linux-kernel - Re: [PATCH V5 2/3] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

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Message-ID: <20190530104203.GC56046@lakrids.cambridge.arm.com>
Date:   Thu, 30 May 2019 11:42:03 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
        catalin.marinas@....com, will.deacon@....com, mhocko@...e.com,
        ira.weiny@...el.com, david@...hat.com, cai@....pw,
        logang@...tatee.com, james.morse@....com, cpandya@...eaurora.org,
        arunks@...eaurora.org, dan.j.williams@...el.com,
        mgorman@...hsingularity.net, osalvador@...e.de,
        ard.biesheuvel@....com
Subject: Re: [PATCH V5 2/3] arm64/mm: Hold memory hotplug lock while walking
 for kernel page table dump

On Wed, May 29, 2019 at 02:46:26PM +0530, 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>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  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();

We need to explicitly include <linux/memory_hotplug.h> to get the
prototypes of {get,put}_online_mems().

With that fixed up:

Acked-by: Mark Rutland <mark.rutland@....com>

I understand from [1] that Michal is ok with using these here, and we're
open to reworking this if/when the hotplug locking is changed.

Mark.

[1] https://lkml.kernel.org/r/20190527072001.GB1658@dhcp22.suse.cz

>  	return 0;
>  }
>  DEFINE_SHOW_ATTRIBUTE(ptdump);
> -- 
> 2.7.4
>