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

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Message-ID: <89aa51c4-20e1-4284-9ab4-bb505f261bea@suse.com>
Date: Tue, 2 Jul 2024 07:29:44 +0200
From: Jürgen Groß <jgross@...e.com>
To: "Kalra, Ashish" <ashish.kalra@....com>,
 "Edgecombe, Rick P" <rick.p.edgecombe@...el.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 01.07.24 21:39, Kalra, Ashish wrote:
> 
> 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() ?

Please add a check for pte_none() after calling lookup_address().


Juergen