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Date:   Wed, 7 Apr 2021 16:35:28 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Michel Lespinasse <michel@lespinasse.org>,
        Linux-MM <linux-mm@kvack.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Michal Hocko <mhocko@suse.com>,
        Rik van Riel <riel@surriel.com>,
        Paul McKenney <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Rom Lemarchand <romlem@google.com>,
        Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 11/37] x86/mm: attempt speculative mm faults first
Message-ID: <20210407153528.GF2531743@casper.infradead.org>
References: <20210407014502.24091-1-michel@lespinasse.org>
 <20210407014502.24091-12-michel@lespinasse.org>
 <YG3GTI8j1ohk4NhS@hirez.programming.kicks-ass.net>
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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.