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

[Matthew Wilcox]

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.