Re: [PATCH 3/3] drm/ttm: remove ttm_bo_wait_unreserved

From: "Thomas Hellström (VMware)" <thomas_os@shipmail.org>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Thomas Hellstrom <thellstrom@vmware.com>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Huang Rui <ray.huang@amd.com>,
	VMware Graphics <linux-graphics-maintainer@vmware.com>,
	Gerd Hoffmann <kraxel@redhat.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	Christian Koenig <christian.koenig@amd.com>
Subject: Re: [PATCH 3/3] drm/ttm: remove ttm_bo_wait_unreserved
Date: Wed, 21 Aug 2019 17:03:34 +0200	[thread overview]
Message-ID: <bdecacf3-cb28-d1a1-efb8-49d347f3ab3f@shipmail.org> (raw)
In-Reply-To: <CAKMK7uHEAkjneZ5k=XfuJ9hftErt7F_UOQoZaUQkpoDkBiCX+w@mail.gmail.com>

On 8/21/19 4:47 PM, Daniel Vetter wrote:
> On Wed, Aug 21, 2019 at 4:27 PM Thomas Hellström (VMware)
> <thomas_os@shipmail.org> wrote:
>> On 8/21/19 4:09 PM, Daniel Vetter wrote:
>>> On Wed, Aug 21, 2019 at 2:47 PM Thomas Hellström (VMware)
>>> <thomas_os@shipmail.org> wrote:
>>>> On 8/21/19 2:40 PM, Thomas Hellström (VMware) wrote:
>>>>> On 8/20/19 4:53 PM, Daniel Vetter wrote:
>>>>>> With nouveau fixed all ttm-using drives have the correct nesting of
>>>>>> mmap_sem vs dma_resv, and we can just lock the buffer.
>>>>>>
>>>>>> Assuming I didn't screw up anything with my audit of course.
>>>>>>
>>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>>>>> Cc: Christian Koenig <christian.koenig@amd.com>
>>>>>> Cc: Huang Rui <ray.huang@amd.com>
>>>>>> Cc: Gerd Hoffmann <kraxel@redhat.com>
>>>>>> Cc: "VMware Graphics" <linux-graphics-maintainer@vmware.com>
>>>>>> Cc: Thomas Hellstrom <thellstrom@vmware.com>
>>>>>> ---
>>>>>>     drivers/gpu/drm/ttm/ttm_bo.c    | 34 ---------------------------------
>>>>>>     drivers/gpu/drm/ttm/ttm_bo_vm.c | 26 +------------------------
>>>>>>     include/drm/ttm/ttm_bo_api.h    |  1 -
>>>>>>     3 files changed, 1 insertion(+), 60 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> index 20ff56f27aa4..a952dd624b06 100644
>>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> @@ -1954,37 +1954,3 @@ void ttm_bo_swapout_all(struct ttm_bo_device
>>>>>> *bdev)
>>>>>>             ;
>>>>>>     }
>>>>>>     EXPORT_SYMBOL(ttm_bo_swapout_all);
>>>>>> -
>>>>>> -/**
>>>>>> - * ttm_bo_wait_unreserved - interruptible wait for a buffer object
>>>>>> to become
>>>>>> - * unreserved
>>>>>> - *
>>>>>> - * @bo: Pointer to buffer
>>>>>> - */
>>>>>> -int ttm_bo_wait_unreserved(struct ttm_buffer_object *bo)
>>>>>> -{
>>>>>> -    int ret;
>>>>>> -
>>>>>> -    /*
>>>>>> -     * In the absense of a wait_unlocked API,
>>>>>> -     * Use the bo::wu_mutex to avoid triggering livelocks due to
>>>>>> -     * concurrent use of this function. Note that this use of
>>>>>> -     * bo::wu_mutex can go away if we change locking order to
>>>>>> -     * mmap_sem -> bo::reserve.
>>>>>> -     */
>>>>>> -    ret = mutex_lock_interruptible(&bo->wu_mutex);
>>>>>> -    if (unlikely(ret != 0))
>>>>>> -        return -ERESTARTSYS;
>>>>>> -    if (!dma_resv_is_locked(bo->base.resv))
>>>>>> -        goto out_unlock;
>>>>>> -    ret = dma_resv_lock_interruptible(bo->base.resv, NULL);
>>>>>> -    if (ret == -EINTR)
>>>>>> -        ret = -ERESTARTSYS;
>>>>>> -    if (unlikely(ret != 0))
>>>>>> -        goto out_unlock;
>>>>>> -    dma_resv_unlock(bo->base.resv);
>>>>>> -
>>>>>> -out_unlock:
>>>>>> -    mutex_unlock(&bo->wu_mutex);
>>>>>> -    return ret;
>>>>>> -}
>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>> index 76eedb963693..505e1787aeea 100644
>>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>>> @@ -125,31 +125,7 @@ static vm_fault_t ttm_bo_vm_fault(struct
>>>>>> vm_fault *vmf)
>>>>>>             &bdev->man[bo->mem.mem_type];
>>>>>>         struct vm_area_struct cvma;
>>>>>>     -    /*
>>>>>> -     * Work around locking order reversal in fault / nopfn
>>>>>> -     * between mmap_sem and bo_reserve: Perform a trylock operation
>>>>>> -     * for reserve, and if it fails, retry the fault after waiting
>>>>>> -     * for the buffer to become unreserved.
>>>>>> -     */
>>>>>> -    if (unlikely(!dma_resv_trylock(bo->base.resv))) {
>>>>>> -        if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
>>>>>> -            if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
>>>>>> -                ttm_bo_get(bo);
>>>>>> - up_read(&vmf->vma->vm_mm->mmap_sem);
>>>>>> -                (void) ttm_bo_wait_unreserved(bo);
>>>>>> -                ttm_bo_put(bo);
>>>>>> -            }
>>>>>> -
>>>>>> -            return VM_FAULT_RETRY;
>>>>>> -        }
>>>>>> -
>>>>>> -        /*
>>>>>> -         * If we'd want to change locking order to
>>>>>> -         * mmap_sem -> bo::reserve, we'd use a blocking reserve here
>>>>>> -         * instead of retrying the fault...
>>>>>> -         */
>>>>> I think you should justify why the above code is removed, since the
>>>>> comments actually outlines what to do if we change locking order.
>>>>>
>>>>> The code that's removed above is not for adjusting locking orders but
>>>>> to decrease the mm latency by releasing the mmap_sem while waiting for
>>>>> bo reserve which in turn might be waiting for GPU. At a minimum we
>>>>> should have a separate patch with justification.
>>>>>
>>>>> Note that the caller here ensures locking progress by adjusting the
>>>>> RETRY flags after a retry.
>>> That would be patches 1&2 in this series.
>>>
>> Hmm? Those seem to touch only dma-buf and nouveau not ttm?  I mean this
>> patch should look along the lines of (based on an older tree) to
>> implement the new locking-order mmap_sem->reservation,
> Only nouveau was breaking was doing copy_*_user or get_user_pages
> while holding dma_resv locks, no one else. So nothing else needed to
> be changed. But patch 1 contains the full audit. I might have missed
> something.
>
>> but to keep the mm latency optimization using the RETRY functionality:
> Still no idea why this is needed? All the comments here and the code
> and history seem like they've been about the mmap_sem vs dma_resv
> inversion between driver ioctls and fault handling here. Once that's
> officially fixed there's no reason to play games here and retry loops
> - previously that was necessary because the old ttm_bo_vm_fault had a
> busy spin and that's definitely not nice. If it's needed I think it
> should be a second patch on top, to keep this all clear. I had to
> audit an enormous amount of code, I'd like to make sure I didn't miss
> anything before we start to make this super fancy again. Further
> patches on top is obviously all fine with me.
> -Daniel

Yes, but there are two different things you remove here. One is the 
workaround for the locking reversal, which is obviously correct.

One is TTM's implementation of the mmap_sem latency optimization, which 
looks like an oversight.

That optimization is why the VM_FAULT_RETRY functionality was added to 
mm in the first place and is intended to be used when drivers expect a 
substantial sleep to avoid keeping the pretty globalish mmap_sem held 
while that sleep is taking place. We do exactly the same while waiting 
for move-fences (ttm_bo_vm_fault_idle) and other drivers that expect 
long waits in the fault handler do the same.

To avoid this accidently happening there was even this comment:

         /*
          * If we'd want to change locking order to
          * mmap_sem -> bo::reserve, we'd use a blocking reserve here
          * instead of retrying the fault...
          */
         return VM_FAULT_NOPAGE;

And I do really think we should avoid accidently removing this just to 
re-add it in a later patch, particularly when I pointed it out at review 
time.

/Thomas

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