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

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Date: Wed, 3 Apr 2024 12:26:43 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>
CC: Nathan Chancellor <nathan@...nel.org>, "linux-mm@...ck.org"
	<linux-mm@...ck.org>, "linux-kernel@...r.kernel.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" <linux-riscv@...ts.infradead.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, 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"
	<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"
	<loongarch@...ts.linux.dev>
Subject: Re: [PATCH v4 05/13] mm/arch: Provide pud_pfn() fallback



Le 03/04/2024 à 14:08, Jason Gunthorpe a écrit :
> On Tue, Apr 02, 2024 at 07:35:45PM -0400, Peter Xu wrote:
>> 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.
> 
> Wow, this is definately an arch issue. You can't define pud_leaf() and
> not have a pud_pfn(). It makes no sense at all..
> 
> I'd say the BUILD_BUG has done it's job and found an issue, fix it by
> not defining pud_leaf? I don't see any calls to pud_leaf in loongarch
> at least

As far as I can see it was added by commit 303be4b33562 ("LoongArch: mm: 
Add p?d_leaf() definitions").

Not sure it was added for a good reason, and I'm not sure what was added 
is correct because arch/loongarch/include/asm/pgtable-bits.h has:

#define	_PAGE_HUGE_SHIFT	6  /* HUGE is a PMD bit */

So I'm not sure it is correct to use that bit for PUD, is it ?

Probably pud_leaf() should always return false.

Christophe