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SubjectRe: [patch 1/2] mm, mmu_notifier: annotate mmu notifiers with blockable invalidate callbacks
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On 2017/12/13 18:34, Christian König wrote:
> Am 12.12.2017 um 22:28 schrieb David Rientjes:
>> On Tue, 12 Dec 2017, Dimitri Sivanich wrote:
>>
>>>> --- a/drivers/misc/sgi-gru/grutlbpurge.c
>>>> +++ b/drivers/misc/sgi-gru/grutlbpurge.c
>>>> @@ -298,6 +298,7 @@ struct gru_mm_struct *gru_register_mmu_notifier(void)
>>>> return ERR_PTR(-ENOMEM);
>>>> STAT(gms_alloc);
>>>> spin_lock_init(&gms->ms_asid_lock);
>>>> + gms->ms_notifier.flags = 0;
>>>> gms->ms_notifier.ops = &gru_mmuops;
>>>> atomic_set(&gms->ms_refcnt, 1);
>>>> init_waitqueue_head(&gms->ms_wait_queue);
>>>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>>> There is a kzalloc() just above this:
>>> gms = kzalloc(sizeof(*gms), GFP_KERNEL);
>>>
>>> Is that not sufficient to clear the 'flags' field?
>>>
>> Absolutely, but whether it is better to explicitly document that the mmu
>> notifier has cleared flags, i.e. there are no blockable callbacks, is
>> another story. I can change it if preferred.
>
> Actually I would invert the new flag, in other words specify that an MMU notifier will never sleep.
>
> The first reason is that we have 8 blocking notifiers and 5 not blocking if I counted right. So it is actually more common to sleep than not to.
>
> The second reason is to be conservative and assume the worst, e.g. that the flag is forgotten when a new notifier is added.

I agree. Some out of tree module might forget to set the flags.

Although you don't need to fix out of tree modules, as a troubleshooting
staff at a support center, I want to be able to identify the careless module.

I guess specifying the flags at register function would be the best, for
an attempt to call register function without knowing this change will
simply results in a build failure.

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