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SubjectRe: [PATCH 01/10] mm/hmm: use reference counting for HMM struct
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On 2/20/19 4:15 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.

thanks,
--
John Hubbard
NVIDIA

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