linux-kernel - Re: [PATCH V7 2/3] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

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Date:   Sun, 15 Sep 2019 12:35:21 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        akpm@...ux-foundation.org, catalin.marinas@....com, will@...nel.org
Cc:     mark.rutland@....com, mhocko@...e.com, ira.weiny@...el.com,
        david@...hat.com, cai@....pw, logang@...tatee.com,
        cpandya@...eaurora.org, arunks@...eaurora.org,
        dan.j.williams@...el.com, mgorman@...hsingularity.net,
        osalvador@...e.de, ard.biesheuvel@....com, steve.capper@....com,
        broonie@...nel.org, valentin.schneider@....com,
        Robin.Murphy@....com, steven.price@....com, suzuki.poulose@....com
Subject: Re: [PATCH V7 2/3] arm64/mm: Hold memory hotplug lock while walking
 for kernel page table dump



On 3/9/19 7:45 pm, Anshuman Khandual wrote:
> The arm64 page table dump code can race with concurrent modification of the
> kernel page tables. When a leaf entries are modified concurrently, the dump
> code may log stale or inconsistent information for a VA range, but this is
> otherwise not harmful.
> 
> When intermediate levels of table are freed, the dump code will continue to
> use memory which has been freed and potentially reallocated for another
> purpose. In such cases, the dump code may dereference bogus addresses,
> leading to a number of potential problems.
> 
> Intermediate levels of table may by freed during memory hot-remove,
> which will be enabled by a subsequent patch. To avoid racing with
> this, take the memory hotplug lock when walking the kernel page table.
> 
> Acked-by: David Hildenbrand <david@...hat.com>
> Acked-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  arch/arm64/mm/ptdump_debugfs.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/mm/ptdump_debugfs.c b/arch/arm64/mm/ptdump_debugfs.c
> index 064163f25592..b5eebc8c4924 100644
> --- a/arch/arm64/mm/ptdump_debugfs.c
> +++ b/arch/arm64/mm/ptdump_debugfs.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/debugfs.h>
> +#include <linux/memory_hotplug.h>
>  #include <linux/seq_file.h>
>  
>  #include <asm/ptdump.h>
> @@ -7,7 +8,10 @@
>  static int ptdump_show(struct seq_file *m, void *v)
>  {
>  	struct ptdump_info *info = m->private;
> +
> +	get_online_mems();
>  	ptdump_walk_pgd(m, info);
> +	put_online_mems();

Looks sane, BTW, checking other arches they might have the same race.
Is there anything special about the arch?

Acked-by: Balbir Singh <bsingharora@...il.com>