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

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Message-ID: <20140917135614.GJ2840@worktop.localdomain>
Date:	Wed, 17 Sep 2014 15:56:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
	Christoph Lameter <cl@...ux.com>,
	linux-mm <linux-mm@...ck.org>, linux-ext4@...r.kernel.org
Subject: Re: Best way to pin a page in ext4?

On Tue, Sep 16, 2014 at 05:07:18PM -0700, 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.

These realtime people are fully aware of this -- they should be at
least, I've been telling them for years.

Also, they would be very happy with means to actually pin pages -- as
per the patches Christoph referred to. The advantage of also having
mpin() and co is that we can migrate the memory into non-movable blocks
before returning etc.

In any case, I think we can (and should) change the behaviour of mlock
to be migratable (possibly with an easy way to revert in -rt for
migratory purposes until we get mpin sorted).
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