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Fri, 08 May 2020 07:26:43 -0700 (PDT) Date: Fri, 8 May 2020 10:26:42 -0400 From: Peter Xu To: Jason Gunthorpe Cc: Alex Williamson , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, cohuck@redhat.com Subject: Re: [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking Message-ID: <20200508142642.GW228260@xz-x1> References: <158871401328.15589.17598154478222071285.stgit@gimli.home> <158871569380.15589.16950418949340311053.stgit@gimli.home> <20200507214744.GP228260@xz-x1> <20200507160334.4c029518@x1.home> <20200507222223.GR228260@xz-x1> <20200507235633.GL26002@ziepe.ca> <20200508021656.GS228260@xz-x1> <20200508120801.GN26002@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200508120801.GN26002@ziepe.ca> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 08, 2020 at 09:08:01AM -0300, Jason Gunthorpe wrote: > On Thu, May 07, 2020 at 10:16:56PM -0400, Peter Xu wrote: > > On Thu, May 07, 2020 at 08:56:33PM -0300, Jason Gunthorpe wrote: > > > On Thu, May 07, 2020 at 06:22:23PM -0400, Peter Xu wrote: > > > > On Thu, May 07, 2020 at 04:03:34PM -0600, Alex Williamson wrote: > > > > > On Thu, 7 May 2020 17:47:44 -0400 > > > > > Peter Xu wrote: > > > > > > > > > > > Hi, Alex, > > > > > > > > > > > > On Tue, May 05, 2020 at 03:54:53PM -0600, Alex Williamson wrote: > > > > > > > +/* > > > > > > > + * Zap mmaps on open so that we can fault them in on access and therefore > > > > > > > + * our vma_list only tracks mappings accessed since last zap. > > > > > > > + */ > > > > > > > +static void vfio_pci_mmap_open(struct vm_area_struct *vma) > > > > > > > +{ > > > > > > > + zap_vma_ptes(vma, vma->vm_start, vma->vm_end - vma->vm_start); > > > > > > > > > > > > A pure question: is this only a safety-belt or it is required in some known > > > > > > scenarios? > > > > > > > > > > It's not required. I originally did this so that I'm not allocating a > > > > > vma_list entry in a path where I can't return error, but as Jason > > > > > suggested I could zap here only in the case that I do encounter that > > > > > allocation fault. However I still like consolidating the vma_list > > > > > handling to the vm_ops .fault and .close callbacks and potentially we > > > > > reduce the zap latency by keeping the vma_list to actual users, which > > > > > we'll get to eventually anyway in the VM case as memory BARs are sized > > > > > and assigned addresses. > > > > > > > > Yes, I don't see much problem either on doing the vma_list maintainance only in > > > > .fault() and .close(). My understandingg is that the worst case is the perf > > > > critical applications (e.g. DPDK) could pre-fault these MMIO region easily > > > > during setup if they want. My question was majorly about whether the vma > > > > should be guaranteed to have no mapping at all when .open() is called. But I > > > > agree with you that it's always good to have that as safety-belt anyways. > > > > > > If the VMA has a mapping then that specific VMA has to be in the > > > linked list. > > > > > > So if the zap is skipped then the you have to allocate something and > > > add to the linked list to track the VMA with mapping. > > > > > > It is not a 'safety belt' > > > > But shouldn't open() only be called when the VMA is created for a memory range? > > If so, does it also mean that the address range must have not been mapped yet? > > open is called whenever a VMA is copied, I don't think it is called > when the VMA is first created? Ah I think you're right. I misunderstood open() which I thought should be always pairing with close(), but it seems not. Thanks, -- Peter Xu