARN_ON(chunk->callocated);
>>>   		list_del(&chunk->list);
>>>   		memunmap_pages(&chunk->pagemap);
>>>   		release_mem_region(chunk->pagemap.range.start,