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Subject: Re: [RFC PATCH 2/2] mm,drm/ttm: Use VM_PFNMAP for TTM vmas
To:     Jason Gunthorpe <jgg@nvidia.com>, dri-devel@lists.freedesktop.org,
        Christian Koenig <christian.koenig@amd.com>,
        David Airlie <airlied@linux.ie>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20210321184529.59006-1-thomas_os@shipmail.org>
 <20210321184529.59006-3-thomas_os@shipmail.org>
 <YFnVTIGKBFnlsGxF@phenom.ffwll.local> <20210323140445.GF2356281@nvidia.com>
From:   =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= 
        <thomas_os@shipmail.org>
Message-ID: <4a47b044-3660-48e6-647d-4a2217cfa287@shipmail.org>
Date:   Tue, 23 Mar 2021 16:51:31 +0100
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On 3/23/21 3:04 PM, Jason Gunthorpe wrote:
> On Tue, Mar 23, 2021 at 12:47:24PM +0100, Daniel Vetter wrote:
>
>>> +static inline bool is_cow_mapping(vm_flags_t flags)
>> Bit a bikeshed, but I wonder whether the public interface shouldn't be
>> vma_is_cow_mapping. Or whether this shouldn't be rejected somewhere else,
>> since at least in drivers/gpu we have tons of cases that don't check for
>> this and get it all kinds of wrong I think.
>>
>> remap_pfn_range handles this for many cases, but by far not for all.
>>
>> Anyway patch itself lgtm:
>>
>> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> I would like it if io_remap_pfn_range() did not allow shared mappings
> at all.

You mean private mappings?

>
> IIRC it doesn't work anyway, the kernel can't reliably copy from IO
> pages eg the "_copy_from_user_inatomic()" under cow_user_page() will
> not work on s390 that requires all IO memory be accessed with special
> instructions.
>
> Unfortunately I have no idea what the long ago special case of
> allowing COW'd IO mappings is. :\

Me neither, but at some point it must have been important enough to 
introduce VM_MIXEDMAP...

/Thomas


> Jason