linux-ext4 - Re: Best way to pin a page in ext4?

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Date:	Wed, 17 Sep 2014 13:37:40 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Christoph Lameter <cl@...ux.com>, Hugh Dickins <hughd@...gle.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger@...ger.ca>,
	linux-mm <linux-mm@...ck.org>, linux-ext4@...r.kernel.org
Subject: 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:
> > On Tue, 16 Sep 2014, Hugh Dickins wrote:
> > 
> > > 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
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