linux-kernel - Re: [PATCHv7 07/16] x86/mm: Return correct level from lookup

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Date: Fri, 23 Feb 2024 10:45:02 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
 Elena Reshetova <elena.reshetova@...el.com>,
 Jun Nakajima <jun.nakajima@...el.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>,
 Tom Lendacky <thomas.lendacky@....com>, "Kalra, Ashish"
 <ashish.kalra@....com>, Sean Christopherson <seanjc@...gle.com>,
 "Huang, Kai" <kai.huang@...el.com>, Baoquan He <bhe@...hat.com>,
 kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 07/16] x86/mm: Return correct level from
 lookup_address() if pte is none

On 2/12/24 02:44, Kirill A. Shutemov wrote:
> lookup_address() only returns correct page table level for the entry if
> the entry is not none.

Currently, lookup_address() returns two things:
  1. A "pte_t" (which might be a p[g4um]d_t)
  2. The 'level' of the page tables where the "pte_t" was found
     (returned via a pointer)

If no pte_t is found, 'level' is essentially garbage.

> Make the helper to always return correct 'level'. It allows to implement
> iterator over kernel page tables using lookup_address().

One nit with this description: What's "correct" isn't immediately
obvious to me.  It wasn't exactly incorrect before.  I think it would be
better to say:

	Always fill out the level.  For NULL "pte_t"s, fill in the level
	where the p*d_none() entry was found mirroring the "found"
	behavior.

	Always filling out the level allows using lookup_address() to
	iterate over kernel page tables.

> Add one more entry into enum pg_level to indicate size of VA covered by
> one PGD entry in 5-level paging mode.

Needs some 'the's:

Add one more entry into enum pg_level to indicate the size of the VA
covered by one PGD entry in 5-level paging mode.

With that fixed:

Reviewed-by: Dave Hansen <dave.hansen@...el.com>