linux-kernel - Re: [RFC PATCH v3 08/16] powerpc/8xx: Rework support for 8M pages using contiguous PTE entries

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Date: Tue, 28 May 2024 10:53:57 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Oscar Salvador <osalvador@...e.de>
CC: Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe
	<jgg@...dia.com>, Peter Xu <peterx@...hat.com>, Michael Ellerman
	<mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH v3 08/16] powerpc/8xx: Rework support for 8M pages
 using contiguous PTE entries



Le 27/05/2024 à 14:10, Oscar Salvador a écrit :
> On Sun, May 26, 2024 at 11:22:28AM +0200, Christophe Leroy wrote:
>> In order to fit better with standard Linux page tables layout, add
>> support for 8M pages using contiguous PTE entries in a standard
>> page table. Page tables will then be populated with 1024 similar
>> entries and two PMD entries will point to that page table.
>>
>> The PMD entries also get a flag to tell it is addressing an 8M page,
>> this is required for the HW tablewalk assistance.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> 
> I did not look close into KSAN bits, and I trust you with the assembly part,
> but other than that looks good to me, so FWIW:
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> 
> Just a nit below:
> 
>> +#define __HAVE_ARCH_HUGE_PTEP_GET
>> +static inline pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
>> +{
>> +	if (ptep_is_8m_pmdp(mm, addr, ptep))
>> +		ptep = pte_offset_kernel((pmd_t *)ptep, 0);
> 
> Would it not be more clear to use pmd_page_vaddr directly there?
> 
> 

Well, the correct way should have been:

	ptep = pte_offset_kernel((pmd_t *)ptep, ALIGN_DOWN(addr, SZ_8M));

Now, is it more clear with:

	ptep = (pte_t *)pmd_page_vaddr(*(pmd_t *)ptep);

I don't know.