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

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Date:   Mon, 23 Oct 2017 14:22:30 +0200
From:   "C.Wehrmeyer" <c.wehrmeyer@....de>
To:     Michal Hocko <mhocko@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: PROBLEM: Remapping hugepages mappings causes kernel to return
 EINVAL

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.