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

[Nadav Amit]

Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> 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.

...

> 
> +#ifndef pmdp_establish
> +#define pmdp_establish pmdp_establish
> +static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd)
> +{
> +	if (IS_ENABLED(CONFIG_SMP)) {
> +		return xchg(pmdp, pmd);
> +	} else {
> +		pmd_t old = *pmdp;
> +		*pmdp = pmd;

I think you may want to use WRITE_ONCE() here - otherwise nobody guarantees
that the compiler will not split writes to *pmdp. Although the kernel uses
similar code to setting PTEs and PMDs, I think that it is best to start
fixing it. Obviously, you might need a different code path for 32-bit
kernels.

Regards,
Nadav