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

From: Peter Zijlstra <peterz@infradead.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
	torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com,
	tglx@linutronix.de, akpm@linux-foundation.org, riel@redhat.com,
	mgorman@suse.de, oleg@redhat.com, mingo@redhat.com,
	minchan@kernel.org, kamezawa.hiroyu@jp.fujitsu.com,
	viro@zeniv.linux.org.uk, laijs@cn.fujitsu.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC][PATCH 0/6] Another go at speculative page faults
Date: Wed, 22 Oct 2014 13:45:58 +0200	[thread overview]
Message-ID: <20141022114558.GC21513@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20141022112925.GH30588@node.dhcp.inet.fi>

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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