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[142.68.57.212]) by smtp.gmail.com with ESMTPSA id m83sm3859503qke.117.2020.05.01.16.48.49 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Fri, 01 May 2020 16:48:50 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1jUfOj-0005t6-HO; Fri, 01 May 2020 20:48:49 -0300 Date: Fri, 1 May 2020 20:48:49 -0300 From: Jason Gunthorpe To: Alex Williamson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, cohuck@redhat.com, peterx@redhat.com Subject: Re: [PATCH 3/3] vfio-pci: Invalidate mmaps and block MMIO access on disabled memory Message-ID: <20200501234849.GQ26002@ziepe.ca> References: <158836742096.8433.685478071796941103.stgit@gimli.home> <158836917028.8433.13715345616117345453.stgit@gimli.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <158836917028.8433.13715345616117345453.stgit@gimli.home> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 01, 2020 at 03:39:30PM -0600, Alex Williamson wrote: > static int vfio_pci_add_vma(struct vfio_pci_device *vdev, > struct vm_area_struct *vma) > { > @@ -1346,15 +1450,49 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf) > { > struct vm_area_struct *vma = vmf->vma; > struct vfio_pci_device *vdev = vma->vm_private_data; > + vm_fault_t ret = VM_FAULT_NOPAGE; > > - if (vfio_pci_add_vma(vdev, vma)) > - return VM_FAULT_OOM; > + /* > + * Zap callers hold memory_lock and acquire mmap_sem, we hold > + * mmap_sem and need to acquire memory_lock to avoid races with > + * memory bit settings. Release mmap_sem, wait, and retry, or fail. > + */ > + if (unlikely(!down_read_trylock(&vdev->memory_lock))) { > + if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) { > + if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT) > + return VM_FAULT_RETRY; > + > + up_read(&vma->vm_mm->mmap_sem); > + > + if (vmf->flags & FAULT_FLAG_KILLABLE) { > + if (!down_read_killable(&vdev->memory_lock)) > + up_read(&vdev->memory_lock); > + } else { > + down_read(&vdev->memory_lock); > + up_read(&vdev->memory_lock); > + } > + return VM_FAULT_RETRY; > + } > + return VM_FAULT_SIGBUS; > + } So, why have the wait? It isn't reliable - if this gets faulted from a call site that can't handle retry then it will SIGBUS anyhow? The weird use of a rwsem as a completion suggest that perhaps using wait_event might improve things: disable: // Clean out the vma list with zap, then: down_read(mm->mmap_sem) mutex_lock(vma_lock); list_for_each_entry_safe() // zap and remove all vmas pause_faults = true; mutex_write(vma_lock); fault: // Already have down_read(mmap_sem) mutex_lock(vma_lock); while (pause_faults) { mutex_unlock(vma_lock) wait_event(..., !pause_faults) mutex_lock(vma_lock) } list_add() remap_pfn() mutex_unlock(vma_lock) enable: pause_faults = false wake_event() The only requirement here is that while inside the write side of memory_lock you cannot touch user pages (ie no copy_from_user/etc) Jason