linux-kernel - Re: [PATCH] x86/mm: fix lookup_address() to handle physical memory holes in direct mapping

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Date: Fri, 28 Jun 2024 21:33:21 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "tglx@...utronix.de" <tglx@...utronix.de>, "ashish.kalra@....com"
	<ashish.kalra@....com>, "peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>, "luto@...nel.org" <luto@...nel.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "bp@...en8.de"
	<bp@...en8.de>
CC: "jgross@...e.com" <jgross@...e.com>, "x86@...nel.org" <x86@...nel.org>,
	"Rodel, Jorg" <jroedel@...e.de>, "mhklinux@...look.com"
	<mhklinux@...look.com>, "hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"peterx@...hat.com" <peterx@...hat.com>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>
Subject: Re: [PATCH] x86/mm: fix lookup_address() to handle physical memory
 holes in direct mapping

On Fri, 2024-06-28 at 16:22 -0500, Kalra, Ashish wrote:
> > > @@ -717,7 +718,11 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd,
> > > unsigned
> > > long address,
> > >         *nx |= pmd_flags(*pmd) & _PAGE_NX;
> > >         *rw &= pmd_flags(*pmd) & _PAGE_RW;
> > >  
> > > -       return pte_offset_kernel(pmd, address);
> > > +       pte = pte_offset_kernel(pmd, address);
> > > +       if (pte_none(*pte))
> > > +               return NULL;
> > > +
> > > +       return pte;
> > The other levels check for pXX_none() before adjusting *level. Not sure what
> > the
> > effect would be, but I think it should be the same behavior for all.
> 
> If we are returning NULL, why should adjusting *level matter.

Well, I think symmetry is enough of a reason, but actually it should be ok.

I was looking at this diff compared to my working tree, but this tip commit
(which is about that scenario) makes it set *level before checking none for all
of them:
https://lore.kernel.org/lkml/171871930159.10875.16081839197437299007.tip-bot2@tip-bot2/

So sorry, nevermind.