linux-kernel - Re: [RFC PATCH 1/8] mm: Provide pagesize to pmd

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Date: Wed, 27 Mar 2024 09:58:35 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.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 1/8] mm: Provide pagesize to pmd_populate()

Le 26/03/2024 à 16:01, Jason Gunthorpe a écrit :
> On Mon, Mar 25, 2024 at 07:05:01PM +0000, Christophe Leroy wrote:
> 
>> Not looked into details yet, but I guess so.
>>
>> By the way there is a wiki dedicated to huge pages on powerpc, you can
>> have a look at it here :
>> https://github.com/linuxppc/wiki/wiki/Huge-pages , maybe you'll find
>> good ideas there to help me.
> 
> There sure are alot of page tables types here
> 
> I'm a bit wondering about terminology, eg on the first diagram "huge
> pte entry" means a PUD entry that is a leaf? Which ones are contiguous
> replications?

Yes, on the first diagram, a huge pte entry covering the same size as 
pud entry means a leaf PUD entry.

Contiguous replications are only on 8xx for the time being and are 
displayed as "consecutive entries".

> 
> Just general remarks on the ones with huge pages:
> 
>   hash 64k and hugepage 16M/16G
>   radix 64k/radix hugepage 2M/1G
>   radix 4k/radix hugepage 2M/1G
>   nohash 32
>    - I think this is just a normal x86 like scheme? PMD/PUD can be a
>      leaf with the same size as a next level table.
> 
>      Do any of these cases need to know the higher level to parse the
>      lower? eg is there a 2M bit in the PUD indicating that the PMD
>      is a table of 2M leafs or does each PMD entry have a bit
>      indicating it is a leaf?

For hash and radix there is a bit that tells it is leaf (_PAGE_PTE)

For nohash32/e500 I think the drawing is not full right, there is a huge 
page directory (hugepd) with a single entry. I think it should be 
possible to change it to a leaf entry, it seems we have bit _PAGE_SW1 
available in the PTE.

> 
>   hash 4k and hugepage 16M/16G
>   nohash 64
>    - How does this work? I guess since 8xx explicitly calls out
>      consecutive this is actually the pgd can point to 512 256M
>      entries or 8 16G entries? Ie the table size at each level is
>      varable? Or is it the same and the table size is still 512 and
>      each 16G entry is replicated 64 times?

For those it is using the huge page directory (hugepd) which can be 
hooked at any level and is a directory of huge pages on its own. There 
is no consecutive entries involved here I think, allthough I'm not 
completely sure.

For hash4k I'm not sure how it works, this was changed by commit 
e2b3d202d1db ("powerpc: Switch 16GB and 16MB explicit hugepages to a 
different page table format")

For the nohash/64, a PGD entry points either to a regular PUD directory 
or to a HUGEPD directory. The size of the HUGEPD directory is encoded in 
the 6 lower bits of the PGD entry.

> 
>      Do the offset accessors already abstract this enough?
> 
>   8xx 4K
>   8xx 16K
>     - As this series does?

This is how it is prior to the series, ie 16k and 512k pages are 
implemented as contiguous PTEs in a standard page table while 8M pages 
are implemented with hugepd and a single entry in it (with two PGD 
entries pointing to the same huge page directory.

Christophe