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SubjectRe: [PATCH v3] mm: pagewalk: Fix walk for hugepage tables
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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" ?

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