Re: Allow migration of mlocked page?

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2012-05-14 at 14:32 +0100, Mel Gorman wrote:

> Embedded does not imply realtime constraints.
> 
> > So, I don't think
> > CMA and compaction are significantly different.
> > 
> 
> CMA is used in cases such as a mobile phone needing to allocate a large
> contiguous range of memory for video decoding. Compaction is used by
> features such as THP with khugepaged potentially using it frequently on
> x86-64 machines. The use cases are different and compaction is used by
> THP a lot more than CMA is used by anything.
> 
> If compaction can move mlocked pages then khugepaged can introduce unexpected
> latencies on mlocked anonymous regions of memory.

I'd like to see CMA used for memcg and things as well, where we only
allocate the shadow page frames on-demand.

This moves CMA out of the crappy hardware-only section and should result
in pretty much everybody using it (except me, since I have cgroup=n).

Anyway, THP isn't an issue for -rt, its impossible to select when you
have PREEMPT_RT.

> > >Compaction on the other hand is during the normal operation of the
> > >machine. There are applications that assume that if anonymous memory
> > >is mlocked() then access to it is close to zero latency. They are
> > >not RT-critical processes (or they would disable THP) but depend on
> > >this. Allowing compaction to migrate mlocked() pages will result in bugs
> > >being reported by these people.
> > >
> > >I've received one bug this year about access latency to mlocked() regions but
> > >it turned out to be a file-backed region and related to when the write-fault
> > >is incurred. The ultimate fix was in the application but we'll get new bug
> > >reports if anonymous mlocked pages do not preserve the current guarantees
> > >on access latency.
> > 
> > Can you please tell us your opinion about autonuma?
> 
> I think it will have the same problem as THP using compaction. If
> mlocked pages can move then there may be unexpected latencies accessing
> mlocked anonymous regions.

numa and rt don't mix anyway.. don't worry about that.

> > I doubt we can keep such
> > mlock guarantee. I think we need to suggest application fix. maybe to introduce
> > MADV_UNMOVABLE is good start. it seems to solve autonuma issue too.
> > 
> 
> That'll regress existing applications. It would be preferable to me that
> it be the other way around to not move mlocked pages unless the user says
> it's allowed.

I'd say go for it, I've been telling everybody who would listen that
mlock() only means no major faults for a very long time now.



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