Re: [PATCH v2] x86/mm: Fix PAT bit missing from page protection modify mask

[Edgecombe, Rick P]

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?

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?