Re: [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

[Dmitry Osipenko]

On 8/29/23 10:29, Boris Brezillon wrote:
> On Tue, 29 Aug 2023 05:34:23 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> 
>> On 8/28/23 13:12, Boris Brezillon wrote:
>>> On Sun, 27 Aug 2023 20:54:43 +0300
>>> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
>>>   
>>>> In a preparation of adding drm-shmem memory shrinker, move all reservation
>>>> locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that
>>>> will resolve spurious lockdep warning about wrong locking order vs
>>>> fs_reclam code paths during freeing of shmem GEM, where lockdep isn't
>>>> aware that it's impossible to have locking contention with the fs_reclam
>>>> at this special time.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>>>> ---
>>>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++---------
>>>>  1 file changed, 25 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> index d96fee3d6166..ca5da976aafa 100644
>>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> @@ -128,6 +128,23 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
>>>>  
>>>> +static void drm_gem_shmem_resv_assert_held(struct drm_gem_shmem_object *shmem)
>>>> +{
>>>> +	/*
>>>> +	 * Destroying the object is a special case.. drm_gem_shmem_free()
>>>> +	 * calls many things that WARN_ON if the obj lock is not held.  But
>>>> +	 * acquiring the obj lock in drm_gem_shmem_free() can cause a locking
>>>> +	 * order inversion between reservation_ww_class_mutex and fs_reclaim.
>>>> +	 *
>>>> +	 * This deadlock is not actually possible, because no one should
>>>> +	 * be already holding the lock when drm_gem_shmem_free() is called.
>>>> +	 * Unfortunately lockdep is not aware of this detail.  So when the
>>>> +	 * refcount drops to zero, we pretend it is already locked.
>>>> +	 */
>>>> +	if (kref_read(&shmem->base.refcount))
>>>> +		drm_gem_shmem_resv_assert_held(shmem);
>>>> +}
>>>> +
>>>>  /**
>>>>   * drm_gem_shmem_free - Free resources associated with a shmem GEM object
>>>>   * @shmem: shmem GEM object to free
>>>> @@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>>>  	if (obj->import_attach) {
>>>>  		drm_prime_gem_destroy(obj, shmem->sgt);
>>>>  	} else if (!shmem->imported_sgt) {
>>>> -		dma_resv_lock(shmem->base.resv, NULL);
>>>> -
>>>>  		drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
>>>>  
>>>>  		if (shmem->sgt) {
>>>> @@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>>>  			drm_gem_shmem_put_pages_locked(shmem);  
>>>
>>> AFAICT, drm_gem_shmem_put_pages_locked() is the only function that's
>>> called in the free path and would complain about resv-lock not being
>>> held. I think I'd feel more comfortable if we were adding a
>>> drm_gem_shmem_free_pages() function that did everything
>>> drm_gem_shmem_put_pages_locked() does except for the lock_held() check
>>> and the refcount dec, and have it called here (and in
>>> drm_gem_shmem_put_pages_locked()). This way we can keep using
>>> dma_resv_assert_held() instead of having our own variant.  
>>
>> It's not only drm_gem_shmem_free_pages(), but any drm-shmem function
>> that drivers may use in the GEM's freeing callback.
>>
>> For example, panfrost_gem_free_object() may unpin shmem BO and then do
>> drm_gem_shmem_free().
> 
> Is this really a valid use case? If the GEM refcount dropped to zero,
> we should certainly not have pages_pin_count > 0 (thinking of vmap-ed
> buffers that might disappear while kernel still has a pointer to the
> CPU-mapped area). The only reason we have this
> drm_gem_shmem_put_pages_locked() in drm_gem_shmem_free() is because of
> this implicit ref hold by the sgt, and IMHO, we should be stricter and
> check that pages_use_count == 1 when sgt != NULL and pages_use_count ==
> 0 otherwise.
> 
> I actually think it's a good thing to try and catch any attempt to call
> functions trying lock the resv in a path they're not supposed to. At
> least we can decide whether these actions are valid or not in this
> context, and provide dedicated helpers for the free path if they are.

To me it's a valid use-case. I was going to do it for the virtio-gpu
driver for a specific BO type that should be permanently pinned in
memory. So I made the BO pinned in the virto_gpu's bo_create() and
unpinned it from the virtio-gpu's gem->free(), this is a perfectly valid
case to me. Though, in the end I switched to another approach that
doesn't require to do the pinning in the virtio-gpu driver.

For now we can do it as you suggested, to use custom put_pages() in the
shmem_free() since neither of drivers need that. Let's try that.

-- 
Best regards,
Dmitry