linux-kernel - Re: [RFC PATCH v2 01/20] mm: Provide pagesize to pmd

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Date: Tue, 21 May 2024 13:57:20 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Christophe Leroy <christophe.leroy@...roup.eu>
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 v2 01/20] mm: Provide pagesize to pmd_populate()

On Mon, May 20, 2024 at 04:24:51PM +0000, Christophe Leroy wrote:
> I had a quick look at that document and it seems to provide a good 
> summary of MMU features and principles. However there are some 
> theoritical information which is not fully right in practice. For 
> instance when they say "Segment attributes. These fields define 
> attributes common to all pages in this segment.". This is right in 
> theory if you consider it from Linux page table topology point of view, 
> hence what they call a segment is a PMD entry for Linux. However, in 
> practice each page has its own L1 and L2 attributes and there is not 
> requirement at HW level to have all L1 attributes of all pages of a 
> segment the same.

Thanks for taking the time Christophe, highly appreciated.

 
> rlwimi = Rotate Left Word Immediate then Mask Insert. Here it rotates 
> r10 by 23 bits to the left (or 9 to the right) then masks with 
> _PMD_PAGE_512K and inserts it into r11.
> 
> It means _PAGE_HUGE bit is copied into lower bit of PS attribute.
> 
> PS takes the following values:
> 
> PS = 00 ==> Small page (4k or 16k)
> PS = 01 ==> 512k page
> PS = 10 ==> Undefined
> PS = 11 ==> 8M page

I see, thanks for the explanation.

> That's a RFC, all ideas are welcome, I needed something to replace 
> hugepd_populate()

The only user interested in pmd_populate() having a sz parameter
is 8xx because it will toggle _PMD_PAGE_8M in case of a 8MB mapping.

Would it be possible for 8xx to encode the 'sz' in the *pmd pointer
prior to calling down the chain? (something like as we do for PTR_ERR()).
Then pmd_populate_{kernel_}size() from 8xx, would extract it like:

 unsigned long sz = PTR_SIZE(pmd)

Then we would not need all these 'sz' parameters scattered.

Can that work?


PD: Do you know a way to emulate a 8xx VM? qemu seems to not have
support support.

Thanks


-- 
Oscar Salvador
SUSE Labs