Re: [PATCHv4 09/12] x86/mm: Provide pmdp_establish() helper

Andrew Morton
2017-12-14T00:09:53Z

On Wed, 13 Dec 2017 13:57:53 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> 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 lose these bits.
>
> On PAE we can avoid expensive cmpxchg8b for cases when new page table
> entry is not present. If it's present, fallback to cpmxchg loop.
>
> ...
>
> --- a/arch/x86/include/asm/pgtable-3level.h
> +++ b/arch/x86/include/asm/pgtable-3level.h
> @@ -158,7 +158,6 @@ static inline pte_t native_ptep_get_and_clear(pte_t *ptep)
> #define native_ptep_get_and_clear(xp) native_local_ptep_get_and_clear(xp)
> #endif
>
> -#ifdef CONFIG_SMP
> union split_pmd {
> struct {
> u32 pmd_low;
> @@ -166,6 +165,8 @@ union split_pmd {
> };
> pmd_t pmd;
> };
> +
> +#ifdef CONFIG_SMP
> static inline pmd_t native_pmdp_get_and_clear(pmd_t *pmdp)
> {
> union split_pmd res, *orig = (union split_pmd *)pmdp;
> @@ -181,6 +182,40 @@ static inline pmd_t native_pmdp_get_and_clear(pmd_t *pmdp)
> #define native_pmdp_get_and_clear(xp) native_local_pmdp_get_and_clear(xp)
> #endif
>
> +#ifndef pmdp_establish
> +#define pmdp_establish pmdp_establish
> +static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> + unsigned long address, pmd_t *pmdp, pmd_t pmd)
> +{
> + pmd_t old;
> +
> + /*
> + * If pmd has present bit cleared we can get away without expensive
> + * cmpxchg64: we can update pmdp half-by-half without racing with
> + * anybody.
> + */
> + if (!(pmd_val(pmd) & _PAGE_PRESENT)) {
> + union split_pmd old, new, *ptr;
> +
> + ptr = (union split_pmd *)pmdp;
> +
> + new.pmd = pmd;
> +
> + /* xchg acts as a barrier before setting of the high bits */
> + old.pmd_low = xchg(&ptr->pmd_low, new.pmd_low);
> + old.pmd_high = ptr->pmd_high;
> + ptr->pmd_high = new.pmd_high;
> + return old.pmd;
> + }
> +
> + {
> + old = *pmdp;
> + } while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);

um, what happened here?

> + return old;
> +}
> +#endif