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
