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

[Alistair Popple <apopple@nvidia.com>]

On Friday, 4 June 2021 1:14:31 PM AEST Hugh Dickins wrote:
> 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).

Oh my bad, I had somehow missed this was reusing an *existing* arch-dependent 
swap bit (_PAGE_SWP_UFFD_WP, although the same argument could apply) even 
though it's in the commit message. Obviously I should have read that more 
carefully, apologies for the noise but thanks for the clarification.

> 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.)

Agree, that's a somewhat subjective debate. Conceptually I don't think this is 
particularly difficult to understand. It just adds another slightly different 
class of special swap pte's to know about.

 - Alistair

> Hugh