Messages in this thread | | | Date | Fri, 29 Apr 2022 15:53:57 +0800 | Subject | Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit | From | Baolu Lu <> |
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On 2022/4/28 16:39, Jean-Philippe Brucker wrote: > 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-b412-5987-2f7a6aa796c3@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.
But I didn't find mmgrab()/mmdrop() get called in both arm and intel IOMMU drivers.
$ git grep mmgrab drivers/iommu/ [no output]
Do we need to add these in a separated fix patch, or I missed anything here?
Best regards, baolu
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