Re: [PATCH RFC 07/30] mm/swap: Introduce the idea of special swap ptes

[Jason Gunthorpe]

On Fri, Jan 15, 2021 at 12:08:44PM -0500, Peter Xu wrote:
> We used to have special swap entries, like migration entries, hw-poison
> entries, device private entries, etc.
> 
> Those "special swap entries" reside in the range that they need to be at least
> swap entries first, and their types are decided by swp_type(entry).
> 
> This patch introduces another idea called "special swap ptes".
> 
> It's very easy to get confused against "special swap entries", but a speical
> swap pte should never contain a swap entry at all.  It means, it's illegal to
> call pte_to_swp_entry() upon a special swap pte.
> 
> Make the uffd-wp special pte to be the first special swap pte.
> 
> Before this patch, is_swap_pte()==true means one of the below:
> 
>    (a.1) The pte has a normal swap entry (non_swap_entry()==false).  For
>          example, when an anonymous page got swapped out.
> 
>    (a.2) The pte has a special swap entry (non_swap_entry()==true).  For
>          example, a migration entry, a hw-poison entry, etc.
> 
> After this patch, is_swap_pte()==true means one of the below, where case (b) is
> added:
> 
>  (a) The pte contains a swap entry.
> 
>    (a.1) The pte has a normal swap entry (non_swap_entry()==false).  For
>          example, when an anonymous page got swapped out.
> 
>    (a.2) The pte has a special swap entry (non_swap_entry()==true).  For
>          example, a migration entry, a hw-poison entry, etc.
> 
>  (b) The pte does not contain a swap entry at all (so it cannot be passed
>      into pte_to_swp_entry()).  For example, uffd-wp special swap pte.

Does the stuff in hmm.c need updating too?

        if (!pte_present(pte)) {
                swp_entry_t entry = pte_to_swp_entry(pte);

Jason