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SubjectRe: [PATCH 01/10] mm/hmm: use reference counting for HMM struct
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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: Jérôme 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 removal
>>>>>>> i get a headache. Switch to reference counting instead it is much
>>>>>>> easier to follow and harder to break. This also remove some code that
>>>>>>> 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 = range->mm->hmm;
>>>>>>> + struct hmm *hmm = hmm_get(range->mm);
>>>>>>> + int ret;
>>>>>>> VM_BUG_ON(!hmm);
>>>>>>> + /* Check if hmm_mm_destroy() was call. */
>>>>>>> + if (hmm->mm == NULL)
>>>>>>> + return 0;
>>>>>>
>>>>>> Let's delete that NULL check. It can't provide true protection. If there
>>>>>> is a way for that to race, we need to take another look at refcounting.
>>>>>
>>>>> 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,
>>> Jérôme
>>
>> I meant the comment that I accidentally deleted, before sending the email!
>> 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 = 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.
>>
>
> What about renaning hmm_get() to mm_get_hmm() instead ?
>

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,
--
John Hubbard
NVIDIA

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