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

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Message-ID: <107ce10f-622c-7914-6269-cff5509b084f@arm.com>
Date:   Wed, 15 May 2019 07:26:32 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mark Rutland <mark.rutland@....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 05/14/2019 09:10 PM, Mark Rutland wrote:
> 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.

Sure will do.