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

[Vlastimil Babka <vbabka@suse.cz>]

On 05/27/2014 01:11 PM, Konstantin Khlebnikov wrote:
> 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.

Mlocking makes sense, as pages won't be uselessly scanned on
non-evictable LRU, no? (Or maybe I just don't see that something else
prevents then from being there already).

Anyway I like the idea of playing nicer with compaction etc.

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