Re: PROBLEM: Remapping hugepages mappings causes kernel to return EINVAL

C.Wehrmeyer
2017-10-23T12:23:17Z

On 2017-10-23 13:42, Michal Hocko wrote:
> I do not remember any such a request either. I can see some merit in the
> described use case. It is not specific on why hugetlb pages are used for
> the allocator memory because that comes with it own issues.

That is yet for the user to specify. As of now hugepages still require a
special setup that not all people might have as of now - to my knowledge
a kernel being compiled with CONFIG_TRANSPARENT_HUGEPAGE=y and a number
of such pages being allocated either through the kernel boot line or
through /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages. I'm
deliberately ignoring 1-GiB pages here because those are only
allocatable during boot, when no processes have been spawned and memory
is still not fragmented.

My point is that I can see people not being too eager to support 1 GiB
pages as of now unless for very specific use case. 2-MiB pages, on the
other hand, shouldn't have those limitations anymore. User-space
programs should be capable of allocating such pages without the need for
the user to fiddle with nr_hugepages beforehand.

Some time ago I've written some code to detect TLB capabilities on my
current testing CPU, those are the results:

[TLB] Instruction TLB: 2M/4M pages, fully associative, 8 entries
[TLB] Data TLB: 4 KByte pages, 4-way set associative, 64 entries
[TLB] Data TLB: 2 MByte or 4 MByte pages, 4-way set associative, 32
entries and a separate array with 1 GByte pages, 4-way set associative,
4 entries
[TLB] Instruction TLB: 4KByte pages, 8-way set associative, 64 entries
[STLB] Shared 2nd-Level TLB: 4 KByte/2MByte pages, 8-way associative,
1024 entries

With the knowledge that allocations in the Mebibyte range aren't
uncommon at all nowadays and that one 2-MiB page eliminates the need for
512 4-KiB pages, we really should make advances towards treating 2-MiB
pages just as casual as older pages. Allocators can still query if the
kernel supports the specified page size, and specifying MAP_HUGETLB |
MAP_HUGE_2MB would still be required in order to not break older
programs, but from my perspective there is a lot to gain here.