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Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is
 MLOCK_ONFAULT
To:     Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-api@vger.kernel.org, emunson@mgebm.net
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From:   Jason Baron <jbaron@akamai.com>
Message-ID: <7a671035-92dc-f9c0-aa7b-ff916d556e82@akamai.com>
Date:   Fri, 15 Jun 2018 15:36:07 -0400
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On 06/13/2018 03:15 AM, Michal Hocko wrote:
> On Wed 13-06-18 08:32:19, Vlastimil Babka wrote:
>> 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.
> 
> But MADV_DONTNEED is an explicit call for data loss. Or do I miss your
> point?
> 
>> The madvise manpage is also quite specific about not allowing
>> MADV_DONTNEED and MADV_FREE for locked pages.
> 
> Yeah, but that seems to describe the state of the art rather than
> explain why.
> 
>> 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?
> 
> That is what Jason wanted but I argued that the two are the same from
> MADV_DONTNEED point of view. I do not see how treating them differently
> would be less confusing or error prone. It's new so we can make it
> behave differently is certainly not an argument.
> 
>>>> 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?
> 
> Well, I wanted to bring this up as well. I guess this would require some
> more hacks to handle the reclaim path correctly because we do rely on
> VM_LOCK at many places for the lazy mlock pages culling.
> 

The point of not allowing MADV_FREE on mlock'd pages for me was that
with mlock and even MLOCK_ON_FAULT, one can always can always determine
if a page is present or not (and thus avoid the major fault). Allowing
MADV_FREE on lock'd pages breaks that assumption.

Thanks,

-Jason