Messages in this thread |  | | Date | Sat, 30 Apr 2022 15:33:17 +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 2022/4/30 06:19, Fenghua Yu wrote: > Hi, Jean and Baolu, > > On Fri, Apr 29, 2022 at 03:34:36PM +0100, Jean-Philippe Brucker wrote: >> On Fri, Apr 29, 2022 at 06:51:17AM -0700, Fenghua Yu wrote: >>> Hi, Baolu, >>> >>> On Fri, Apr 29, 2022 at 03:53:57PM +0800, Baolu Lu wrote: >>>> On 2022/4/28 16:39, Jean-Philippe Brucker wrote: >>>>>> 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? >>> >>> On both ARM and X86, sva_bind() calls mmu_notifier_register()->mmgrab() and >>> sva_unbind() calls mmu_notifier_unregister()/mmu_notifier_put()->mmdrop(). >> >> Yes, although for Arm I realized the mmu_notifier grab wasn't sufficient >> so I sent a separate fix that should go in 5.18 as well >> https://lore.kernel.org/linux-iommu/20220426130444.300556-1-jean-philippe@linaro.org/ >> The Arm driver still touches the arch mm context after mmu_notifier_put(). >> I don't think X86 has that problem. > > I think so too. On X86, the mm is not used after mmu_notifier_unregister(). > > In case of supervisor mode SVM (i.e. svm is bound to init_mm), the code > is right too because init_mm and its PASID cannot be dropped and > mmu_notifier_register()/mmu_notifier_unregister() are not called. > > So I think no extra fix patch is needed on X86.
Thanks, Fenghua and Jean. It's clear to me now.
Jean, another quick question about the iommu_sva_bind_device()
/** * iommu_sva_bind_device() - Bind a process address space to a device * @dev: the device * @mm: the mm to bind, caller must hold a reference to it * @drvdata: opaque data pointer to pass to bind callback
This interface requires the caller to take a reference to mm. Which reference should it take, mm->mm_count or mm->mm_users? It's better to make it explicit in this comment.
Best regards, baolu
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