linux-kernel - Re: [PATCH v5 02/18] mm: Define __pte_leaf

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Message-ID: <ZmhcepJrkDpJ7mSC@x1n>
Date: Tue, 11 Jun 2024 10:17:30 -0400
From: Peter Xu <peterx@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 02/18] mm: Define __pte_leaf_size() to also take a PMD
 entry

Oscar,

On Tue, Jun 11, 2024 at 11:34:23AM +0200, Oscar Salvador wrote:
> Which means that they would be caught in the following code:
> 
>         ptl = pmd_huge_lock(pmd, vma);
>         if (ptl) {
> 	        - 8MB hugepages will be handled here
>                 smaps_pmd_entry(pmd, addr, walk);
>                 spin_unlock(ptl);
>         }
> 	/* pte stuff */
> 	...

Just one quick comment: I think there's one challenge though as this is
also not a generic "pmd leaf", but a pgtable page underneath.  I think it
means smaps_pmd_entry() won't trivially work here, e.g., it will start to
do this:

	if (pmd_present(*pmd)) {
		page = vm_normal_page_pmd(vma, addr, *pmd);

Here vm_normal_page_pmd() will only work if pmd_leaf() satisfies its
definition as:

 * - It should contain a huge PFN, which points to a huge page larger than
 *   PAGE_SIZE of the platform.  The PFN format isn't important here.

But now it's a pgtable page, containing cont-ptes.  Similarly, I think most
pmd_*() helpers will stop working there if we report it as a leaf.

Thanks,

-- 
Peter Xu