linux-kernel - Re: [RFC][PATCH 0/6] Another go at speculative page faults

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Message-ID: <20141022114558.GC21513@worktop.programming.kicks-ass.net>
Date:	Wed, 22 Oct 2014 13:45:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Davidlohr Bueso <dave@...olabs.net>, torvalds@...ux-foundation.org,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	akpm@...ux-foundation.org, riel@...hat.com, mgorman@...e.de,
	oleg@...hat.com, mingo@...hat.com, minchan@...nel.org,
	kamezawa.hiroyu@...fujitsu.com, viro@...iv.linux.org.uk,
	laijs@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC][PATCH 0/6] Another go at speculative page faults

On Wed, Oct 22, 2014 at 02:29:25PM +0300, Kirill A. Shutemov wrote:
> On Wed, Oct 22, 2014 at 12:34:49AM -0700, Davidlohr Bueso wrote:
> > On Mon, 2014-10-20 at 23:56 +0200, Peter Zijlstra wrote:
> > > Hi,
> > > 
> > > I figured I'd give my 2010 speculative fault series another spin:
> > > 
> > >   https://lkml.org/lkml/2010/1/4/257
> > > 
> > > Since then I think many of the outstanding issues have changed sufficiently to
> > > warrant another go. In particular Al Viro's delayed fput seems to have made it
> > > entirely 'normal' to delay fput(). Lai Jiangshan's SRCU rewrite provided us
> > > with call_srcu() and my preemptible mmu_gather removed the TLB flushes from
> > > under the PTL.
> > > 
> > > The code needs way more attention but builds a kernel and runs the
> > > micro-benchmark so I figured I'd post it before sinking more time into it.
> > > 
> > > I realize the micro-bench is about as good as it gets for this series and not
> > > very realistic otherwise, but I think it does show the potential benefit the
> > > approach has.
> > > 
> > > (patches go against .18-rc1+)
> > 
> > I think patch 2/6 is borken:
> > 
> > error: patch failed: mm/memory.c:2025
> > error: mm/memory.c: patch does not apply
> > 
> > and related, as you mention, I would very much welcome having the
> > introduction of 'struct faut_env' as a separate cleanup patch. May I
> > suggest renaming it to fault_cxt?
> 
> What about extend start using 'struct vm_fault' earlier by stack?

I'm not sure we should mix the environment for vm_ops::fault, which
acquires the page, and the fault path, which deals with changing the
PTE. Ideally we should not expose the page-table information to file
ops, its a layering violating if nothing else, drivers should not have
access to the page tables.
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