linux-kernel - Re: [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking

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Date:   Fri, 8 May 2020 10:26:42 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, cohuck@...hat.com
Subject: Re: [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking

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 <peterx@...hat.com> 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