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

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Date: Mon, 25 Mar 2024 19:05:01 +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 25/03/2024 à 17:19, Jason Gunthorpe a écrit :
> On Mon, Mar 25, 2024 at 03:55:54PM +0100, Christophe Leroy wrote:
>> Unlike many architectures, powerpc 8xx hardware tablewalk requires
>> a two level process for all page sizes, allthough second level only
>> has one entry when pagesize is 8M.
>>
>> To fit with Linux page table topology and without requiring special
>> page directory layout like hugepd, the page entry will be replicated
>> 1024 times in the standard page table. However for large pages it is
>> necessary to set bits in the level-1 (PMD) entry. At the time being,
>> for 512k pages the flag is kept in the PTE and inserted in the PMD
>> entry at TLB miss exception, that is necessary because we can have
>> pages of different sizes in a page table. However the 12 PTE bits are
>> fully used and there is no room for an additional bit for page size.
>>
>> For 8M pages, there will be only one page per PMD entry, it is
>> therefore possible to flag the pagesize in the PMD entry, with the
>> advantage that the information will already be at the right place for
>> the hardware.
>>
>> To do so, add a new helper called pmd_populate_size() which takes the
>> page size as an additional argument, and modify __pte_alloc() to also
>> take that argument. pte_alloc() is left unmodified in order to
>> reduce churn on callers, and a pte_alloc_size() is added for use by
>> pte_alloc_huge().
>>
>> When an architecture doesn't provide pmd_populate_size(),
>> pmd_populate() is used as a fallback.
> 
> I think it would be a good idea to document what the semantic is
> supposed to be for sz?
> 
> Just a general remark, probably nothing for this, but with these new
> arguments the historical naming seems pretty tortured for
> pte_alloc_size().. Something like pmd_populate_leaf(size) as a naming
> scheme would make this more intuitive. Ie pmd_populate_leaf() gives
> you a PMD entry where the entry points to a leaf page table able to
> store folios of at least size.
> 
> Anyhow, I thought the edits to the mm helpers were fine, certainly
> much nicer than hugepd. Do you see a path to remove hugepd entirely
> from here?

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.

Christophe