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

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On 6/10/20 5:30 PM, Daniel Vetter wrote:
On Wed, Jun 10, 2020 at 04:05:04PM +0200, Christian König wrote:
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.
Only if you have a drm_dev_enter/exit pair in your fault handler.
Otherwise you're still open to the races Thomas described. But aside from
that the drm_dev_unplug stuff has all the barriers and stuff to make sure
nothing escapes.

Failure to drm_dev_enter could then also trigger the special case where we
put a dummy page in place.
-Daniel

Hmm, Yes, indeed advertizing the flag before the call to unmap_mapping_range isn't enough, since there might be fault handlers running that haven't picked up the flag when unmap_mapping_range is launched.

For the special case of syncing a full address-space unmap_mapping_range() with fault handlers regardless of the reason for the full address-space unmap_mapping_range() one could either traverse the address space (drm_vma_manager) and grab *all* bo reservations around the unmap_mapping_range(), or grab the i_mmap_lock in read mode in the fault handler. (It's taken in write mode in unmap_mapping_range). While the latter may seem like a simple solution, one should probably consider the overhead both in run-time and scaling ability.

/Thomas


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