linux-kernel - Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct

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Date:   Wed, 20 Feb 2019 16:32:09 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct

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@...hat.com wrote:
>>>>> From: Jérôme Glisse <jglisse@...hat.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.

thanks,
-- 
John Hubbard
NVIDIA