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

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Date: Mon, 1 Jul 2024 14:39:34 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "jgross@...e.com" <jgross@...e.com>, "luto@...nel.org" <luto@...nel.org>,
 "bp@...en8.de" <bp@...en8.de>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
 "peterz@...radead.org" <peterz@...radead.org>,
 "mingo@...hat.com" <mingo@...hat.com>,
 "tglx@...utronix.de" <tglx@...utronix.de>
Cc: "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 7/1/2024 2:13 PM, Edgecombe, Rick P wrote:
> On Mon, 2024-07-01 at 13:59 -0500, Kalra, Ashish wrote:
>> Then what is the caller supposed to do in this case ?
>>
>> As the return from lookup_address() is non-NULL in this case, accessing it
>> causes a fatal #PF.
>>
>> Is the caller supposed to add the check for a valid PTE using pte_none(*pte) ?
> I did a quick look at the callers, and some do their own check for pte_none().
> But some don't. Some also assume the return can't be NULL.
>
> Can you elaborate on your goal for this change? Just a cleanup?

Hit this issue while implementing and testing SNP guest kexec.

So trying to understand if need a generic fix for this issue or do i need to add my own check for pte_none() ?

Thanks, Ashish