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

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Date:   Wed, 07 Jun 2023 23:33:05 +0200
From:   Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "Gross, Jurgen" <jgross@...e.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi.shyti@...ux.intel.com" <andi.shyti@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>
Subject: Re: [PATCH v2] x86/mm: Fix PAT bit missing from page protection modify mask

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