linux-kernel - Re: [PATCH 2/2] x86/mm/KASLR: Adjust the vmemmap size according to paging mode

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Message-ID: <20180829120530.6dhgsautjjrj4ztt@kshutemo-mobl1>
Date:   Wed, 29 Aug 2018 15:05:30 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Baoquan He <bhe@...hat.com>
Cc:     tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
        thgarnie@...gle.com, kirill.shutemov@...ux.intel.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/mm/KASLR: Adjust the vmemmap size according to
 paging mode

On Wed, Aug 29, 2018 at 10:17:54AM +0800, Baoquan He wrote:
> Vmemmap area has different base and size depending on paging mode.
> Now we just hardcode its size as 1TB in memory KASLR, it's not
> right for 5-level paging mode.
> 
> Adjust it according to paging mode and use it during memory KASLR.
> 

I think 512TiB is wasteful for 5-level paging. We don't need that much.

1TiB limit with 4-level paging is required to fit struct pages for all
64TiB of physical memory, assuming each struct page is 64 bytes.

With 5-level paging the limit on physical memory is not 512-times bigger:
we cap at 52-bit physical address space. So it's just 64 times bigger and
we need only 64TiB in worst case.

I think we can limit it further by taking into account memory_tb. Most of
machines will be fine with 1TiB there and we save few more bits from
KASLR.

-- 
 Kirill A. Shutemov