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

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Date: Tue, 20 Feb 2024 14:36:48 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Baoquan He <bhe@...hat.com>
Cc: 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, 
	"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>, 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 Tue, Feb 20, 2024 at 06:25:43PM +0800, Baoquan He wrote:
> > I am not sure what part of the comment you see doesn't reflect the
> > behaviour. From my PoV, changed code matches the comment closer that
> > original.
> 
> Oh, I didn't make it clear. I mean update the code comment for
> lookup_address(), and add code comment for lookup_address_in_pgd() to
> mention the level thing. Maybe it's a chance to do that.
> 
> ===1>
> *
>  * Lookup the page table entry for a virtual address. Return a pointer
>  * to the entry and the level of the mapping.
>  *
>  * Note: We return pud and pmd either when the entry is marked large
>                    ~~~~~~~~~~~ seems we return p4d too
>  * or when the present bit is not set. Otherwise we would return a
>  * pointer to a nonexisting mapping.
>               ~~~~~~~~~~~~~~~ NULL?
>  */                          
> pte_t *lookup_address(unsigned long address, unsigned int *level)
> {
>         return lookup_address_in_pgd(pgd_offset_k(address), address, level);
> }
> EXPORT_SYMBOL_GPL(lookup_address);
> ===
> 
> ===2>
> /*
>  * Lookup the page table entry for a virtual address in a specific pgd.
>  * Return a pointer to the entry and the level of the mapping.
>    ~~ also could return NULL if it's none entry. And do we need to
> mention the level thing?
>  */
> pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
>                              unsigned int *level)
> ...
> }
> 

What about this fixup:

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 3612e3167147..425ff6e192e6 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -657,7 +657,8 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
 
 /*
  * Lookup the page table entry for a virtual address in a specific pgd.
- * Return a pointer to the entry and the level of the mapping.
+ * Return a pointer to the entry and the level of the mapping (or NULL if
+ * the entry is none) and level of the entry.
  */
 pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
 			     unsigned int *level)
@@ -704,9 +705,8 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
  * Lookup the page table entry for a virtual address. Return a pointer
  * to the entry and the level of the mapping.
  *
- * Note: We return pud and pmd either when the entry is marked large
- * or when the present bit is not set. Otherwise we would return a
- * pointer to a nonexisting mapping.
+ * Note: the function returns p4d, pud and pmd either when the entry is marked
+ * large or when the present bit is not set. Otherwise it returns NULL.
  */
 pte_t *lookup_address(unsigned long address, unsigned int *level)
 {
-- 
  Kiryl Shutsemau / Kirill A. Shutemov