linux-kernel - Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

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Message-ID: <6effcd33-8cc3-a4e0-3608-b9cef7a76da7@collabora.com>
Date:   Mon, 15 Aug 2022 18:54:47 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Christian König <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>, Huang Rui <ray.huang@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Trigger Huang <Trigger.Huang@...il.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Antonio Caggiano <antonio.caggiano@...labora.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>, kvm@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

On 8/15/22 17:57, Dmitry Osipenko wrote:
> On 8/15/22 16:53, Christian König wrote:
>> Am 15.08.22 um 15:45 schrieb Dmitry Osipenko:
>>> [SNIP]
>>>> Well that comment sounds like KVM is doing the right thing, so I'm
>>>> wondering what exactly is going on here.
>>> KVM actually doesn't hold the page reference, it takes the temporal
>>> reference during page fault and then drops the reference once page is
>>> mapped, IIUC. Is it still illegal for TTM? Or there is a possibility for
>>> a race condition here?
>>>
>>
>> Well the question is why does KVM grab the page reference in the first
>> place?
>>
>> If that is to prevent the mapping from changing then yes that's illegal
>> and won't work. It can always happen that you grab the address, solve
>> the fault and then immediately fault again because the address you just
>> grabbed is invalidated.
>>
>> If it's for some other reason than we should probably investigate if we
>> shouldn't stop doing this.
> 
> CC: +Paolo Bonzini who introduced this code
> 
> commit add6a0cd1c5ba51b201e1361b05a5df817083618
> Author: Paolo Bonzini <pbonzini@...hat.com>
> Date:   Tue Jun 7 17:51:18 2016 +0200
> 
>     KVM: MMU: try to fix up page faults before giving up
> 
>     The vGPU folks would like to trap the first access to a BAR by setting
>     vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault
> handler
>     then can use remap_pfn_range to place some non-reserved pages in the
> VMA.
> 
>     This kind of VM_PFNMAP mapping is not handled by KVM, but follow_pfn
>     and fixup_user_fault together help supporting it.  The patch also
> supports
>     VM_MIXEDMAP vmas where the pfns are not reserved and thus subject to
>     reference counting.
> 
> @Paolo,
> https://lore.kernel.org/dri-devel/73e5ed8d-0d25-7d44-8fa2-e1d61b1f5a04@amd.com/T/#m7647ce5f8c4749599d2c6bc15a2b45f8d8cf8154
> 

If we need to bump the refcount only for VM_MIXEDMAP and not for
VM_PFNMAP, then perhaps we could add a flag for that to the kvm_main
code that will denote to kvm_release_page_clean whether it needs to put
the page?

-- 
Best regards,
Dmitry