linux-kernel - Re: [PATCH v3] mm: pagewalk: Fix walk for hugepage tables

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Date:   Mon, 28 Jun 2021 07:56:20 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Steven Price <steven.price@....com>, linux-mm@...ck.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, dja@...ens.net,
        Oliver O'Halloran <oohall@...il.com>,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: pagewalk: Fix walk for hugepage tables

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@...roup.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" ?