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?
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