On Tue, Apr 26, 2022 at 04:31:57PM -0700, Dave Hansen wrote: > On 4/26/22 09:48, Jean-Philippe Brucker wrote: > > On Tue, Apr 26, 2022 at 08:27:00AM -0700, Dave Hansen wrote: > >> On 4/25/22 09:40, Jean-Philippe Brucker wrote: > >>> The problem is that we'd have to request the device driver to stop DMA > >>> before we can destroy the context and free the PASID. We did consider > >>> doing this in the release() MMU notifier, but there were concerns about > >>> blocking mmput() for too long (for example > >>> https://lore.kernel.org/linux-iommu/4d68da96-0ad5-b412-5987-2f7a6aa79...@amd.com/ > >>> though I think there was a more recent discussion). We also need to drain > >>> the PRI and fault queues to get rid of all references to that PASID. > >> Is the concern truly about blocking mmput() itself? Or, is it about > >> releasing the resources associated with the mm? > > The latter I think, this one was about releasing pages as fast as possible > > if the process is picked by the OOM killer. > > We're tying the PASID to the life of the mm itself, not the mm's address > space. That means the PASID should be tied to > mmgrab()/mmdrop()/mm->mm_count. > > The address space is what the OOM killer is after. That gets refcounted > with mmget()/mmput()/mm->mm_users. The OOM killer is satiated by the > page freeing done in __mmput()->exit_mmap(). > > Also, all the VMAs should be gone after exit_mmap(). So, even if > vma->vm_file was holding a reference to a device driver, that reference > should be gone by the time __mmdrop() is actually freeing the PASID.
I agree with all that. The concern was about tearing down the PASID in the IOMMU and device from the release() MMU notifier, which would happen in exit_mmap(). But doing the teardown at or before __mmdrop() is fine. And since the IOMMU drivers need to hold mm->mm_count anyway between bind() and unbind(), I think Fenghua's fix works. Thanks, Jean _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
