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

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Date:   Thu, 08 Jun 2023 15:19:12 +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 Thursday, 8 June 2023 00:47:36 CEST Edgecombe, Rick P wrote:
> On Wed, 2023-06-07 at 23:33 +0200, Janusz Krzysztofik wrote:
> > > 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.
> 
> _PAGE_PSE is already in _HPAGE_CHG_MASK though, right? I'm confused.

Yes, it is, sorry for confusion.  I should have said: we may still want to 
keep _PAGE_PSE explicitly added to _HPAGE_CHK_MASK to have the reason for 
including that bit documented.

Thanks,
Janusz


> 
> > 
> > > 
> > > 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.
> 
> When you say "vel", this is similar to the english acronym "AKA" I
> think?
> 
> So I think you mean, when you add _PAGE_PAT to _PAGE_CHG_MASK, you are
> also adding _PAGE_PSE to it. So does that cause any problems? I see,
> good question... 
> 
> vm_page_prot is used when creating PTEs and huge PMDs, and the setter
> only uses _PAGE_CHG_MASK, even though it won't actually know where that
> prot is going to end up.
> 
> Having _PAGE_PAT/PSE in _PAGE_CHG_MASK certainly doesn't make it easier
> to think about. One thing it's favor though is vm_page_prot is not
> applied to page table entries that are pointing to other page table
> entries (PSE = 0). So you shouldn't accidentally set PSE=1. And
> _PAGE_PSE shouldn't be being set in there, so you also shouldn't
> accidentally be setting PAT=1.
> 
> But yea, I see why you are concerned. I would /guess/ it would be ok
> functionally. That probably doesn't help much...
>