Re: [PATCH] mm: use standard page table accessors

Wei Yang
2025-11-26T12:15:39Z

On Wed, Nov 26, 2025 at 11:28:37AM +0100, Christophe Leroy (CS GROUP) wrote:
>
>
>Le 26/11/2025 à 11:03, Wei Yang a écrit :
>> On Wed, Nov 26, 2025 at 09:15:24AM +0100, Christophe Leroy (CS GROUP) wrote:
>> >
>> >
>> > Le 26/11/2025 à 07:47, Wei Yang a écrit :
>> > > Use standard page table accessors i.e pxdp_get() to get the value of
>> > > pxdp.
>> >
>> > Please provide more detail of why you want to do that and how you are sure it
>> > doesn't break existing implementation.
>> >
>> > There was similar tentative in the past already which proved to give
>> > suboptimal results, see discussion here:
>> >
>> > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2Ff40ea8bf-0862-41a7-af19-70bfbd838568%40csgroup.eu%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C63a7608567de4df0a16308de2cd31076%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638997482180349327%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=ly4LRRjkKqQNBY6ZpkbHS7%2B0xuu2c1r0op1eysPtbSQ%3D&reserved=0
>>
>> Interesting.
>>
>> The reason is P4D/PUD/PMD on PPC32 actually just has PGD/PMD. So it is not
>> necessary to retrieve P4D and PUD.
>
>More exactly, PPC32 just has PGD and PTE, no PMD either:
>
>arch/powerpc/include/asm/book3s/32/pgtable.h:#include
><asm-generic/pgtable-nopmd.h>
>arch/powerpc/include/asm/nohash/32/pgtable.h:#include
><asm-generic/pgtable-nopmd.h>
>
>
>>
>> If this is the case, sounds other platform with less pagetable level would be
>> affected too.
>
>Indeed.
>
>>
>> And this looks only affect when we walk the pagetable from pgd. If we just
>> iterating pmd/pte, we won't have this issue, right?
>
>Maybe. I have not looked at your changes in details, it just lit up a memory
>in my mind.
>

Thanks for noticing me this, I learned a lesson from you :-)

>Christophe

--
Wei Yang
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