linux-kernel - Re: [Patch v3] x86/head/64: remove redundant check on level2_kernel_pgt's _PAGE

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Date: Tue, 11 Jun 2024 01:01:21 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Wei Yang <richard.weiyang@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, Steve Wahl <steve.wahl@....com>
Subject: Re: [Patch v3] x86/head/64: remove redundant check on
 level2_kernel_pgt's _PAGE_PRESENT bit

On Mon, Jun 03, 2024 at 11:50:06AM -0700, Dave Hansen wrote:
>On 5/23/24 05:35, Wei Yang wrote:
>> --- a/arch/x86/kernel/head64.c
>> +++ b/arch/x86/kernel/head64.c
>> @@ -260,8 +260,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
>>  
>>  	/* fixup pages that are part of the kernel image */
>>  	for (; i <= pmd_index((unsigned long)_end); i++)
>> -		if (pmd[i] & _PAGE_PRESENT)
>> -			pmd[i] += load_delta;
>> +		pmd[i] += load_delta;
>
>So, I think this is correct.  But, man, I wish folks would go through
>the git history and make it clear that they understand _how_ the code
>got the way it is.
>
>I suspect that the original _PAGE_PRESENT check wasn't even necessary if
>cleanup_highmap() really did fix things up.  But this commit:
>
>	2aa85f246c18 ("x86/boot/64: Make level2_kernel_pgt pages invalid
>		       outside kernel area")
>
>tweaked things to actively clear out PMDs that weren't populated in
>Kirill's original loop.  It didn't touch the _PAGE_PRESENT check.  But
>it certainly did imply that the PMD doesn't have any holes in it and
>there's nothing int he middle that needs _PAGE_PRESENT cleared.
>
>> level2_kernel_pgt compiled with _PAGE_PRESENT set. The check is
>> redundant
>
>This isn't super reassuring.  It also depends on nothing having munged
>the page tables up to this point.  The code is also a bit cruel in that
>it manipulates two different sets of PMDs with the same 'pmd' variable.
>
>Also, is this comment still accurate after '2aa85f246c18'?
>
>>          * Fixup the kernel text+data virtual addresses. Note that
>>          * we might write invalid pmds, when the kernel is relocated
>>          * cleanup_highmap() fixes this up along with the mappings
>>          * beyond _end.

Hi, Dave

Do you have other suggestions? What do I expect to do next?

-- 
Wei Yang
Help you, Help me