[PATCH] drm/sched: fix potential page fault in drm_sched_job_init()

[Luben Tuikov]

On 2023-11-08 19:09, Danilo Krummrich wrote:
> On 11/8/23 06:46, Luben Tuikov wrote:
>> Hi,
>>
>> Could you please use my gmail address, the one one I'm responding from--I don't want
>> to miss any DRM scheduler patches. BTW, the luben.tuikov at amd.com email should bounce
>> as undeliverable.
>>
>> On 2023-11-07 21:26, Danilo Krummrich wrote:
>>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>>> order to indicate that the given entity has no runq, however at this
>>> time job->sched is not yet set, likely to be NULL initialized, and hence
>>> shouldn't be used.
>>>
>>> Replace the corresponding drm_err() call with pr_err() to avoid a
>>> potential page fault.
>>>
>>> While at it, extend the documentation of drm_sched_job_init() to
>>> indicate that job->sched is not a valid pointer until
>>> drm_sched_job_arm() has been called.
>>>
>>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>>> Signed-off-by: Danilo Krummrich <dakr at redhat.com>
>>> ---
>>>   drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 27843e37d9b7..dd28389f0ddd 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>>    * This function returns -ENOENT in this case (which probably should be -EIO as
>>>    * a more meanigful return value).
>>>    *
>>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>>> + * been called.
>>> + *
>>
>> Good catch!
>>
>> Did you actually get this to page-fault and have a kernel log?
> 
> No, I just found it because I was about to make the same mistake.
> 
>>
>> I'm asking because we see it correctly set in this kernel log coming from AMD,
> 
> I think that's because amdgpu just sets job->sched to *some* scheduler instance after
> job allocation [1].
> 
> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108
> 
>>
>> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
>>
>> in this email,
>> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
>>
>>>    * Returns 0 for success, negative error code otherwise.
>>>    */
>>>   int drm_sched_job_init(struct drm_sched_job *job,
>>> @@ -691,7 +694,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>>   		 * or worse--a blank screen--leave a trail in the
>>>   		 * logs, so this can be debugged easier.
>>>   		 */
>>> -		drm_err(job->sched, "%s: entity has no rq!\n", __func__);
>>> +		pr_err("%s: entity has no rq!\n", __func__);
>>
>> Is it feasible to do something like the following?
>>
>> 		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);
> 
> I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
> structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
> drm_sched_job_arm() is called.

Okay. However, when using pr_err() we're losing "[drm] *ERROR* " prefix and we scan for that
in the logs to quickly find the cause of the error.

Perhaps we can define pr_fmt() and also include "*ERROR*" so that we can get the desired result
as the attached patch shows?
-- 
Regards,
Luben
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