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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 27, 2014 at 09:34:22AM -0500, Christoph Lameter wrote:
> On Tue, 27 May 2014, Peter Zijlstra wrote:
> 
> > The things I care about for VM_PINNED are long term pins, like the IB
> > stuff, which sets up its RDMA buffers at the start of a program and
> > basically leaves them in place for the entire duration of said program.
> 
> Ok that also means the pages are not to be allocated from ZONE_MOVABLE?

Well, like with IB, they start out as normal userspace pages, and will
be from ZONE_MOVABLE.

> I expected the use of a page flag. With a vma flag we may have a situation
> that mapping a page into a vma changes it to pinned and terminating a
> process may unpin a page. That means the zone that the page should be
> allocated from changes.

So the only way to 'map' something into pinned is what perf does (have
the f_ops->mmap call set VM_PINNED). But that way already ensures we
have full control over the allocation since its a custom file.

And in fact the perf buffer is allocated with GFP_KERNEL and is thus
already not from MOVABLE.

Any other use, like (again) the IB stuff, will go through
get_user_pages() which will ensure all the pages are mapped and present.

So I don't think this is a real problem and certainly not one that
requires a page flag.

> Pinned pages in ZONE_MOVABLE are not a good idea. But since "kernelcore"
> is rarely used maybe that is not an issue?

Well, the idea was to migrate pages to a more suitable location on
mm_mpin(). We could choose to move them out again on mm_munpin() or not.

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