linux-kernel - Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

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Date:   Tue, 14 May 2019 16:40:00 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        akpm@...ux-foundation.org, catalin.marinas@....com,
        will.deacon@....com, mhocko@...e.com, mgorman@...hsingularity.net,
        james.morse@....com, robin.murphy@....com, cpandya@...eaurora.org,
        arunks@...eaurora.org, dan.j.williams@...el.com, osalvador@...e.de,
        david@...hat.com, cai@....pw, logang@...tatee.com,
        ira.weiny@...el.com
Subject: Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking
 for kernel page table dump

On Tue, May 14, 2019 at 02:30:05PM +0530, Anshuman Khandual wrote:
> The arm64 pagetable 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 addressses,
> leading to a number of potential problems.
> 
> Intermediate levels of table may by freed during memory hot-remove, or when
> installing a huge mapping in the vmalloc region. To avoid racing with these
> cases, take the memory hotplug lock when walking the kernel page table.
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>

Can we please move this after the next patch (which addresses the huge
vmap case), and change the last paragraph to:

  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.

With that, this looks good to me.

Thanks,
Mark.

> ---
>  arch/arm64/mm/ptdump_debugfs.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/mm/ptdump_debugfs.c b/arch/arm64/mm/ptdump_debugfs.c
> index 064163f..80171d1 100644
> --- a/arch/arm64/mm/ptdump_debugfs.c
> +++ b/arch/arm64/mm/ptdump_debugfs.c
> @@ -7,7 +7,10 @@
>  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();
>  	return 0;
>  }
>  DEFINE_SHOW_ATTRIBUTE(ptdump);
> -- 
> 2.7.4
>