Re: [PATCH 1/6] drm/ttm: Add unampping of the entire device address space

[Christian König <christian.koenig@xxxxxxx>]

Am 10.06.20 um 15:54 schrieb Andrey Grodzovsky:

On 6/10/20 6:15 AM, Thomas Hellström (Intel) wrote:

On 6/9/20 7:21 PM, Koenig, Christian wrote:

Am 09.06.2020 18:37 schrieb "Grodzovsky, Andrey" <Andrey.Grodzovsky@xxxxxxx>:

On 6/5/20 2:40 PM, Christian König wrote:
> Am 05.06.20 um 16:29 schrieb Andrey Grodzovsky:
>>
>> On 5/11/20 2:45 AM, Christian König wrote:
>>> Am 09.05.20 um 20:51 schrieb Andrey Grodzovsky:
>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
>>>> ---
>>>>   drivers/gpu/drm/ttm/ttm_bo.c    | 22 +++++++++++++++++++++-
>>>>   include/drm/ttm/ttm_bo_driver.h |  2 ++
>>>>   2 files changed, 23 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
>>>> b/drivers/gpu/drm/ttm/ttm_bo.c
>>>> index c5b516f..eae61cc 100644
>>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>>> @@ -1750,9 +1750,29 @@ void ttm_bo_unmap_virtual(struct
>>>> ttm_buffer_object *bo)
>>>>       ttm_bo_unmap_virtual_locked(bo);
>>>>       ttm_mem_io_unlock(man);
>>>>   }
>>>> +EXPORT_SYMBOL(ttm_bo_unmap_virtual);
>>>>   +void ttm_bo_unmap_virtual_address_space(struct ttm_bo_device *bdev)
>>>> +{
>>>> +    struct ttm_mem_type_manager *man;
>>>> +    int i;
>>>>   -EXPORT_SYMBOL(ttm_bo_unmap_virtual);
>>>
>>>> +    for (i = 0; i < TTM_NUM_MEM_TYPES; i++) {
>>>> +        man = &bdev->man[i];
>>>> +        if (man->has_type && man->use_type)
>>>> +            ttm_mem_io_lock(man, false);
>>>> +    }
>>>
>>> You should drop that it will just result in a deadlock warning for
>>> Nouveau and has no effect at all.
>>>
>>> Apart from that looks good to me,
>>> Christian.
>>
>>
>> As I am considering to re-include this in V2 of the patchsets, can
>> you clarify please why this will have no effect at all ?
>
> The locks are exclusive for Nouveau to allocate/free the io address
> space.
>
> Since we don't do this here we don't need the locks.
>
> Christian.


So basically calling unmap_mapping_range doesn't require any extra
locking around it and whatever locks are taken within the function
should be enough ?

I think so, yes.

Christian.

Yes, that's true. However, without the bo reservation, nothing stops a PTE from being immediately re-faulted back again. Even while unmap_mapping_range() is running.

Can you explain more on this - specifically, which function to reserve the BO, why BO reservation would prevent re-fault of the PTE ?

Thomas is talking about ttm_bo_reserver()/ttm_bo_unreserve(), but we don't need this because we unmap everything because the whole device is gone and not just manipulate a single BO.

So the device removed flag needs to be advertized before this function is run,

I indeed intend to call this  right after calling drm_dev_unplug from amdgpu_pci_remove while adding drm_dev_enter/exit in ttm_bo_vm_fault (or in amdgpu specific wrapper since I don't see how can I access struct drm_device from ttm_bo_vm_fault) and this in my understanding should stop a PTE from being re-faulted back as you pointed out - so again I don't see how  bo reservation would prevent it so it looks like I am missing something...

(perhaps with a memory barrier pair).

drm_dev_unplug and drm_dev_enter/exit are RCU synchronized and so I don't think require any extra memory barriers for visibility of the removed flag being set

As far as I can see that should be perfectly sufficient.

Christian.

Andrey

That should probably be added to the function documentation.

(Other than that, please add a commit message if respinning).

/Thomas

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