From: John Hubbard <jhubbard@nvidia.com>
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>
Subject: Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct
Date: Wed, 20 Feb 2019 16:42:45 -0800 [thread overview]
Message-ID: <1ccab0d3-7e90-8e39-074d-02ffbfc68480@nvidia.com> (raw)
In-Reply-To: <20190221003716.GD24489@redhat.com>
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
next prev parent reply other threads:[~2019-02-21 0:43 UTC|newest]
Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-29 16:54 [PATCH 00/10] HMM updates for 5.1 jglisse
2019-01-29 16:54 ` [PATCH 01/10] mm/hmm: use reference counting for HMM struct jglisse
2019-02-20 23:47 ` John Hubbard
2019-02-20 23:59 ` Jerome Glisse
2019-02-21 0:06 ` John Hubbard
2019-02-21 0:15 ` Jerome Glisse
2019-02-21 0:32 ` John Hubbard
2019-02-21 0:37 ` Jerome Glisse
2019-02-21 0:42 ` John Hubbard [this message]
2019-01-29 16:54 ` [PATCH 02/10] mm/hmm: do not erase snapshot when a range is invalidated jglisse
2019-02-20 23:58 ` John Hubbard
2019-01-29 16:54 ` [PATCH 03/10] mm/hmm: improve and rename hmm_vma_get_pfns() to hmm_range_snapshot() jglisse
2019-02-21 0:25 ` John Hubbard
2019-02-21 0:28 ` Jerome Glisse
2019-01-29 16:54 ` [PATCH 04/10] mm/hmm: improve and rename hmm_vma_fault() to hmm_range_fault() jglisse
2019-01-29 16:54 ` [PATCH 05/10] mm/hmm: improve driver API to work and wait over a range jglisse
2019-01-29 16:54 ` [PATCH 06/10] mm/hmm: add default fault flags to avoid the need to pre-fill pfns arrays jglisse
2019-01-29 16:54 ` [PATCH 07/10] mm/hmm: add an helper function that fault pages and map them to a device jglisse
2019-03-18 20:21 ` Dan Williams
2019-03-18 20:41 ` Jerome Glisse
2019-03-18 21:30 ` Dan Williams
2019-03-18 22:15 ` Jerome Glisse
2019-03-19 3:29 ` Dan Williams
2019-03-19 13:30 ` Jerome Glisse
2019-03-19 8:44 ` Ira Weiny
2019-03-19 17:10 ` Jerome Glisse
2019-03-19 14:10 ` Ira Weiny
2019-01-29 16:54 ` [PATCH 08/10] mm/hmm: support hugetlbfs (snap shoting, faulting and DMA mapping) jglisse
2019-01-29 16:54 ` [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX backed filesystem jglisse
2019-01-29 18:41 ` Dan Williams
2019-01-29 19:31 ` Jerome Glisse
2019-01-29 20:51 ` Dan Williams
2019-01-29 21:21 ` Jerome Glisse
2019-01-30 2:32 ` Dan Williams
2019-01-30 3:03 ` Jerome Glisse
2019-01-30 17:25 ` Dan Williams
2019-01-30 18:36 ` Jerome Glisse
2019-01-31 3:28 ` Dan Williams
2019-01-31 4:16 ` Jerome Glisse
2019-01-31 5:44 ` Dan Williams
2019-03-05 22:16 ` Andrew Morton
2019-03-06 4:20 ` Dan Williams
2019-03-06 15:51 ` Jerome Glisse
2019-03-06 15:57 ` Dan Williams
2019-03-06 16:03 ` Jerome Glisse
2019-03-06 16:06 ` Dan Williams
2019-03-07 17:46 ` Andrew Morton
2019-03-07 18:56 ` Jerome Glisse
2019-03-12 3:13 ` Dan Williams
2019-03-12 15:25 ` Jerome Glisse
2019-03-12 16:06 ` Dan Williams
2019-03-12 19:06 ` Jerome Glisse
2019-03-12 19:30 ` Dan Williams
2019-03-12 20:34 ` Dave Chinner
2019-03-13 1:06 ` Dan Williams
2019-03-12 21:52 ` Andrew Morton
2019-03-13 0:10 ` Jerome Glisse
2019-03-13 0:46 ` Dan Williams
2019-03-13 1:00 ` Jerome Glisse
2019-03-13 16:06 ` Andrew Morton
2019-03-13 18:39 ` Jerome Glisse
2019-03-06 15:49 ` Jerome Glisse
2019-03-06 22:18 ` Andrew Morton
2019-03-07 0:36 ` Jerome Glisse
2019-01-29 16:54 ` [PATCH 10/10] mm/hmm: add helpers for driver to safely take the mmap_sem jglisse
2019-02-20 21:59 ` John Hubbard
2019-02-20 22:19 ` Jerome Glisse
2019-02-20 22:40 ` John Hubbard
2019-02-20 23:09 ` Jerome Glisse
2019-02-20 23:17 ` [PATCH 00/10] HMM updates for 5.1 John Hubbard
2019-02-20 23:36 ` Jerome Glisse
2019-02-22 23:31 ` Ralph Campbell
2019-03-13 1:27 ` Jerome Glisse
2019-03-13 16:10 ` Andrew Morton
2019-03-13 18:01 ` Jason Gunthorpe
2019-03-13 18:33 ` Jerome Glisse
2019-03-18 17:00 ` Kuehling, Felix
2019-03-18 17:04 ` Jerome Glisse
2019-03-18 18:30 ` Dan Williams
2019-03-18 18:54 ` Jerome Glisse
2019-03-18 19:18 ` Dan Williams
2019-03-18 19:28 ` Jerome Glisse
2019-03-18 19:36 ` Dan Williams
2019-03-19 16:40 ` Andrew Morton
2019-03-19 16:58 ` Jerome Glisse
2019-03-19 17:12 ` Andrew Morton
2019-03-19 17:18 ` Jerome Glisse
2019-03-19 17:33 ` Dan Williams
2019-03-19 17:45 ` Jerome Glisse
2019-03-19 18:42 ` Dan Williams
2019-03-19 19:05 ` Jerome Glisse
2019-03-19 19:13 ` Dan Williams
2019-03-19 14:18 ` Ira Weiny
2019-03-19 22:24 ` Jerome Glisse
2019-03-19 19:18 ` Jerome Glisse
2019-03-19 20:25 ` Jerome Glisse
2019-03-19 21:51 ` Stephen Rothwell
2019-03-19 18:51 ` Deucher, Alexander
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