Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT

From: Konstantin Khlebnikov
Date: Mon Aug 24 2015 - 06:17:56 EST

On Fri, Aug 21, 2015 at 9:31 PM, Eric B Munson <emunson@xxxxxxxxxx> wrote:
> On Fri, 21 Aug 2015, Michal Hocko wrote:
>> On Thu 20-08-15 13:03:09, Eric B Munson wrote:
>> > On Thu, 20 Aug 2015, Michal Hocko wrote:
>> >
>> > > On Wed 19-08-15 17:33:45, Eric B Munson wrote:
>> > > [...]
>> > > > The group which asked for this feature here
>> > > > wants the ability to distinguish between LOCKED and LOCKONFAULT regions
>> > > > and without the VMA flag there isn't a way to do that.
>> > >
>> > > Could you be more specific on why this is needed?
>> >
>> > They want to keep metrics on the amount of memory used in a LOCKONFAULT
>> > region versus the address space of the region.
>> /proc/<pid>/smaps already exports that information AFAICS. It exports
>> VMA flags including VM_LOCKED and if rss < size then this is clearly
>> LOCKONFAULT because the standard mlock semantic is to populate. Would
>> that be sufficient?
>> Now, it is true that LOCKONFAULT wouldn't be distinguishable from
>> MAP_LOCKED which failed to populate but does that really matter? It is
>> LOCKONFAULT in a way as well.
> Does that matter to my users? No, they do not use MAP_LOCKED at all so
> any VMA with VM_LOCKED set and rss < size is lock on fault. Will it
> matter to others? I suspect so, but these are likely to be the same
> group of users which will be suprised to learn that MAP_LOCKED does not
> guarantee that the entire range is faulted in on return from mmap.
>> > > > Do we know that these last two open flags are needed right now or is
>> > > > this speculation that they will be and that none of the other VMA flags
>> > > > can be reclaimed?
>> > >
>> > > I do not think they are needed by anybody right now but that is not a
>> > > reason why it should be used without a really strong justification.
>> > > If the discoverability is really needed then fair enough but I haven't
>> > > seen any justification for that yet.
>> >
>> > To be completely clear you believe that if the metrics collection is
>> > not a strong enough justification, it is better to expand the mm_struct
>> > by another unsigned long than to use one of these bits right?
>> A simple bool is sufficient for that. And yes I think we should go with
>> per mm_struct flag rather than the additional vma flag if it has only
>> the global (whole address space) scope - which would be the case if the
>> LOCKONFAULT is always an mlock modifier and the persistance is needed
>> only for MCL_FUTURE. Which is imho a sane semantic.
> I am in the middle of implementing lock on fault this way, but I cannot
> see how we will hanlde mremap of a lock on fault region. Say we have
> the following:
> addr = mmap(len, MAP_ANONYMOUS, ...);
> mlock(addr, len, MLOCK_ONFAULT);
> ...
> mremap(addr, len, 2 * len, ...)
> There is no way for mremap to know that the area being remapped was lock
> on fault so it will be locked and prefaulted by remap. How can we avoid
> this without tracking per vma if it was locked with lock or lock on
> fault?

remap can count filled ptes and prefault only completely populated areas.

There might be a problem after failed populate: remap will handle them
as lock on fault. In this case we can fill ptes with swap-like non-present
entries to remember that fact and count them as should-be-locked pages.
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