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

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Date:   Thu, 7 May 2020 16:03:34 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        cohuck@...hat.com, jgg@...pe.ca
Subject: Re: [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking

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.

> In all cases:
> 
> Reviewed-by: Peter Xu <peterx@...hat.com>

Thanks!
Alex