linux-kernel - Re: [RFC PATCH] mm: Add helpers for locked

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Date:	Fri, 01 Aug 2014 20:08:02 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alexey Kardashevskiy <aik@...abs.ru>,
	Davidlohr Bueso <davidlohr@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Jo\"rn Engel" <joern@...fs.org>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Alex Williamson <alex.williamson@...hat.com>,
	Alexander Graf <agraf@...e.de>,
	Michael Ellerman <michael@...erman.id.au>
Subject: Re: [RFC PATCH] mm: Add helpers for locked_vm

On Wed, 2014-07-30 at 14:47 +0200, Peter Zijlstra wrote:
> On Wed, Jul 30, 2014 at 10:30:48PM +1000, Alexey Kardashevskiy wrote:
> > 
> > No, this is not my intention here. Here I only want to increment the counter.
> 
> Full and hard nack on that. It should always be tied to actual pages, we
> should not detach this and make it 'a number'.

But this is the only way. We *cannot* go through the whole per-page
locking logic every time the guest puts a translation into the IOMMU,
this will completely kill guest performances for pass-through devices.

Worse, for performances, because populating the iommu is a hypercall,
we want to do it in "real mode" (special MMU-off environment) where we
cannot rely on most normal kernel services such as normal locks, vmalloc
space isn't accessible etc...

So we don't have a choice. Either we let guests randomly pin arbitrary
amounts of system memory, or we have a way to predictively account for
the maximum that *can* be mapped/pinned in the iommu table to enable
the fast path.

Another problem with the mlock logic is that it doesn't refcount how
many time a page has been locked, while the guest can map a given page
multiple time in the iommu.

Ben.

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