Re: [PATCH 6/7] nouveau/dmem: Evict device private memory during release

[Lyude Paul <lyude@redhat.com>]

On Tue, 2022-09-27 at 11:39 +1000, Alistair Popple wrote:
> Felix Kuehling <felix.kuehling@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@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.

Mhm, TBH I feel like this was going to happen eventually anyway but there's
another reason for nouveau to start using the managed versions of these
functions at some point

> 
> > commit 22f4f4faf337d5fb2d2750aff13215726814273e
> > Author: Philip Yang <Philip.Yang@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@amd.com>
> >     Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
> >     Signed-off-by: Alex Deucher <alexander.deucher@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,
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat