linux-kernel - Re: [PATCH 2/7] x86/mm: Drop page table entry address output from pxd

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Message-ID: <9d3286bd-7ad4-4472-aa26-2fb7d166fceb@arm.com>
Date: Mon, 16 Sep 2024 08:24:17 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 "Mike Rapoport (IBM)" <rppt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 x86@...nel.org, linux-m68k@...ts.linux-m68k.org,
 linux-fsdevel@...r.kernel.org, kasan-dev@...glegroups.com,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 2/7] x86/mm: Drop page table entry address output from
 pxd_ERROR()



On 9/13/24 22:51, Dave Hansen wrote:
> On 9/13/24 01:44, Anshuman Khandual wrote:
>> This drops page table entry address output from all pxd_ERROR() definitions
>> which now matches with other architectures. This also prevents build issues
>> while transitioning into pxdp_get() based page table entry accesses.
> 
> Could you be a _little_ more specific than "build issues"?  Is it that
> you want to do:
> 
>  void pmd_clear_bad(pmd_t *pmd)
>  {
> -        pmd_ERROR(*pmd);
> +        pmd_ERROR(pmdp_get(pmd));
>          pmd_clear(pmd);
>  }
> 
> But the pmd_ERROR() macro would expand that to:
> 
> 	&pmdp_get(pmd)
> 
> which is nonsense?

Yes, that's the one which fails the build with the following warning.

error: lvalue required as unary '&' operand

Will update the commit message with these details about the build problem.

> 
> Having the PTEs' kernel addresses _is_ handy, but I guess they're
> scrambled on most end users' systems now and anybody that's actively
> debugging can just use a kprobe or something to dump the pmd_clear_bad()
> argument directly.

Right.