From: "Christian König" <ckoenig.leichtzumerken@gmail.com>
To: Michal Hocko <mhocko@kernel.org>, christian.koenig@amd.com
Cc: kvm@vger.kernel.org, "Radim Krčmář" <rkrcmar@redhat.com>,
"Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
"Joonas Lahtinen" <joonas.lahtinen@linux.intel.com>,
"Sudeep Dutt" <sudeep.dutt@intel.com>,
dri-devel@lists.freedesktop.org, linux-mm@kvack.org,
"Andrea Arcangeli" <aarcange@redhat.com>,
"David (ChunMing) Zhou" <David1.Zhou@amd.com>,
"Dimitri Sivanich" <sivanich@sgi.com>,
"Jason Gunthorpe" <jgg@ziepe.ca>,
linux-rdma@vger.kernel.org, amd-gfx@lists.freedesktop.org,
"David Airlie" <airlied@linux.ie>,
"Doug Ledford" <dledford@redhat.com>,
"David Rientjes" <rientjes@google.com>,
xen-devel@lists.xenproject.org, intel-gfx@lists.freedesktop.org,
"Jani Nikula" <jani.nikula@linux.intel.com>,
"Leon Romanovsky" <leonro@mellanox.com>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Rodrigo Vivi" <rodrigo.vivi@intel.com>,
"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
"Juergen Gross" <jgross@suse.com>,
"Mike Marciniszyn" <mike.marciniszyn@intel.com>,
"Dennis Dalessandro" <dennis.dalessandro@intel.com>,
LKML <linux-kernel@vger.kernel.org>,
"Ashutosh Dixit" <ashutosh.dixit@intel.com>,
"Alex Deucher" <alexander.deucher@amd.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Felix Kuehling" <felix.kuehling@amd.com>
Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers
Date: Fri, 24 Aug 2018 13:57:52 +0200 [thread overview]
Message-ID: <a27ad1a3-34bd-6b7d-fd09-7737ec3c888d@gmail.com> (raw)
In-Reply-To: <20180824115226.GK29735@dhcp22.suse.cz>
Am 24.08.2018 um 13:52 schrieb Michal Hocko:
> On Fri 24-08-18 13:43:16, Christian König wrote:
>> Am 24.08.2018 um 13:32 schrieb Michal Hocko:
>>> On Fri 24-08-18 19:54:19, Tetsuo Handa wrote:
>>>> Two more worries for this patch.
>>>>
>>>>
>>>>
>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
>>>>> @@ -178,12 +178,18 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
>>>>> *
>>>>> * @amn: our notifier
>>>>> */
>>>>> -static void amdgpu_mn_read_lock(struct amdgpu_mn *amn)
>>>>> +static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
>>>>> {
>>>>> - mutex_lock(&amn->read_lock);
>>>>> + if (blockable)
>>>>> + mutex_lock(&amn->read_lock);
>>>>> + else if (!mutex_trylock(&amn->read_lock))
>>>>> + return -EAGAIN;
>>>>> +
>>>>> if (atomic_inc_return(&amn->recursion) == 1)
>>>>> down_read_non_owner(&amn->lock);
>>>> Why don't we need to use trylock here if blockable == false ?
>>>> Want comment why it is safe to use blocking lock here.
>>> Hmm, I am pretty sure I have checked the code but it was quite confusing
>>> so I might have missed something. Double checking now, it seems that
>>> this read_lock is not used anywhere else and it is not _the_ lock we are
>>> interested about. It is the amn->lock (amdgpu_mn_lock) which matters as
>>> it is taken in exclusive mode for expensive operations.
>> The write side of the lock is only taken in the command submission IOCTL.
>>
>> So you actually don't need to change anything here (even the proposed
>> changes are overkill) since we can't tear down the struct_mm while an IOCTL
>> is still using.
> I am not so sure. We are not in the mm destruction phase yet. This is
> mostly about the oom context which might fire right during the IOCTL. If
> any of the path which is holding the write lock blocks for unbound
> amount of time or even worse allocates a memory then we are screwed. So
> we need to back of when blockable = false.
Oh, yeah good point. Haven't thought about that possibility.
>
>>> Is that correct Christian? If this is correct then we need to update the
>>> locking here. I am struggling to grasp the ref counting part. Why cannot
>>> all readers simply take the lock rather than rely on somebody else to
>>> take it? 1ed3d2567c800 didn't really help me to understand the locking
>>> scheme here so any help would be appreciated.
>> That won't work like this there might be multiple
>> invalidate_range_start()/invalidate_range_end() pairs open at the same time.
>> E.g. the lock might be taken recursively and that is illegal for a
>> rw_semaphore.
> I am not sure I follow. Are you saying that one invalidate_range might
> trigger another one from the same path?
No, but what can happen is:
invalidate_range_start(A,B);
invalidate_range_start(C,D);
...
invalidate_range_end(C,D);
invalidate_range_end(A,B);
Grabbing the read lock twice would be illegal in this case.
Regards,
Christian.
next prev parent reply other threads:[~2018-08-24 11:57 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-16 11:50 [PATCH] mm, oom: distinguish blockable mode for mmu notifiers Michal Hocko
2018-07-16 23:12 ` Andrew Morton
2018-07-17 4:03 ` Leon Romanovsky
2018-07-17 8:12 ` Michal Hocko
2018-07-20 23:01 ` Andrew Morton
2018-07-23 8:43 ` Michal Hocko
2018-07-19 9:12 ` Michal Hocko
2018-07-21 0:09 ` Andrew Morton
2018-07-23 7:03 ` Michal Hocko
2018-07-23 7:11 ` Michal Hocko
2018-07-23 8:11 ` Michal Hocko
2018-07-24 14:17 ` Michal Hocko
2018-07-24 19:53 ` Andrew Morton
2018-07-25 6:17 ` Michal Hocko
2018-07-24 21:07 ` David Rientjes
2018-07-25 6:13 ` Michal Hocko
2018-08-24 10:54 ` Tetsuo Handa
2018-08-24 11:32 ` Michal Hocko
2018-08-24 11:43 ` Christian König
2018-08-24 11:52 ` Michal Hocko
2018-08-24 11:57 ` Christian König [this message]
2018-08-24 12:03 ` Michal Hocko
2018-08-24 12:18 ` Christian König
2018-08-24 12:33 ` Michal Hocko
2018-08-24 12:52 ` Christian König
2018-08-24 13:01 ` Michal Hocko
2018-08-24 13:10 ` Christian König
2018-08-24 13:24 ` Michal Hocko
2018-08-24 13:28 ` Christian König
2018-08-24 13:40 ` Michal Hocko
2018-08-24 13:44 ` Christian König
2018-08-24 13:52 ` Michal Hocko
2018-08-26 8:40 ` Tetsuo Handa
2018-08-27 7:41 ` Christian König
2018-09-06 22:46 ` Tetsuo Handa
2018-08-24 15:08 ` Jerome Glisse
2018-08-24 11:36 ` Michal Hocko
2018-08-24 13:02 ` Tetsuo Handa
2018-08-24 13:32 ` Michal Hocko
2018-08-24 14:52 ` Tetsuo Handa
2018-08-24 15:12 ` Jerome Glisse
2018-08-24 16:40 ` Michal Hocko
2018-08-24 17:33 ` Jerome Glisse
2018-08-24 16:38 ` Michal Hocko
2018-08-24 14:40 ` Jerome Glisse
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