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

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Message-ID: <ZdR+JzfHjES3zytp@MiWiFi-R3L-srv>
Date: Tue, 20 Feb 2024 18:25:43 +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 02/19/24 at 03:52pm, Kirill A. Shutemov wrote:
> On Mon, Feb 19, 2024 at 01:12:32PM +0800, Baoquan He wrote:
> > On 02/12/24 at 12:44pm, Kirill A. Shutemov wrote:
> > > lookup_address() only returns correct page table level for the entry if
> > > the entry is not none.
> > > 
> > > Make the helper to always return correct 'level'. It allows to implement
> > > iterator over kernel page tables using lookup_address().
> > > 
> > > Add one more entry into enum pg_level to indicate size of VA covered by
> > > one PGD entry in 5-level paging mode.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> > > ---
> > >  arch/x86/include/asm/pgtable_types.h | 1 +
> > >  arch/x86/mm/pat/set_memory.c         | 8 ++++----
> > >  2 files changed, 5 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> > > index 0b748ee16b3d..3f648ffdfbe5 100644
> > > --- a/arch/x86/include/asm/pgtable_types.h
> > > +++ b/arch/x86/include/asm/pgtable_types.h
> > > @@ -548,6 +548,7 @@ enum pg_level {
> > >  	PG_LEVEL_2M,
> > >  	PG_LEVEL_1G,
> > >  	PG_LEVEL_512G,
> > > +	PG_LEVEL_256T,
> > >  	PG_LEVEL_NUM
> > >  };
> > >  
> > > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > > index f92da8c9a86d..3612e3167147 100644
> > > --- a/arch/x86/mm/pat/set_memory.c
> > > +++ b/arch/x86/mm/pat/set_memory.c
> > > @@ -666,32 +666,32 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> > 
> > LGTM,
> > 
> > Reviewed-by: Baoquan He <bhe@...hat.com>
> > 
> > By the way, we may need update the code comment above function
> > lookup_address_in_pgd() and function lookup_address() since they don't
> > reflect the latest behaviour of them.
> 
> 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)
..
}