Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take the mmap_sem v2

[John Hubbard <jhubbard@nvidia.com>]

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: Jérôme Glisse <jglisse@redhat.com>
[...]
>>
>>>>
>>>> If you insist on having this wrapper, I think it should have approximately 
>>>> this form:
>>>>
>>>> void hmm_mirror_mm_down_read(...)
>>>> {
>>>> 	WARN_ON(...)
>>>> 	down_read(...)
>>>> } 
>>>
>>> I do insist as it is useful and use by both RDMA and nouveau and the
>>> above would kill the intent. The intent is do not try to take the lock
>>> if the process is dying.
>>
>> Could you provide me a link to those examples so I can take a peek? I
>> am still convinced that this whole thing is a race condition at best.
> 
> The race is fine and ok see:
> 
> https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-odp-v2&id=eebd4f3095290a16ebc03182e2d3ab5dfa7b05ec
> 
> which has been posted and i think i provided a link in the cover
> letter to that post. The same patch exist for nouveau i need to
> cleanup that tree and push it.

Thanks for that link, and I apologize for not keeping up with that
other review thread.

Looking it over, hmm_mirror_mm_down_read() is only used in one place.
So, what you really want there is not a down_read() wrapper, but rather,
something like

	hmm_sanity_check()

, that ib_umem_odp_map_dma_pages() calls.


> 
>>>
>>>
>>>>
>>>>> +{
>>>>> +	struct mm_struct *mm;
>>>>> +
>>>>> +	/* Sanity check ... */
>>>>> +	if (!mirror || !mirror->hmm)
>>>>> +		return -EINVAL;
>>>>> +	/*
>>>>> +	 * Before trying to take the mmap_sem make sure the mm is still
>>>>> +	 * alive as device driver context might outlive the mm lifetime.
>>>>
>>>> Let's find another way, and a better place, to solve this problem.
>>>> Ref counting?
>>>
>>> This has nothing to do with refcount or use after free or anthing
>>> like that. It is just about checking wether we are about to do
>>> something pointless. If the process is dying then it is pointless
>>> to try to take the lock and it is pointless for the device driver
>>> to trigger handle_mm_fault().
>>
>> Well, what happens if you let such pointless code run anyway? 
>> Does everything still work? If yes, then we don't need this change.
>> If no, then we need a race-free version of this change.
> 
> Yes everything work, nothing bad can happen from a race, it will just
> do useless work which never hurt anyone.
> 

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.


thanks,
-- 
John Hubbard
NVIDIA