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

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Date:   Wed, 29 Aug 2018 20:18:56 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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 08/29/18 at 03:05pm, Kirill A. Shutemov wrote:
> 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.

Oh, do you mean to make a calculation according to the actual size of
system RAM? And still taking 1TB as default, then adapt later by RAM? 

Then, no need to introduce VMEMMAP_SIZE_TB, this also looks good to me.

Thanks
Baoquan