linux-kernel - Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

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Date:   Thu, 25 Mar 2021 14:26:50 +0100
From:   Christian König <christian.koenig@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>, David Airlie <airlied@...ux.ie>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

Am 25.03.21 um 14:17 schrieb Jason Gunthorpe:
> On Thu, Mar 25, 2021 at 02:05:14PM +0100, Christian König wrote:
>>
>> Am 25.03.21 um 13:42 schrieb Jason Gunthorpe:
>>> On Thu, Mar 25, 2021 at 01:09:14PM +0100, Christian König wrote:
>>>> Am 25.03.21 um 13:01 schrieb Jason Gunthorpe:
>>>>> On Thu, Mar 25, 2021 at 12:53:15PM +0100, Thomas Hellström (Intel) wrote:
>>>>>
>>>>>> Nope. The point here was that in this case, to make sure mmap uses the
>>>>>> correct VA to give us a reasonable chance of alignement, the driver might
>>>>>> need to be aware of and do trickery with the huge page-table-entry sizes
>>>>>> anyway, although I think in most cases a standard helper for this can be
>>>>>> supplied.
>>>>> Of course the driver needs some way to influence the VA mmap uses,
>>>>> gernally it should align to the natural page size of the device
>>>> Well a mmap() needs to be aligned to the page size of the CPU, but not
>>>> necessarily to the one of the device.
>>>>
>>>> So I'm pretty sure the device driver should not be involved in any way the
>>>> choosing of the VA for the CPU mapping.
>>> No, if the device wants to use huge pages it must influence the mmap
>>> VA or it can't form huge pgaes.
>> No, that's the job of the core MM and not of the individual driver.
> The core mm doesn't know the page size of the device, only which of
> several page levels the arch supports. The device must be involevd
> here.

Why? See you can have a device which has for example 256KiB pages, but 
it should perfectly work that the CPU mapping is aligned to only 4KiB.

As long as you don't do things like shared virtual memory between device 
and CPU the VA addresses used on the CPU should be completely irrelevant 
for the device.

Regards,
Christian.

>
> Jason