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Subject: Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct
To:     Jerome Glisse <jglisse@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Wed, 20 Feb 2019 16:42:45 -0800
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On 2/20/19 4:37 PM, Jerome Glisse wrote:
> On Wed, Feb 20, 2019 at 04:32:09PM -0800, John Hubbard wrote:
>> On 2/20/19 4:15 PM, Jerome Glisse wrote:
>>> On Wed, Feb 20, 2019 at 04:06:50PM -0800, John Hubbard wrote:
>>>> On 2/20/19 3:59 PM, Jerome Glisse wrote:
>>>>> On Wed, Feb 20, 2019 at 03:47:50PM -0800, John Hubbard wrote:
>>>>>> On 1/29/19 8:54 AM, jglisse@redhat.com wrote:
>>>>>>> From: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
>>>>>>>
>>>>>>> Every time i read the code to check that the HMM structure does not
>>>>>>> vanish before it should thanks to the many lock protecting its remo=
val
>>>>>>> i get a headache. Switch to reference counting instead it is much
>>>>>>> easier to follow and harder to break. This also remove some code th=
at
>>>>>>> is no longer needed with refcounting.
>>>>>>
>>>>>> Hi Jerome,
>>>>>>
>>>>>> That is an excellent idea. Some review comments below:
>>>>>>
>>>>>> [snip]
>>>>>>
>>>>>>>      static int hmm_invalidate_range_start(struct mmu_notifier *mn,
>>>>>>>      			const struct mmu_notifier_range *range)
>>>>>>>      {
>>>>>>>      	struct hmm_update update;
>>>>>>> -	struct hmm *hmm =3D range->mm->hmm;
>>>>>>> +	struct hmm *hmm =3D hmm_get(range->mm);
>>>>>>> +	int ret;
>>>>>>>      	VM_BUG_ON(!hmm);
>>>>>>> +	/* Check if hmm_mm_destroy() was call. */
>>>>>>> +	if (hmm->mm =3D=3D NULL)
>>>>>>> +		return 0;
>>>>>>
>>>>>> Let's delete that NULL check. It can't provide true protection. If t=
here
>>>>>> is a way for that to race, we need to take another look at refcounti=
ng.
>>>>>
>>>>> I will do a patch to delete the NULL check so that it is easier for
>>>>> Andrew. No need to respin.
>>>>
>>>> (Did you miss my request to make hmm_get/hmm_put symmetric, though?)
>>>
>>> Went over my mail i do not see anything about symmetric, what do you
>>> mean ?
>>>
>>> Cheers,
>>> J=C3=A9r=C3=B4me
>>
>> I meant the comment that I accidentally deleted, before sending the emai=
l!
>> doh. Sorry about that. :) Here is the recreated comment:
>>
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index a04e4b810610..b9f384ea15e9 100644
>>
>> --- a/mm/hmm.c
>>
>> +++ b/mm/hmm.c
>>
>> @@ -50,6 +50,7 @@
>>
>>   static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
>>
>>    */
>>   struct hmm {
>>   	struct mm_struct	*mm;
>> +	struct kref		kref;
>>   	spinlock_t		lock;
>>   	struct list_head	ranges;
>>   	struct list_head	mirrors;
>>
>> @@ -57,6 +58,16 @@
>>
>>   struct hmm {
>>
>>   	struct rw_semaphore	mirrors_sem;
>>   };
>>
>> +static inline struct hmm *hmm_get(struct mm_struct *mm)
>> +{
>> +	struct hmm *hmm =3D READ_ONCE(mm->hmm);
>> +
>> +	if (hmm && kref_get_unless_zero(&hmm->kref))
>> +		return hmm;
>> +
>> +	return NULL;
>> +}
>> +
>>
>> So for this, hmm_get() really ought to be symmetric with
>> hmm_put(), by taking a struct hmm*. And the null check is
>> not helping here, so let's just go with this smaller version:
>>
>> static inline struct hmm *hmm_get(struct hmm *hmm)
>> {
>> 	if (kref_get_unless_zero(&hmm->kref))
>> 		return hmm;
>>
>> 	return NULL;
>> }
>>
>> ...and change the few callers accordingly.
>>
>=20
> What about renaning hmm_get() to mm_get_hmm() instead ?
>=20

For a get/put pair of functions, it would be ideal to pass
the same argument type to each. It looks like we are passing
around hmm*, and hmm retains a reference count on hmm->mm,
so I think you have a choice of using either mm* or hmm* as
the argument. I'm not sure that one is better than the other
here, as the lifetimes appear to be linked pretty tightly.

Whichever one is used, I think it would be best to use it
in both the _get() and _put() calls.

thanks,
--=20
John Hubbard
NVIDIA