Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Jun 19, 2017 at 07:00:05PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jun 19, 2017 at 04:22:29PM +0100, Catalin Marinas wrote:
> > On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote:
> > > We need an atomic way to setup pmd page table entry, avoiding races with
> > > CPU setting dirty/accessed bits. This is required to implement
> > > pmdp_invalidate() that doesn't loose these bits.
> > > 
> > > On PAE we have to use cmpxchg8b as we cannot assume what is value of new pmd and
> > > setting it up half-by-half can expose broken corrupted entry to CPU.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > Cc: Ingo Molnar <mingo@kernel.org>
> > > Cc: H. Peter Anvin <hpa@zytor.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > 
> > I'll look at this from the arm64 perspective. It would be good if we can
> > have a generic atomic implementation based on cmpxchg64 but I need to
> > look at the details first.
> 
> Unfortunately, I'm not sure it's possbile.
> 
> The format of a page table is defined per-arch. We cannot assume much about
> it in generic code.
> 
> I guess we could make it compile by casting to 'unsigned long', but is it
> useful?
> Every architecture manintainer still has to validate that this assumption
> is valid for the architecture.

You are right, not much gained in doing this.

Maybe a stupid question but can we not implement pmdp_invalidate() with
something like pmdp_get_and_clear() (usually reusing the ptep_*
equivalent). Or pmdp_clear_flush() (again, reusing ptep_clear_flush())?

In my quick grep on pmdp_invalidate, it seems to be followed by
set_pmd_at() or pmd_populate() already and the *pmd value after
mknotpresent isn't any different from 0 to the hardware (at least on
ARM). That's unless Linux expects to see some non-zero value here if
walking the page tables on another CPU.

> > > +static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd)
> > > +{
> > > +	pmd_t old;
> > > +
> > > +	/*
> > > +	 * We cannot assume what is value of pmd here, so there's no easy way
> > > +	 * to set if half by half. We have to fall back to cmpxchg64.
> > > +	 */
> > > +	{
> > 
> > BTW, you are missing a "do" here (and it probably compiles just fine
> > without it, though different behaviour).
> 
> Ouch. Thanks.
> 
> Hm, what is semantics of the construct without a "do"?

You can just ignore the brackets:

	old = *pmdp;
	while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd)
		;

-- 
Catalin

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