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

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Date: Wed, 21 Feb 2024 10:37:29 +0800
From: Baoquan He <bhe@...hat.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 02/20/24 at 02:36pm, Kirill A. Shutemov wrote:
> 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:

Some nitpicks.
> 
> 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.
                       ^ this right parenthesis may need be moved to the end.


=======
 * 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
                                          ~~~
                                           ^ s/and/or/
> + * 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
>