linux-kernel - Re: [PATCH v6 06/18] asm-generic/tlb: Conditionally provide tlb_migrate

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Date:   Wed, 20 Feb 2019 07:02:53 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        aneesh.kumar@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        npiggin@...il.com, linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux@...linux.org.uk,
        heiko.carstens@...ibm.com, riel@...riel.com, tony.luck@...el.com
Subject: Re: [PATCH v6 06/18] asm-generic/tlb: Conditionally provide
 tlb_migrate_finish()

On Wed, Feb 20, 2019 at 02:47:05PM +0000, Will Deacon wrote:
> On Tue, Feb 19, 2019 at 02:41:47PM +0100, Peter Zijlstra wrote:
> > On Tue, Feb 19, 2019 at 12:47:38PM +0000, Will Deacon wrote:
> > > Fine for now, but I agree that we should drop the hook altogether. AFAICT,
> > > this only exists to help an ia64 optimisation which looks suspicious to
> > > me since it uses:
> > > 
> > >     mm == current->active_mm && atomic_read(&mm->mm_users) == 1
> > > 
> > > to identify a "single-threaded fork()" and therefore perform only local TLB
> > > invalidation. Even if this was the right thing to do, it's not clear to me
> > > that tlb_migrate_finish() is called on the right CPU anyway.
> > > 
> > > So I'd be keen to remove this hook before it spreads, but in the meantime:
> > 
> > Agreed :-)
> > 
> > The obvious slash and kill patch ... untested
> 
> I'm also unable to test this, unfortunately. Can we get it into next after
> the merge window and see if anybody reports issues?

While I do have a pair of Itanium systems in my basement, neither are
sn2 machines, which was the only sub-architecture that implemented
tlb_migrate_finish().  I see NASA decomissioned Columbia in 2013, and
I imagine most sn2 machines have been similarly scrapped.