Messages in this thread | | | Date | Wed, 29 Aug 2018 15:26:47 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH 2/2] x86/mm/KASLR: Adjust the vmemmap size according to paging mode |
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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
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