linux-kernel - Re: [PATCH v2 02/11] mm/hmm: use reference counting for HMM struct v2

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Date:   Fri, 29 Mar 2019 13:07:45 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>, Ira Weiny <ira.weiny@...el.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 02/11] mm/hmm: use reference counting for HMM struct v2

On 3/28/19 7:25 PM, Jerome Glisse wrote:
[...]
>> The input value is not the problem.  The problem is in the naming.
>>
>> obj = get_obj( various parameters );
>> put_obj(obj);
>>
>>
>> The problem is that the function is named hmm_register() either "gets" a
>> reference to _or_ creates and gets a reference to the hmm object.
>>
>> What John is probably ready to submit is something like.
>>
>> struct hmm *get_create_hmm(struct mm *mm);
>> void put_hmm(struct hmm *hmm);
>>
>>
>> So when you are reading the code you see...
>>
>> foo(...) {
>> 	struct hmm *hmm = get_create_hmm(mm);
>>
>> 	if (!hmm)
>> 		error...
>>
>> 	do stuff...
>>
>> 	put_hmm(hmm);
>> }
>>
>> Here I can see a very clear get/put pair.  The name also shows that the hmm is
>> created if need be as well as getting a reference.
>>
> 
> You only need to create HMM when you either register a mirror or
> register a range. So they two pattern:
> 
>     average_foo() {
>         struct hmm *hmm = mm_get_hmm(mm);
>         ...
>         hmm_put(hmm);
>     }
> 
>     register_foo() {
>         struct hmm *hmm = hmm_register(mm);
>         ...
>         return 0;
>     error:
>         ...
>         hmm_put(hmm);
>     }
> 

1. Looking at this fresh this morning, Ira's idea of just a single rename
actually clarifies things a lot more than I expected. I think the following
tiny patch would suffice here (I've updated documentation to match, and added
a missing "@Return:" line too):

iff --git a/mm/hmm.c b/mm/hmm.c
index fd143251b157..37b1c5803f1e 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -50,14 +50,17 @@ static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
 }
 
 /*
- * hmm_register - register HMM against an mm (HMM internal)
+ * hmm_get_create - returns an HMM object, either by referencing the existing
+ * (per-process) object, or by creating a new one.
  *
- * @mm: mm struct to attach to
+ * @mm: the mm_struct to attach to
+ * @Return: a pointer to the HMM object, or NULL upon failure. This pointer must
+ * be released, when done, via hmm_put().
  *
- * This is not intended to be used directly by device drivers. It allocates an
- * HMM struct if mm does not have one, and initializes it.
+ * This is an internal HMM function, and is not intended to be used directly by
+ * device drivers.
  */
-static struct hmm *hmm_register(struct mm_struct *mm)
+static struct hmm *hmm_get_create(struct mm_struct *mm)
 {
        struct hmm *hmm = mm_get_hmm(mm);
        bool cleanup = false;
@@ -288,7 +291,7 @@ int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
        if (!mm || !mirror || !mirror->ops)
                return -EINVAL;
 
-       mirror->hmm = hmm_register(mm);
+       mirror->hmm = hmm_get_create(mm);
        if (!mirror->hmm)
                return -ENOMEM;
 
@@ -915,7 +918,7 @@ int hmm_range_register(struct hmm_range *range,
        range->start = start;
        range->end = end;
 
-       range->hmm = hmm_register(mm);
+       range->hmm = hmm_get_create(mm);
        if (!range->hmm)
                return -EFAULT;




2. A not directly related point: did you see my minor comment on patch 0001? I think it might have been missed in all the threads yesterday.



thanks,
-- 
John Hubbard
NVIDIA