Re: [PATCH v4 05/13] mm/arch: Provide pud_pfn() fallback

From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.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>,
	linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Rik van Riel <riel@surriel.com>,
	linux-arm-kernel@lists.infradead.org,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	James Houghton <jthoughton@google.com>,
	Mike Rapoport <rppt@kernel.org>,
	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
Date: Wed, 3 Apr 2024 14:25:20 -0400	[thread overview]
Message-ID: <Zg2fEP4eEeLhgDwE@x1n> (raw)
In-Reply-To: <20240403120841.GB1723999@nvidia.com>

On Wed, Apr 03, 2024 at 09:08:41AM -0300, Jason Gunthorpe wrote:
> 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

Yes, that sounds better too to me, however it means we may also risk other
archs that can fail another defconfig build.. and I worry I bring trouble
to multiple such cases.  Fundamentally it's indeed my patch that broke
those builds, so I still sent the change and leave that for arch developers
to decide the best for the archs.

I think if wanted, we can add that BUILD_BUG() back when we're sure no arch
will break with it.  So such changes from arch can still be proposed
alongside of removal of BUILD_BUG() (and I'd guess some other arch will
start to notice such build issue soon if existed.. so it still more or less
has similar effect of a reminder..).

Thanks,

-- 
Peter Xu

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