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Subject: Re: [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking
To:     Peter Xu <peterx@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Alex Williamson <alex.williamson@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, cohuck@redhat.com
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <0ee2fd04-d544-d03b-0a7c-90c22275aac9@redhat.com>
Date:   Fri, 8 May 2020 14:44:44 +0800
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On 2020/5/8 上午10:16, 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@redhat.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?


Probably not, e.g when VMA is being split.

Thanks


>
> Thanks,
>