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

From: Vlastimil Babka
Date: Wed Jun 13 2018 - 02:32:32 EST

On 06/12/2018 04:11 PM, Jason Baron wrote:
> 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.

I think more concerning than guaranteeing no later major fault is
possible data loss, e.g. replacing data with zero-filled pages.

The madvise manpage is also quite specific about not allowing
MADV_DONTNEED and MADV_FREE for locked pages.

So I don't think we should risk changing that for all mlocked pages.
Maybe we can risk MCL_ONFAULT, since it's relatively new and has few users?

>> 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.

A new niche case flag? Sad :(

BTW I didn't get why we should allow this for MADV_DONTNEED but not
MADV_FREE. Can you expand on that?

> Thanks,
> -Jason
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