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

[Kirill A. Shutemov]

On Wed, Aug 29, 2018 at 08:18:56PM +0800, Baoquan He wrote:
> 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? 

Right, actual system RAM plus padding for memory hotplug.

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

There's a tricky part that we ignore now. struct page might be larger than
64 bytes depending on debug options enabled. We may include the actual
size of struct page in calculation.

-- 
 Kirill A. Shutemov