Re: [RFC][PATCH 0/5] VM_PINNED

[Peter Zijlstra]

On Tue, May 27, 2014 at 12:29:09PM +0200, Peter Zijlstra wrote:
> On Tue, May 27, 2014 at 12:49:08AM +0400, Konstantin Khlebnikov wrote:
> > Another suggestion. VM_RESERVED is stronger than VM_LOCKED and extends
> > its functionality.
> > Maybe it's easier to add VM_DONTMIGRATE and use it together with VM_LOCKED.
> > This will make accounting easier. No?
> 
> I prefer the PINNED name because the not being able to migrate is only
> one of the desired effects of it, not the primary effect. We're really
> looking to keep physical pages in place and preserve mappings.
> 
> The -rt people for example really want to avoid faults (even minor
> faults), and DONTMIGRATE would still allow unmapping.
> 
> Maybe always setting VM_PINNED and VM_LOCKED together is easier, I
> hadn't considered that. The first thing that came to mind is that that
> might make the fork() semantics difficult, but maybe it works out.
> 
> And while we're on the subject, my patch preserves PINNED over fork()
> but maybe we don't actually need that either.

So pinned_vm is userspace exposed, which means we have to maintain the
individual counts, and doing the fully orthogonal accounting is 'easier'
than trying to get the boundary cases right.

That is, if we have a program that does mlockall() and then does the IB
ioctl() to 'pin' a region, we'd have to make mm_mpin() do munlock()
after it splits the vma, and then do the pinned accounting.

Also, we'll have lost the LOCKED state and unless MCL_FUTURE was used,
we don't know what to restore the vma to on mm_munpin().

So while the accounting looks tricky, it has simpler semantics.

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