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

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Date:   Wed, 20 Feb 2019 14:47:05 +0000
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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 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?

Will