[PATCH 17/35] drm/amdkfd: register HMM device private zone

[Thomas Hellström (Intel)]

On 3/1/21 9:32 AM, Daniel Vetter wrote:
> On Wed, Jan 06, 2021 at 10:01:09PM -0500, Felix Kuehling wrote:
>> From: Philip Yang <Philip.Yang at amd.com>
>>
>> Register vram memory as MEMORY_DEVICE_PRIVATE type resource, to
>> allocate vram backing pages for page migration.
>>
>> Signed-off-by: Philip Yang <Philip.Yang at amd.com>
>> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
> So maybe I'm getting this all wrong, but I think that the current ttm
> fault code relies on devmap pte entries (especially for hugepte entries)
> to stop get_user_pages. But this only works if the pte happens to not
> point at a range with devmap pages.

I don't think that's in TTM yet, but the proposed fix, yes (see email I 
just sent in another thread),
but only for huge ptes.

>
> This patch here changes that, and so probably breaks this devmap pte hack
> ttm is using?
>
> If I'm not wrong here then I think we need to first fix up the ttm code to
> not use the devmap hack anymore, before a ttm based driver can register a
> dev_pagemap. Also adding Thomas since that just came up in another
> discussion.

It doesn't break the ttm devmap hack per se, but it indeed allows gup to 
the range registered, but here's where my lack of understanding why we 
can't allow gup-ing TTM ptes if there indeed is a backing struct-page? 
Because registering MEMORY_DEVICE_PRIVATE implies that, right?

/Thomas

> -Daniel
>