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Subject: Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take
 the mmap_sem v2
To:     Jerome Glisse <jglisse@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Thu, 28 Mar 2019 16:34:04 -0700
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On 3/28/19 4:24 PM, Jerome Glisse wrote:
> On Thu, Mar 28, 2019 at 04:20:37PM -0700, John Hubbard wrote:
>> On 3/28/19 4:05 PM, Jerome Glisse wrote:
>>> On Thu, Mar 28, 2019 at 03:43:33PM -0700, John Hubbard wrote:
>>>> On 3/28/19 3:40 PM, Jerome Glisse wrote:
>>>>> On Thu, Mar 28, 2019 at 03:25:39PM -0700, John Hubbard wrote:
>>>>>> On 3/28/19 3:08 PM, Jerome Glisse wrote:
>>>>>>> On Thu, Mar 28, 2019 at 02:41:02PM -0700, John Hubbard wrote:
>>>>>>>> On 3/28/19 2:30 PM, Jerome Glisse wrote:
>>>>>>>>> On Thu, Mar 28, 2019 at 01:54:01PM -0700, John Hubbard wrote:
>>>>>>>>>> On 3/25/19 7:40 AM, jglisse@redhat.com wrote:
>>>>>>>>>>> From: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
>>>>>> [...]
>>>>>> OK, so let's either drop this patch, or if merge windows won't allow=
 that,
>>>>>> then *eventually* drop this patch. And instead, put in a hmm_sanity_=
check()
>>>>>> that does the same checks.
>>>>>
>>>>> RDMA depends on this, so does the nouveau patchset that convert to ne=
w API.
>>>>> So i do not see reason to drop this. They are user for this they are =
posted
>>>>> and i hope i explained properly the benefit.
>>>>>
>>>>> It is a common pattern. Yes it only save couple lines of code but dow=
n the
>>>>> road i will also help for people working on the mmap_sem patchset.
>>>>>
>>>>
>>>> It *adds* a couple of lines that are misleading, because they look lik=
e they
>>>> make things safer, but they don't actually do so.
>>>
>>> It is not about safety, sorry if it confused you but there is nothing a=
bout
>>> safety here, i can add a big fat comment that explains that there is no=
 safety
>>> here. The intention is to allow the page fault handler that potential h=
ave
>>> hundred of page fault queue up to abort as soon as it sees that it is p=
ointless
>>> to keep faulting on a dying process.
>>>
>>> Again if we race it is _fine_ nothing bad will happen, we are just doin=
g use-
>>> less work that gonna be thrown on the floor and we are just slowing dow=
n the
>>> process tear down.
>>>
>>
>> In addition to a comment, how about naming this thing to indicate the ab=
ove=20
>> intention?  I have a really hard time with this odd down_read() wrapper,=
 which
>> allows code to proceed without really getting a lock. It's just too wron=
g-looking.
>> If it were instead named:
>>
>> 	hmm_is_exiting()
>=20
> What about: hmm_lock_mmap_if_alive() ?
>=20

That's definitely better, but I want to vote for just doing a check, not=20
taking any locks.

I'm not super concerned about the exact name, but I really want a routine t=
hat
just checks (and optionally asserts, via WARN or BUG), and that's *all*. Th=
en
drivers can scatter that around like pixie dust as they see fit. Maybe righ=
t before
taking a lock, maybe in other places. Decoupled from locking.

thanks,
--=20
John Hubbard
NVIDIA