Re: [PATCH v3 04/27] mm/userfaultfd: Introduce special pte for unmapped file-backed mem

[Hugh Dickins]

On Fri, 4 Jun 2021, Alistair Popple wrote:
> 
> The detail which is perhaps less important is whether to implement this using 
> a new swap entry type or arch-specific swap bit. The argument for using a swap 
> type is it will work across architectures due to the use of pte_to_swp_entry() 
> and swp_entry_to_pte() to convert to and from the arch-dependent and 
> independent representations.
> 
> The argument against seems to have been that it is wasting a swap type. 
> However if I'm understanding correctly that's not true for all architectures, 
> and needing to reserve a bit is more wasteful than using a swap type.

I'm on the outside, not paying much attention here,
but thought Peter would have cleared this up already.

My understanding is that it does *not* use an additional arch-dependent
bit, but puts the _PAGE_UFFD_WP bit (already set aside by any architecture
implementing UFFD WP) to an additional use.  That's why I called this
design (from Andrea) more elegant than mine (swap type business).

If I've got that wrong, and yet another arch-dependent bit is needed,
then I very much agree with you: finding arch-dependent pte bits is a
much tougher job than another play with swap type.

(And "more elegant" might not be "easier to understand": you decide.)

Hugh