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

[Janusz Krzysztofik]

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.

Thanks,
Janusz