Re: [RFC 1/4] mm: Introduce lazy exec permission setting on a page

[Matthew Wilcox <willy@infradead.org>]

On Wed, Feb 13, 2019 at 01:36:28PM +0530, Anshuman Khandual wrote:
> +#ifdef CONFIG_ARCH_SUPPORTS_LAZY_EXEC
> +static inline pte_t maybe_mkexec(pte_t entry, struct vm_area_struct *vma)
> +{
> +	if (unlikely(vma->vm_flags & VM_EXEC))
> +		return pte_mkexec(entry);
> +	return entry;
> +}
> +#else
> +static inline pte_t maybe_mkexec(pte_t entry, struct vm_area_struct *vma)
> +{
> +	return entry;
> +}
> +#endif

> +++ b/mm/memory.c
> @@ -2218,6 +2218,8 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
>  	flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
>  	entry = pte_mkyoung(vmf->orig_pte);
>  	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +	if (vmf->flags & FAULT_FLAG_INSTRUCTION)
> +		entry = maybe_mkexec(entry, vma);

I don't understand this bit.  We have a fault based on an instruction
fetch.  But we're only going to _maybe_ set the exec bit?  Why not call
pte_mkexec() unconditionally?