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 14:31:20 +0100
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     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

Hi,

On 3/25/21 2:02 PM, Christian König wrote:
>
>
> Am 25.03.21 um 13:36 schrieb Thomas Hellström (Intel):
>>
>> On 3/25/21 1:09 PM, 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.
>>>
>>> Christian.
>>>
>> We've had this discussion before and at that time I managed to 
>> convince you by pointing to the shmem helper for this, 
>> shmem_get_umapped_area().
>
> No, you didn't convinced me. I was just surprised that this is 
> something under driver control.
>
>>
>> Basically there are two ways to do this. Either use a standard helper 
>> similar to shmem's, and then the driver needs to align physical 
>> (device) huge page boundaries to address space offset huge page 
>> boundaries. If you don't do that you can just as well use a custom 
>> function that adjusts for you not doing that 
>> (drm_get_unmapped_area()). Both require driver knowledge of the size 
>> of huge pages.
>
> And once more, at least for GPU drivers that looks like the totally 
> wrong approach to me.
>
> Aligning the VMA so that huge page allocations become possible is the 
> job of the MM subsystem and not that of the drivers.
>
Previous discussion here

https://www.spinics.net/lists/linux-mm/msg205291.html

>>
>> Without a function to adjust, mmap will use it's default (16 byte?) 
>> alignment and chance of alignment becomes very small.
>
> Well it's 4KiB at least.
Yes :/ ...
>
> Regards,
> Christian.
>
Thanks,

Thomas