Re: [PATCH v4 2/4] mm: Add function to support extra actions on swap in/out

[Dave Hansen]

On 01/11/2017 08:12 AM, Khalid Aziz wrote:
> +#ifndef set_swp_pte_at
> +#define set_swp_pte_at(mm, addr, ptep, pte, oldpte)	\
> +		set_pte_at(mm, addr, ptep, pte)
> +#endif

BTW, thanks for the *much* improved description of the series.  This is
way easier to understand.

I really don't think this is the interface we want, though.
set_swp_pte_at() is really doing *two* things:
1. Detecting _PAGE_MCD_4V and squirreling the MCD data away at swap-out
2. Reading back in the MCD data at swap-on

You're effectively using (!pte_none(pte) && !pte_present(pte)) to
determine whether you're at swap in or swap out time.  That's goofy, IMNHO.

It isn't obvious from the context, but this hunk is creating a migration
PTE.  Why is ADI tag manipulation needed?  We're just changing the
physical address of the underlying memory, but neither the
application-visible contents nor the tags are changing.

> @@ -1539,7 +1539,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  		swp_pte = swp_entry_to_pte(entry);
>  		if (pte_soft_dirty(pteval))
>  			swp_pte = pte_swp_mksoft_dirty(swp_pte);
> -		set_pte_at(mm, address, pte, swp_pte);
> +		set_swp_pte_at(mm, address, pte, swp_pte, pteval);
>  	} else if (PageAnon(page)) {
>  		swp_entry_t entry = { .val = page_private(page) };
>  		pte_t swp_pte;

Which means you're down to a single call that does swap-out, and a
single call that does swap-in.  There's no reason to hide all your code
behind set_pte_at().

Just add a new arch-specific call that takes the VMA and the swap PTE
and stores the ADI bit in there, here:

> @@ -1572,7 +1572,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  		swp_pte = swp_entry_to_pte(entry);
>  		if (pte_soft_dirty(pteval))
>  			swp_pte = pte_swp_mksoft_dirty(swp_pte);
> -		set_pte_at(mm, address, pte, swp_pte);
> +		set_swp_pte_at(mm, address, pte, swp_pte, pteval);
>  	} else

and in do_swap_page(), do the opposite with a second, new call.