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Gerald Schaefer 00a34d5a99 s390: select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP 
Enable HUGETLB_PAGE_OPTIMIZE_VMEMMAP for s390.

With this, vmemmap pages used to back struct pages for compound tail
pages of hugetlb pages are freed and remapped to compound head page
frame as RO, see also Documentation/vm/vmemmap_dedup.rst.

For 1M hugetlb pages, this results in freeing 3 of 4 vmemmap pages,
saving 12K of memory for each 1M hugetlb page (~1.2%).
/sys/kernel/debug/kernel_page_tables will show the impact:

---[ vmemmap Area Start ]---
[...]
0x0000037202d84000-0x0000037202d85000         4K PTE RW NX
0x0000037202d85000-0x0000037202d88000        12K PTE RO NX

For 2G hugetlb pages, this results in freeing 8191 of 8192 vmemmap
pages, saving 32764K of memory for each 2G hugetlb page (~1.6%)
/sys/kernel/debug/kernel_page_tables will show the impact:

---[ vmemmap Area Start ]---
[...]
0x000003720a000000-0x000003720a001000         4K PTE RW NX
0x000003720a001000-0x000003720c000000     32764K PTE RO NX

The memory savings come with some costs:
- vmemmap mapping for compound hugetlb pages is not a PMD mapping any
  more, but split to 4K PTE mappings, and it will not be coalesced back
  to PMD mapping after freeing hugetlb pages from the pool.
  Apart from theoretical performance impact, this will also (slightly)
  relativize the memory savings because of additional 2K PTE pagetable
  allocations.
- Workload using "on the fly" hugetlb allocations via
  "nr_overcommit_hugepages" instead of using the hugetlb pool via
  "nr_hugepages" will suffer from considerably increased fault handling
  time, see also description from commit 78f39084b4
  ("mm: hugetlb_vmemmap: add hugetlb_optimize_vmemmap sysctl").
- Freeing hugetlb pages from the pool will require re-allocation of the
  freed struct pages, and therefore needs some memory available to the
  kernel. This might fail in memory constrained scenarios.
- For the same reason, memory offline might fail even for ZONE_MOVABLE
  when hugetlb pages are present (but not for s390, since we do not
  support ARCH_ENABLE_HUGEPAGE_MIGRATION, and therefore cannot have
  hugetlb pages in ZONE_MOVABLE).
- General increased complexity and overhead in kernel handling of
  compound (head) pages.

Therefore, this feature is disabled by default, and has to be enabled
explicitly either by adding "hugetlb_free_vmemmap=on" kernel parameter,
or during run-time via "/proc/sys/vm/hugetlb_optimize_vmemmap" sysctl.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
2022-11-10 08:00:41 +01:00
arch s390: select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP 2022-11-10 08:00:41 +01:00
block block-6.1-2022-10-20 2022-10-21 15:14:14 -07:00
certs certs: make system keyring depend on built-in x509 parser 2022-09-24 04:31:18 +09:00
crypto treewide: use get_random_bytes() when possible 2022-10-11 17:42:58 -06:00
Documentation s390/con3215: Drop console data printout when buffer full 2022-10-26 14:46:51 +02:00
drivers s390/con3215: Fix white space errors 2022-10-26 14:46:51 +02:00
fs First batch of EFI fixes for v6.1 2022-10-21 18:02:36 -07:00
include RISC-V: 2022-10-23 15:00:43 -07:00
init init: Kconfig: fix spelling mistake "satify" -> "satisfy" 2022-10-20 21:27:22 -07:00
io_uring io_uring/net: fail zc sendmsg when unsupported by socket 2022-10-22 08:43:03 -06:00
ipc - hfs and hfsplus kmap API modernization from Fabio Francesco 2022-10-12 11:00:22 -07:00
kernel kernel/utsname_sysctl.c: Fix hostname polling 2022-10-23 12:01:01 -07:00
lib Random number generator fixes for Linux 6.1-rc1. 2022-10-16 15:27:07 -07:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/huge_memory: do not clobber swp_entry_t during THP split 2022-10-20 21:27:24 -07:00
net io_uring-6.1-2022-10-22 2022-10-23 09:55:50 -07:00
rust Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
samples VFIO updates for v6.1-rc1 2022-10-12 14:46:48 -07:00
scripts Kbuild fixes for v6.1 2022-10-16 11:12:22 -07:00
security selinux: enable use of both GFP_KERNEL and GFP_ATOMIC in convert_context() 2022-10-19 09:55:53 -04:00
sound sound fixes for 6.1-rc1 2022-10-14 13:22:14 -07:00
tools RISC-V: 2022-10-23 15:00:43 -07:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt kvm: Add support for arch compat vm ioctls 2022-10-22 05:15:23 -04:00
.clang-format PCI/DOE: Add DOE mailbox support functions 2022-07-19 15:38:04 -07:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
.mailmap mailmap: update email for Qais Yousef 2022-10-20 21:27:21 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS drm for 5.20/6.0 2022-08-03 19:52:08 -07:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS pci-v6.1-fixes-2 2022-10-22 15:52:36 -07:00
Makefile Linux 6.1-rc2 2022-10-23 15:27:33 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.