linux-kernel - Re: [PATCH 09/13] mm/powerpc: Redefine pXd_huge() with pXd

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Date: Thu, 14 Mar 2024 08:53:57 -0400
From: Peter Xu <peterx@...hat.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	Matthew Wilcox <willy@...radead.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"x86@...nel.org" <x86@...nel.org>, Mike Rapoport <rppt@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
Subject: Re: [PATCH 09/13] mm/powerpc: Redefine pXd_huge() with pXd_leaf()

On Thu, Mar 14, 2024 at 08:45:34AM +0000, Christophe Leroy wrote:
> 
> 
> Le 13/03/2024 à 22:47, peterx@...hat.com a écrit :
> > From: Peter Xu <peterx@...hat.com>
> > 
> > PowerPC book3s 4K mostly has the same definition on both, except pXd_huge()
> > constantly returns 0 for hash MMUs.  As Michael Ellerman pointed out [1],
> > it is safe to check _PAGE_PTE on hash MMUs, as the bit will never be set so
> > it will keep returning false.
> > 
> > As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to create
> > such huge mappings for 4K hash MMUs.  Meanwhile, the major powerpc hugetlb
> > pgtable walker __find_linux_pte() already used pXd_leaf() to check hugetlb
> > mappings.
> > 
> > The goal should be that we will have one API pXd_leaf() to detect all kinds
> > of huge mappings.  AFAICT we need to use the pXd_leaf() impl (rather than
> > pXd_huge() ones) to make sure ie. THPs on hash MMU will also return true.
> 
> All kinds of huge mappings ?
> 
> pXd_leaf() will detect only leaf mappings (like pXd_huge() ). There are 
> also huge mappings through hugepd. On powerpc 8xx we have 8M huge pages 
> and 512k huge pages. A PGD entry covers 4M so pgd_leaf() won't report 
> those huge pages.

Ah yes, I should always mention this is in the context of leaf huge pages
only.  Are the examples you provided all fall into hugepd category?  If so
I can reword the commit message, as:

        As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to
        create such huge mappings for 4K hash MMUs.  Meanwhile, the major
        powerpc hugetlb pgtable walker __find_linux_pte() already used
        pXd_leaf() to check leaf hugetlb mappings.

        The goal should be that we will have one API pXd_leaf() to detect
        all kinds of huge mappings except hugepd.  AFAICT we need to use
        the pXd_leaf() impl (rather than pXd_huge() ones) to make sure
        ie. THPs on hash MMU will also return true.

Does this look good to you?

Thanks,

-- 
Peter Xu