linux-kernel - Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take the mmap

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Date:   Thu, 28 Mar 2019 16:20:37 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take
 the mmap_sem v2

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@...hat.com wrote:
>>>>>>>>> From: Jérôme Glisse <jglisse@...hat.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 new 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 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.
> 
> It is not about safety, sorry if it confused you but there is nothing about
> 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 have
> hundred of page fault queue up to abort as soon as it sees that it is pointless
> to keep faulting on a dying process.
> 
> 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.
> 

In addition to a comment, how about naming this thing to indicate the above 
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 wrong-looking.
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,
-- 
John Hubbard
NVIDIA