linux-kernel - Re: [PATCH v3 02/16] mm: introduce leaf entry type and use to simplify leaf entry logic

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Message-ID: <a948b201-30e0-4ffe-ba81-e033afd94ba3@lucifer.local>
Date: Wed, 12 Nov 2025 15:36:43 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zi Yan <ziy@...dia.com>
Cc: "David Hildenbrand (Red Hat)" <david@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
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Subject: Re: [PATCH v3 02/16] mm: introduce leaf entry type and use to
 simplify leaf entry logic

On Tue, Nov 11, 2025 at 11:26:03AM -0500, Zi Yan wrote:
> On 11 Nov 2025, at 8:06, David Hildenbrand (Red Hat) wrote:
>
> > On 11.11.25 04:25, Zi Yan wrote:
> >> On 10 Nov 2025, at 17:21, Lorenzo Stoakes wrote:
> >>
> >>> The kernel maintains leaf page table entries which contain either:
> >>>
> >>> - Nothing ('none' entries)
> >>> - Present entries (that is stuff the hardware can navigate without fault)
> >>
> >> This is not true for:
> >>
> >> 1. pXX_protnone(), where _PAGE_PROTNONE flag also means pXX_present() is
> >> true, but hardware would still trigger a fault.
> >> 2. pmd_present() where _PAGE_PSE also means a present PMD (see the comment
> >> in pmd_present()).
> >
> > I'll note that pte_present/pmd_present etc is always about "soft-present".
> >
> > For example, if the hardware does not have a hw-managed access bit, doing a pte_mkyoung() would also clear the hw-valid/hw-present bit because we have to catch any next access done by hardware.
> >
> > [fun fact: some hardware has an invalid bit instead of a valid/present bit :) IIRC s390x falls into that category]
> >
> > Similar things happen on ordinary PROT_NONE of course (independent of pte_protnone).
> >
> > A better description might be "there is a page/pfn mapped here, but it might not be accessible by the CPU right now".
> >
> > We have device-exclusive/device-private nonswap (before this series) entries that fall into the same category, unfortunately ("there is something mapped there that is not accessible by the CPU")
>
> I agree. I am fine with the categorization using pte_none(), pte_present(),
> and softleaf. It is “hardware can navigate without fault” that causes
> confusion. Removing this comment would work for me, since people can look
> at the definition of pXX_present() for further clarification.

Well I proposed a suggestion :) I think referencing PFN is problematic ecause of
those softleaf entries that may also have a PFN.

I think better definition is that the entry is valid modulo the present bit.

>
> Best Regards,
> Yan, Zi

Cheers, Lorenzo