mirror of
https://github.com/torvalds/linux.git
synced 2026-08-01 20:22:08 +02:00
Linux kernel source tree
912aa82595
Patch series "ksm: perform a range-walk to jump over holes in break_ksm",
v4.
When unmerging an address range, unmerge_ksm_pages function walks every
page address in the specified range to locate ksm pages. This becomes
highly inefficient when scanning large virtual memory areas that contain
mostly unmapped regions, causing the process to get blocked for several
minutes.
This patch makes break_ksm, function called by unmerge_ksm_pages for every
page in an address range, perform a range walk, allowing it to skip over
entire unmapped holes in a VMA, avoiding unnecessary lookups.
As pointed out by David Hildenbrand in [1], unmerge_ksm_pages() is called
from:
* ksm_madvise() through madvise(MADV_UNMERGEABLE). There are not a lot
of users of that function.
* __ksm_del_vma() through ksm_del_vmas(). Effectively called when
disabling KSM for a process either through the sysctl or from s390x gmap
code when enabling storage keys for a VM.
Consider the following test program which creates a 32 TiB mapping in the
virtual address space but only populates a single page:
#include <unistd.h>
#include <stdio.h>
#include <sys/mman.h>
/* 32 TiB */
const size_t size = 32ul * 1024 * 1024 * 1024 * 1024;
int main() {
char *area = mmap(NULL, size, PROT_READ | PROT_WRITE,
MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0);
if (area == MAP_FAILED) {
perror("mmap() failed\n");
return -1;
}
/* Populate a single page such that we get an anon_vma. */
*area = 0;
/* Enable KSM. */
madvise(area, size, MADV_MERGEABLE);
madvise(area, size, MADV_UNMERGEABLE);
return 0;
}
Without this patch, this program takes 9 minutes to finish, while with
this patch it finishes in less then 5 seconds.
This patch (of 3):
This reverts commit
|
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| 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 reStructuredText 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.