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Date: Tue, 2 Apr 2024 19:53:20 -0300
From: Jason Gunthorpe <jgg@nvidia.com>
To: Peter Xu <peterx@redhat.com>
Cc: Nathan Chancellor <nathan@kernel.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Yang Shi <shy828301@gmail.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Andrew Jones <andrew.jones@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
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	Axel Rasmussen <axelrasmussen@google.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>, loongarch@lists.linux.dev
Subject: Re: [PATCH v4 05/13] mm/arch: Provide pud_pfn() fallback
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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.

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

Jason