linux-kernel - Re: [RFC PATCH 11/37] x86/mm: attempt speculative mm faults first

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Message-ID: <20210407153528.GF2531743@casper.infradead.org>
Date:   Wed, 7 Apr 2021 16:35:28 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 11/37] x86/mm: attempt speculative mm faults first

On Wed, Apr 07, 2021 at 04:48:44PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 06, 2021 at 06:44:36PM -0700, Michel Lespinasse wrote:
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1219,6 +1219,8 @@ void do_user_addr_fault(struct pt_regs *regs,
> >  	struct mm_struct *mm;
> >  	vm_fault_t fault;
> >  	unsigned int flags = FAULT_FLAG_DEFAULT;
> > +	struct vm_area_struct pvma;
> 
> That's 200 bytes on-stack... I suppose that's just about acceptible, but
> perhaps we need a comment in struct vm_area_struct to make people aware
> this things lives on-stack and size really is an issue now.

Michel's gone off and done his own thing here.

The rest of us (Laurent, Liam & I) are working on top of the maple tree
which shrinks vm_area_struct by five pointers, so just 160 bytes.

Also, our approach doesn't involve copying VMAs in order to handle a fault.