[PATCH] drm/ttm: Don't inherit GEM object VMAs in child process

Bhardwaj, Rajneesh rajneesh.bhardwaj at amd.com
Thu Dec 23 01:49:47 UTC 2021 

Adding Adrian Rebel who is the CRIU maintainer and CRIU list

On 12/22/2021 3:53 PM, Daniel Vetter wrote:
> On Mon, Dec 20, 2021 at 01:12:51PM -0500, Bhardwaj, Rajneesh wrote:
>> On 12/20/2021 4:29 AM, Daniel Vetter wrote:
>>> On Fri, Dec 10, 2021 at 07:58:50AM +0100, Christian König wrote:
>>>> Am 09.12.21 um 19:28 schrieb Felix Kuehling:
>>>>> Am 2021-12-09 um 10:30 a.m. schrieb Christian König:
>>>>>> That still won't work.
>>>>>>
>>>>>> But I think we could do this change for the amdgpu mmap callback only.
>>>>> If graphics user mode has problems with it, we could even make this
>>>>> specific to KFD BOs in the amdgpu_gem_object_mmap callback.
>>>> I think it's fine for the whole amdgpu stack, my concern is more about
>>>> radeon, nouveau and the ARM stacks which are using this as well.
>>>>
>>>> That blew up so nicely the last time we tried to change it and I know of at
>>>> least one case where radeon was/is used with BOs in a child process.
>>> I'm way late and burried again, but I think it'd be good to be consistent


I had committed this change into our amd-staging-drm-next branch last 
week after I got the ACK and RB from Felix and Christian.


>>> here across drivers. Or at least across drm drivers. And we've had the vma
>>> open/close refcounting to make fork work since forever.
>>>
>>> I think if we do this we should really only do this for mmap() where this
>>> applies, but reading through the thread here I'm honestly confused why
>>> this is a problem. If CRIU can't handle forked mmaps it needs to be
>>> thought that, not hacked around. Or at least I'm not understanding why
>>> this shouldn't work ...
>>> -Daniel
>>>
>> Hi Daniel
>>
>> In the v2
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2Fa1a865f5-ad2c-29c8-cbe4-2635d53eceb6%40amd.com%2FT%2F&data=04%7C01%7Crajneesh.bhardwaj%40amd.com%7Ce4634a16c37149da173408d9c58d1338%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637758031981907821%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=h0z4sO19bsJecMqeHGdz%2BHZElKuyzK%2BW%2FMbLWA79I10%3D&reserved=0
>> I pretty much limited the scope of the change to KFD BOs on mmap. Regarding
>> CRIU, I think its not a CRIU problem as CRIU on restore, only tries to
>> recreate all the child processes and then mmaps all the VMAs it sees (as per
>> checkpoint snapshot) in the new process address space after the VMA
>> placements are finalized in the position independent code phase. Since the
>> inherited VMAs don't have access rights the criu mmap fails.
> Still sounds funky. I think minimally we should have an ack from CRIU
> developers that this is officially the right way to solve this problem. I
> really don't want to have random one-off hacks that don't work across the
> board, for a problem where we (drm subsystem) really shouldn't be the only
> one with this problem. Where "this problem" means that the mmap space is
> per file description, and not per underlying inode or real device or
> whatever. That part sounds like a CRIU problem, and I expect CRIU folks
> want a consistent solution across the board for this. Hence please grab an
> ack from them.
>
> Cheers, Daniel


Maybe Adrian can share his views on this.

Hi Adrian - For the context, on CRIU restore we see mmap failures ( in 
PIE restore phase) due to permission issues on the (render node) VMAs 
that were inherited since the application that check pointed had 
forked.  The VMAs ideally should not be in the child process but the 
smaps file shows these VMAs in the child address space. We didn't want 
to use madvise to avoid this copy and rather change in the kernel mode 
to limit the impact to our user space library thunk. Based on my 
understanding, during PIE restore phase, after the VMA placements are 
finalized, CRIU does a sys_mmap on all the VMA it sees in the VmaEntry 
list and I think its not an issue as per CRIU design but do you think we 
could handle this corner case better inside CRIU?


>
>> Regards,
>>
>> Rajneesh
>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> Regards,
>>>>>      Felix
>>>>>
>>>>>
>>>>>> Regards,
>>>>>> Christian.
>>>>>>
>>>>>> Am 09.12.21 um 16:29 schrieb Bhardwaj, Rajneesh:
>>>>>>> Sounds good. I will send a v2 with only ttm_bo_mmap_obj change. Thank
>>>>>>> you!
>>>>>>>
>>>>>>> On 12/9/2021 10:27 AM, Christian König wrote:
>>>>>>>> Hi Rajneesh,
>>>>>>>>
>>>>>>>> yes, separating this from the drm_gem_mmap_obj() change is certainly
>>>>>>>> a good idea.
>>>>>>>>
>>>>>>>>> The child cannot access the BOs mapped by the parent anyway with
>>>>>>>>> access restrictions applied
>>>>>>>> exactly that is not correct. That behavior is actively used by some
>>>>>>>> userspace stacks as far as I know.
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Christian.
>>>>>>>>
>>>>>>>> Am 09.12.21 um 16:23 schrieb Bhardwaj, Rajneesh:
>>>>>>>>> Thanks Christian. Would it make it less intrusive if I just use the
>>>>>>>>> flag for ttm bo mmap and remove the drm_gem_mmap_obj change from
>>>>>>>>> this patch? For our use case, just the ttm_bo_mmap_obj change
>>>>>>>>> should suffice and we don't want to put any more work arounds in
>>>>>>>>> the user space (thunk, in our case).
>>>>>>>>>
>>>>>>>>> The child cannot access the BOs mapped by the parent anyway with
>>>>>>>>> access restrictions applied so I wonder why even inherit the vma?
>>>>>>>>>
>>>>>>>>> On 12/9/2021 2:54 AM, Christian König wrote:
>>>>>>>>>> Am 08.12.21 um 21:53 schrieb Rajneesh Bhardwaj:
>>>>>>>>>>> When an application having open file access to a node forks, its
>>>>>>>>>>> shared
>>>>>>>>>>> mappings also get reflected in the address space of child process
>>>>>>>>>>> even
>>>>>>>>>>> though it cannot access them with the object permissions applied.
>>>>>>>>>>> With the
>>>>>>>>>>> existing permission checks on the gem objects, it might be
>>>>>>>>>>> reasonable to
>>>>>>>>>>> also create the VMAs with VM_DONTCOPY flag so a user space
>>>>>>>>>>> application
>>>>>>>>>>> doesn't need to explicitly call the madvise(addr, len,
>>>>>>>>>>> MADV_DONTFORK)
>>>>>>>>>>> system call to prevent the pages in the mapped range to appear in
>>>>>>>>>>> the
>>>>>>>>>>> address space of the child process. It also prevents the memory
>>>>>>>>>>> leaks
>>>>>>>>>>> due to additional reference counts on the mapped BOs in the child
>>>>>>>>>>> process that prevented freeing the memory in the parent for which
>>>>>>>>>>> we had
>>>>>>>>>>> worked around earlier in the user space inside the thunk library.
>>>>>>>>>>>
>>>>>>>>>>> Additionally, we faced this issue when using CRIU to checkpoint
>>>>>>>>>>> restore
>>>>>>>>>>> an application that had such inherited mappings in the child which
>>>>>>>>>>> confuse CRIU when it mmaps on restore. Having this flag set for the
>>>>>>>>>>> render node VMAs helps. VMAs mapped via KFD already take care of
>>>>>>>>>>> this so
>>>>>>>>>>> this is needed only for the render nodes.
>>>>>>>>>> Unfortunately that is most likely a NAK. We already tried
>>>>>>>>>> something similar.
>>>>>>>>>>
>>>>>>>>>> While it is illegal by the OpenGL specification and doesn't work
>>>>>>>>>> for most userspace stacks, we do have some implementations which
>>>>>>>>>> call fork() with a GL context open and expect it to work.
>>>>>>>>>>
>>>>>>>>>> Regards,
>>>>>>>>>> Christian.
>>>>>>>>>>
>>>>>>>>>>> Cc: Felix Kuehling<Felix.Kuehling at amd.com>
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: David Yat Sin<david.yatsin at amd.com>
>>>>>>>>>>> Signed-off-by: Rajneesh Bhardwaj<rajneesh.bhardwaj at amd.com>
>>>>>>>>>>> ---
>>>>>>>>>>>      drivers/gpu/drm/drm_gem.c       | 3 ++-
>>>>>>>>>>>      drivers/gpu/drm/ttm/ttm_bo_vm.c | 2 +-
>>>>>>>>>>>      2 files changed, 3 insertions(+), 2 deletions(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>>>>>>>>>>> index 09c820045859..d9c4149f36dd 100644
>>>>>>>>>>> --- a/drivers/gpu/drm/drm_gem.c
>>>>>>>>>>> +++ b/drivers/gpu/drm/drm_gem.c
>>>>>>>>>>> @@ -1058,7 +1058,8 @@ int drm_gem_mmap_obj(struct drm_gem_object
>>>>>>>>>>> *obj, unsigned long obj_size,
>>>>>>>>>>>                  goto err_drm_gem_object_put;
>>>>>>>>>>>              }
>>>>>>>>>>>      -        vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND |
>>>>>>>>>>> VM_DONTDUMP;
>>>>>>>>>>> +        vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND
>>>>>>>>>>> +                | VM_DONTDUMP | VM_DONTCOPY;
>>>>>>>>>>>              vma->vm_page_prot =
>>>>>>>>>>> pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
>>>>>>>>>>>              vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>>>>>>>>>>>          }
>>>>>>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>>>>>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>>>>>>> index 33680c94127c..420a4898fdd2 100644
>>>>>>>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>>>>>>> @@ -566,7 +566,7 @@ int ttm_bo_mmap_obj(struct vm_area_struct
>>>>>>>>>>> *vma, struct ttm_buffer_object *bo)
>>>>>>>>>>>            vma->vm_private_data = bo;
>>>>>>>>>>>      -    vma->vm_flags |= VM_PFNMAP;
>>>>>>>>>>> +    vma->vm_flags |= VM_PFNMAP | VM_DONTCOPY;
>>>>>>>>>>>          vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
>>>>>>>>>>>          return 0;
>>>>>>>>>>>      }
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