On Wednesday, 7 June 2023 23:12:13 CEST Edgecombe, Rick P wrote:
> On Wed, 2023-06-07 at 19:11 +0200, Janusz Krzysztofik wrote:
> > On Wednesday, 7 June 2023 17:31:24 CEST Dave Hansen wrote:
> > > On 6/7/23 08:23, Janusz Krzysztofik wrote:
> > > >
> > > > Extend bitmask used by pgprot_modify() for selecting bits to be
> > > > preserved
> > > > with _PAGE_PAT bit. However, since that bit can be reused as
> > > > _PAGE_PSE,
> > > > and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is
> > > > likely
> > > > intentionally defined with that bit not set, keep that symbol
> > > > unchanged.
> > >
> > > I'm really having a hard time parsing what that last sentence is
> > > saying.
> > >
> > > Could you try again, please?
> >
> > OK, but then I need to get my doubts addressed by someone first,
> > otherwise I'm
> > not able to provide a better justification from my heart.
> >
> > The issue needs to be fixed by including _PAGE_PAT bit into a bitmask
> > used
> > by pgprot_modify() for selecting bits to be preserved. We can either
> > do
> > that internally to pgprot_modify() (my initial proposal, which my
> > poorly
> > worded paragraph was still trying to describe and justify), or by
> > making
> > _PAGE_PAT a part of _PAGE_CHG_MASK, as suggested by Borislav and
> > reflected in
> > my v2 changelog. But for the latter, I think we need to make sure
> > that we
> > don't break other users of _PAGE_CHG_MASK. Maybe Borislav can
> > confirm that's
> > the case.
> >
> > Since _PAGE_PAT is the same as _PAGE_PSE, _HPAGE_CHG_MASK -- a huge
> > pmds'
> > counterpart of _PAGE_CHG_MASK, introduced by commit c489f1257b8c
> > ("thp: add
> > pmd_modify"), defined as (_PAGE_CHG_MASK | _PAGE_PSE) -- will no
> > longer differ
> > from _PAGE_CHG_MASK as soon as we add _PAGE_PAT bit to the latter.
> > If such
> > modification of _PAGE_CHG_MASK was irrelevant to its users then one
> > may ask
> > why a new symbol was introduced instead of reusing the existing one
> > with that
> > otherwise irrelevant bit (_PAGE_PSE in that case) added. I've
> > initially
> > assumed that keeping _PAGE_CHG_MASK without _PAGE_PSE (vel _PAGE_PAT)
> > included
> > into it was intentional for some reason. Maybe Johannes Weiner, the
> > author of
> > that patch (adding him to Cc:), could shed more light on that.
>
> So since _PAGE_PSE is actually the same value as _PAGE_PAT, you don't
> actually need to have _PAGE_PSE in _HPAGE_CHG_MASK in order to get
> functional correctness. Is that right?
As soon as we add _PAGE_PAT to _PAGE_CHG_MASK -- yes, that's right. But we
may still want to add _PAGE_PSE to _HPAGE_CHG_MASK to have the need for that
bit explicitly documented.
>
> I think it is still a little hidden (even before this) and I wonder
> about separating out the common bits into, like, _COMMON_PAGE_CHG_MASK
> or something. Then setting specific PAGE and HPAGE bits (like
> _PAGE_PAT, _PAGE_PSE and _PAGE_PAT_LARGE) in their specific define.
> Would it be more readable that way?
Yes, I think that's a good idea, and I can use it in my patch.
The question if _PAGE_PAT vel _PAGE_PSE added to _PAGE_CHG_MASK is really
harmless for pte_modify() and its users is still open for me though.
Thanks,
Janusz
