On Mon, Jun 19, 2017 at 04:22:29PM +0100, Catalin Marinas wrote:
> Hi Kirill,
>
> 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@xxxxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> 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.
> > +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"?
>
> > + old = *pmdp;
> > + } while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);
> > +
> > + return old;
> > +}
>
> --
> Catalin
>
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Kirill A. Shutemov
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