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Subject: Re: [PATCH v3] mm: pagewalk: Fix walk for hugepage tables
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Steven Price <steven.price@arm.com>, linux-mm@kvack.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, dja@axtens.net,
        Oliver O'Halloran <oohall@gmail.com>,
        linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
References: <38d04410700c8d02f28ba37e020b62c55d6f3d2c.1624597695.git.christophe.leroy@csgroup.eu>
 <20210627181226.983d899cc30c02420e1a6af5@linux-foundation.org>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <ecf71f68-d25e-c0c6-4c2a-b181b836ac43@csgroup.eu>
Date:   Mon, 28 Jun 2021 07:56:20 +0200
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Le 28/06/2021 à 03:12, Andrew Morton a écrit :
> On Fri, 25 Jun 2021 05:10:12 +0000 (UTC) Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
> 
>> Pagewalk ignores hugepd entries and walk down the tables
>> as if it was traditionnal entries, leading to crazy result.
>>
>> Add walk_hugepd_range() and use it to walk hugepage tables.
> 
> More details, please?  I assume "crazy result" is userspace visible?
> For how long has this bug existed?  Is a -stable backport needed?  Has
> a Fixes: commit been identified?  etcetera!
> 

I discovered the problem while porting powerpc to generic page table dump.
The generic page table dump uses walk_page_range_novma() .

Yes, "crazy result" is that when dumping /sys/kernel/debug/kernel_page_tables, you get random 
entries because at the time being the pagewalk code sees huge page directories as standard page tables.

The bug has always existed as far as I can see, but as no other architectures than powerpc use huge 
page directories, it only pops up now when powerpc is trying to use that generic page walking code.

So I don't think it is worth a backport to -stable, and about a Fixes: tag I don't know.

IIUC, hugepd was introduced for the first time in mm by commit cbd34da7dc9a ("mm: move the powerpc 
hugepd code to mm/gup.c")

Before that, hugepd was internal to powerpc.

I guess you are asking about Fixes: tag and backporting because of the patch subject.
Should I reword the page subject to something like "mm: enable the generic page walk code to walk 
huge page directories" ?