Re: Best way to pin a page in ext4?

On Wed, 17 Sep 2014, Peter Zijlstra wrote:

On Tue, Sep 16, 2014 at 10:31:24PM -0500, Christoph Lameter wrote:

quoted

On Tue, 16 Sep 2014, Hugh Dickins wrote:

quoted

On the page migration issue: it's not quite as straightforward as
Christoph suggests.  He and I agree completely that mlocked pages
should be migratable, but some real-time-minded people disagree:
so normal compaction is still forbidden to migrate mlocked pages in
the vanilla kernel (though we in Google patch that prohibition out).
So pinning by refcount is no worse for compaction than mlocking,
in the vanilla kernel.

Note though that compaction is not the only mechanism that uses page
migration.

True: offhand, I think memory hotremove, and CMA, and explicit mempolicy
changes, are all (for good reason) allowed to migrate mlocked pages; but
the case which most interests many is migration for compaction.

Agreed, and not all migration paths check for mlocked iirc. ISTR it is
very much possible for mlocked pages to get migrated in mainline.

I think all the checks are for unevictable; and certainly we permit
races whereby an mlocked page may miss the unevictable LRU, until
subsequent reclaim corrects the omission.  But I think that's the
extent to which mlocked pages might be migrated for compaction at
present.

Hugh