linux-kernel - Re: [PATCH v4 05/13] mm/arch: Provide pud

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Date: Tue, 2 Apr 2024 19:35:45 -0400
From: Peter Xu <peterx@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Nathan Chancellor <nathan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Yang Shi <shy828301@...il.com>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	John Hubbard <jhubbard@...dia.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Andrew Jones <andrew.jones@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Matthew Wilcox <willy@...radead.org>,
	Rik van Riel <riel@...riel.com>,
	linux-arm-kernel@...ts.infradead.org,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Hildenbrand <david@...hat.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	James Houghton <jthoughton@...gle.com>,
	Mike Rapoport <rppt@...nel.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Huacai Chen <chenhuacai@...nel.org>,
	WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev
Subject: Re: [PATCH v4 05/13] mm/arch: Provide pud_pfn() fallback

On Tue, Apr 02, 2024 at 07:53:20PM -0300, Jason Gunthorpe wrote:
> On Tue, Apr 02, 2024 at 06:43:56PM -0400, Peter Xu wrote:
> 
> > I actually tested this without hitting the issue (even though I didn't
> > mention it in the cover letter..).  I re-kicked the build test, it turns
> > out my "make alldefconfig" on loongarch will generate a config with both
> > HUGETLB=n && THP=n, while arch/loongarch/configs/loongson3_defconfig has
> > THP=y (which I assume was the one above build used).  I didn't further
> > check how "make alldefconfig" generated the config; a bit surprising that
> > it didn't fetch from there.
> 
> I suspect it is weird compiler variations.. Maybe something is not
> being inlined.
> 
> > (and it also surprises me that this BUILD_BUG can trigger.. I used to try
> >  triggering it elsewhere but failed..)
> 
> As the pud_leaf() == FALSE should result in the BUILD_BUG never being
> called and the optimizer removing it.

Good point, for some reason loongarch defined pud_leaf() without defining
pud_pfn(), which does look strange.

#define pud_leaf(pud)		((pud_val(pud) & _PAGE_HUGE) != 0)

But I noticed at least MIPS also does it..  Logically I think one arch
should define either none of both.

> 
> Perhaps the issue is that the pud_leaf() is too far from the pud_pfn?

My understanding is follow_pud_mask() should completely get optimized and
follow_huge_pud() will be dropped in the compiler output if pud_leaf()==false.

Thanks,

-- 
Peter Xu