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

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Date:	Wed, 22 Oct 2014 14:55:40 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Peter Zijlstra <peterz@...radead.org>
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 01:45:58PM +0200, Peter Zijlstra wrote:
> 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.

We already have this for ->map_pages() :-P
I have asked if it's considered layering violation and seems nobody
cares...

-- 
 Kirill A. Shutemov
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