Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT

[Jason Baron]

On 06/12/2018 03:46 AM, Michal Hocko wrote:
> On Mon 11-06-18 12:23:58, Jason Baron wrote:
>> On 06/11/2018 11:03 AM, Michal Hocko wrote:
>>> So can we start discussing whether we want to allow MADV_DONTNEED on
>>> mlocked areas and what downsides it might have? Sure it would turn the
>>> strong mlock guarantee to have the whole vma resident but is this
>>> acceptable for something that is an explicit request from the owner of
>>> the memory?
>>>
>>
>> If its being explicity requested by the owner it makes sense to me. I
>> guess there could be a concern about this breaking some userspace that
>> relied on MADV_DONTNEED not freeing locked memory?
> 
> Yes, this is always the fear when changing user visible behavior.  I can
> imagine that a userspace allocator calling MADV_DONTNEED on free could
> break. The same would apply to MLOCK_ONFAULT/MCL_ONFAULT though. We
> have the new flag much shorter so the probability is smaller but the
> problem is very same. So I _think_ we should treat both the same because
> semantically they are indistinguishable from the MADV_DONTNEED POV. Both
> remove faulted and mlocked pages. Mlock, once applied, should guarantee
> no later major fault and MADV_DONTNEED breaks that obviously.
> 
> So the more I think about it the more I am worried about this but I am
> more and more convinced that making ONFAULT special is just a wrong way
> around this.
> 

Ok, I share the concern that there is a chance that userspace is relying
on MADV_DONTNEED not free'ing locked memory. In that case, what if we
introduce a MADV_DONTNEED_FORCE, which does everything that
MADV_DONTNEED currently does but in addition will also free mlock areas.
That way there is no concern about breaking something.

Thanks,

-Jason