[PATCH 2/2] drm/ttm: Fix COW check

[Christian König]

Am 09.07.21 um 21:31 schrieb Felix Kuehling:
> On 2021-07-09 2:38 a.m., Christian König wrote:
>>
>>
>> Am 08.07.21 um 21:36 schrieb Alex Deucher:
>>> From: Felix Kuehling <Felix.Kuehling at amd.com>
>>>
>>> KFD Thunk maps invisible VRAM BOs with PROT_NONE, MAP_PRIVATE.
>>> is_cow_mapping returns true for these mappings. Add a check for
>>> vm_flags & VM_WRITE to avoid mmap failures on private read-only or
>>> PROT_NONE mappings.
>>
>> I'm pretty sure that this is not working as expected.
>
> Not sure what you mean. Debugger access to the memory through the 
> PROT_NONE VMAs is definitely working, with both ptrace and 
> /proc/<pid>/mem.
>
>
>>
>>>
>>> Fixes: f91142c62161 ("drm/ttm: nuke VM_MIXEDMAP on BO mappings v3")
>>> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
>>> Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
>>> ---
>>> drivers/gpu/drm/ttm/ttm_bo_vm.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c 
>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>> index f56be5bc0861..a75e90c7d4aa 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>> @@ -552,7 +552,7 @@ static const struct vm_operations_struct 
>>> ttm_bo_vm_ops = {
>>>   int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct 
>>> ttm_buffer_object *bo)
>>>   {
>>>       /* Enforce no COW since would have really strange behavior 
>>> with it. */
>>> -    if (is_cow_mapping(vma->vm_flags))
>>> +    if (is_cow_mapping(vma->vm_flags) && (vma->vm_flags & VM_WRITE))
>>
>> is_cow_mapping() already checks for VM_MAYWRITE, so this here 
>> shouldn't be necessary.
>
> AFAICT, VM_MAYWRITE is not based on the PROT_... bits used to create 
> the VMA, but based on the permissions of the file. So as long as the 
> render node is writable, VM_MAYWRITE is set for all VMAs that map it.
>
> I would agree that it's probably a bad idea for the Thunk to map these 
> VMAs with MAP_PRIVATE. We can try changing the Thunk to use MAP_SHARED 
> for these PROT_NONE mappings. But that doesn't change the fact that 
> this kernel patch broke existing usermode.

Yeah, but see the discussion around MAP_PRIVATE and COW regarding this. 
What Thunk did here was a really bad idea.

I think we have only two options here:
1. Accept the breakage of thunk.
2. Add a workaround in amdgpu/amdkfd to force MAP_PRIVATE into 
MAP_SHARED in the kernel.

Regards,
Christian.

>
> Regards,
>   Felix
>
>
>>
>> Christian.
>>
>>>           return -EINVAL;
>>>         ttm_bo_get(bo);
>>