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

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Message-ID: <20210407201453.GA25738@lespinasse.org>
Date:   Wed, 7 Apr 2021 13:14:53 -0700
From:   Michel Lespinasse <michel@...pinasse.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>,
        Matthew Wilcox <willy@...radead.org>,
        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.

Right, I agree that having the vma copy on-stack is not ideal.

I think what really should be done, is to copy just the attributes of
the vma that will be needed during the page fault. Things like vm_mm,
vm_page_prot, vm_flags, vm_ops, vm_pgoff, vm_file, vm_private_data,
vm_policy. We definitely do not need rbtree and rmap fields such as
vm_prev, vm_next, vm_rb, rb_subtree_gap, shared, anon_vma_chain etc...

The reason I did things this way, is because changing the entire fault
handler to use attributes stored in struct vm_fault, rather than in
the original vma, would be quite intrusive. I think it would be a
reasonable step to consider once there is agreement on the rest of the
speculative fault patch set, but it's practical doing it before then.