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 16:22:59 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "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>,
 "mhklinux@...look.com" <mhklinux@...look.com>, "Rodel, Jorg"
 <jroedel@...e.de>, "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 6/28/2024 3:58 PM, Edgecombe, Rick P wrote:
> On Fri, 2024-06-28 at 20:52 +0000, Ashish Kalra wrote:
>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>> index 443a97e515c0..be8b5bf3bc3f 100644
>> --- a/arch/x86/mm/pat/set_memory.c
>> +++ b/arch/x86/mm/pat/set_memory.c
>> @@ -672,6 +672,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned
>> long address,
>>         p4d_t *p4d;
>>         pud_t *pud;
>>         pmd_t *pmd;
>> +       pte_t *pte;
>>  
>>         *level = PG_LEVEL_256T;
>>         *nx = false;
>> @@ -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.

Thanks, Ashish