[PATCH] drm/ttm: Temporarily disable the huge_fault() callback

[Christian König]

Am 08.04.20 um 15:49 schrieb Thomas Hellström (VMware):
> On 4/8/20 2:19 PM, Christian König wrote:
>> Am 08.04.20 um 14:01 schrieb Thomas Hellström (VMware):
>>> Hi, Christian,
>>>
>>> On 4/8/20 1:53 PM, Thomas Hellström (VMware) wrote:
>>>> From: "Thomas Hellstrom (VMware)" <thomas_os at shipmail.org>
>>>>
>>>> With amdgpu and CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS=y, there are
>>>> errors like:
>>>> BUG: non-zero pgtables_bytes on freeing mm
>>>> and:
>>>> BUG: Bad rss-counter state
>>>> with TTM transparent huge-pages.
>>>> Until we've figured out what other TTM drivers do differently 
>>>> compared to
>>>> vmwgfx, disable the huge_fault() callback, eliminating transhuge
>>>> page-table entries.
>>>>
>>>> Cc: Christian König <christian.koenig at amd.com>
>>>> Signed-off-by: Thomas Hellstrom (VMware) <thomas_os at shipmail.org>
>>>> Reported-by: Alex Xu (Hello71) <alex_y_xu at yahoo.ca>
>>>> Tested-by: Alex Xu (Hello71) <alex_y_xu at yahoo.ca>
>>
>> Acked-by: Christian König <christian.koenig at amd.com>
>>
>>>> ---
>>>
>>> Without being able to test and track this down on amdgpu there's 
>>> little more than this I can do at the moment. Hopefully I'll be able 
>>> to test on nouveau/ttm after getting back from vacation to see if I 
>>> can reproduce.
>>>
>>> It looks like some part of the kernel mistakes a huge page-table 
>>> entry for a page directory, and that would be a path that is not hit 
>>> with vmwgfx.
>>
>> Well that looks like an ugly one and I don't know enough about the 
>> page table handling to hunt this down either.
>>
>> BTW: Have you seen the coverity warning about  "WARN_ON_ONCE(ret = 
>> VM_FAULT_FALLBACK);"?
>
> Yes, that's a false warning but it might be that it should be 
> rewritten for clarity like so:
>
> ret = VM_FAULT_FALLBACK;
> WARN_ON_ONCE(true);

Using an additional () also usually works, but I'm not even sure if 
WARN_ON_ONCE() isn't compiled to a no-op under some circumstances.

So better save than sorry,
Christian.

>
> /Thomas
>
>
>
>>
>> Regards,
>> Christian.
>>
>>>
>>> /Thomas
>>>
>>>
>