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

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Tue, May 27, 2014 at 2:54 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> 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.

Ah, I just mixed it up.

>>
>> 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.

What if VM_PINNED will require VM_LOCKED?
I.e. user must mlock it before pining and cannot munlock vma while it's pinned.

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