Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active

[Vlastimil Babka]

On 06/02/2017 11:10 PM, Andrew Morton wrote:
> On Fri, 2 Jun 2017 22:55:12 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
> 
>> On 06/02/2017 10:40 PM, Andrew Morton wrote:
>>> On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
>>>>> Perhaps we should be adding new prctl modes to select this new
>>>>> behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is?
>>>>
>>>> I think we can reasonably assume that most users of the prctl do just
>>>> the fork() & exec() thing, so they will be unaffected.
>>>
>>> That sounds optimistic.  Perhaps people are using the current behaviour
>>> to set on particular mapping to MMF_DISABLE_THP, with
>>>
>>> 	prctl(PR_SET_THP_DISABLE)
>>> 	mmap()
>>> 	prctl(PR_CLR_THP_DISABLE)
>>>
>>> ?
>>>
>>> Seems a reasonable thing to do.
>>
>> Using madvise(MADV_NOHUGEPAGE) seems reasonabler to me, with the same
>> effect. And it's older (2.6.38).
>>
>>> But who knows - people do all sorts of
>>> inventive things.
>>
>> Yeah :( but we can hope they don't even know that the prctl currently
>> behaves they way it does - man page doesn't suggest it would, and most
>> of us in this thread found it surprising.
> 
> Well.  There might be such people and sometimes we do make people
> unhappy.  it partly depends on how traumatic it would be to leave the
> current behaviour as-is.  Have you evaluated such a patch?

You mean introducing a new prctl instead of changing the existing one? I
can evaluate that as being ugly :)
Well, maybe we could use arg3, because currently we have:
        case PR_SET_THP_DISABLE:
                if (arg3 || arg4 || arg5)
                        return -EINVAL;

We could make non-zero arg3 (or specific value of arg3) set the new
"immediate" behavior. This would also take care of the discovery of
kernels that support the fixed/altered behavior, without having to check
uname etc - just check if we got -EINVAL.

I'm just not sure how to implement PR_GET_THP_DISABLE properly in such
scenario. Or what happens when somebody calls SET with arg3==0 and then
arg3==1 (or vice versa). But we would have to think about it even when
we introduced a newly named option. Reminds me of the MLOCK_ONFAULT
discussions...

>>>> And as usual, if
>>>> somebody does complain in the end, we revert and try the other way?
>>>
>>> But by then it's too late - the new behaviour will be out in the field.
>>
>> Revert in stable then?
>> But I don't think this patch should go to stable. I understand right
>> that CRIU will switch to the UFFDIO_COPY approach and doesn't need the
>> prctl change/new madvise anymore?
> 
> What I mean is that the new behaviour will go out in 4.12 and it may
> be many months before we find out that we broke someone.  By then, we
> can't go back because others may be assuming the new behaviour.
>