Re: Allow migration of mlocked page?

[Christoph Lameter <cl@linux.com>]

On Mon, 14 May 2012, Peter Zijlstra wrote:

> 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.

We could introduce a new page flag PG_pinned (it already exists for Xen)
that would mean no faults on the page?

The situation with pinned pages is not clean right now because page count
increases should only signal temporary references to a page but subsystems
use an elevated page count to pin pages for good (f.e. Infiniband memory
registration). The reclaim logic has no way to differentiate between a
pinned page and a temporary reference count increase for page handling.

Therefore f.e. the page migration logic will repeatedly try to move the
page and always fail to account for all references.

A PG_pinned could allow us to make that distinction to avoid overhead in
the reclaim and page migration logic and also we could add some semantics
that avoid page faults.

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