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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 16:20:37 -0700
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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 t=
hat,
>>>> then *eventually* drop this patch. And instead, put in a hmm_sanity_ch=
eck()
>>>> that does the same checks.
>>>
>>> RDMA depends on this, so does the nouveau patchset that convert to new =
API.
>>> So i do not see reason to drop this. They are user for this they are po=
sted
>>> and i hope i explained properly the benefit.
>>>
>>> It is a common pattern. Yes it only save couple lines of code but down =
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 like =
they
>> make things safer, but they don't actually do so.
>=20
> It is not about safety, sorry if it confused you but there is nothing abo=
ut
> safety here, i can add a big fat comment that explains that there is no s=
afety
> here. The intention is to allow the page fault handler that potential hav=
e
> hundred of page fault queue up to abort as soon as it sees that it is poi=
ntless
> to keep faulting on a dying process.
>=20
> Again if we race it is _fine_ nothing bad will happen, we are just doing =
use-
> less work that gonna be thrown on the floor and we are just slowing down =
the
> process tear down.
>=20

In addition to a comment, how about naming this thing to indicate the above=
=20
intention?  I have a really hard time with this odd down_read() wrapper, wh=
ich
allows code to proceed without really getting a lock. It's just too wrong-l=
ooking.
If it were instead named:

	hmm_is_exiting()

and had a comment about why racy is OK, then I'd be a lot happier. :)


thanks,
--=20
John Hubbard
NVIDIA