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

On Fri, 4 Jun 2021, Alistair Popple wrote:

quoted

The detail which is perhaps less important is whether to implement this 

using

quoted

a new swap entry type or arch-specific swap bit. The argument for using a 

swap

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type is it will work across architectures due to the use of 

pte_to_swp_entry()

quoted

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,

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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