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

[Baoquan He <bhe@redhat.com>]

On 08/29/18 at 03:26pm, Kirill A. Shutemov wrote:
> 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.

OK, this may need be applied on top of Masa's padding adjusting patches.

x86/mm: Add an option to change the padding used for the physical memory mapping
lkml.kernel.org/r/20180821212305.20214-1-msys.mizuma@gmail.com

> 
> > 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.

OK, I will consider this.

Thanks
Baoquan