mm: introduce idle page tracking

	- a brief summary of hugetlbpage support in the Linux kernel.

hwpoison.txt

	- explains what hwpoison is

idle_page_tracking.txt

	- description of the idle page tracking feature.

ksm.txt

	- how to use the Kernel Samepage Merging feature.

numa

MOTIVATION

The idle page tracking feature allows to track which memory pages are being

accessed by a workload and which are idle. This information can be useful for

estimating the workload's working set size, which, in turn, can be taken into

account when configuring the workload parameters, setting memory cgroup limits,

or deciding where to place the workload within a compute cluster.

It is enabled by CONFIG_IDLE_PAGE_TRACKING=y.

USER API

The idle page tracking API is located at /sys/kernel/mm/page_idle. Currently,

it consists of the only read-write file, /sys/kernel/mm/page_idle/bitmap.

The file implements a bitmap where each bit corresponds to a memory page. The

bitmap is represented by an array of 8-byte integers, and the page at PFN #i is

mapped to bit #i%64 of array element #i/64, byte order is native. When a bit is

set, the corresponding page is idle.

A page is considered idle if it has not been accessed since it was marked idle

(for more details on what "accessed" actually means see the IMPLEMENTATION

DETAILS section). To mark a page idle one has to set the bit corresponding to

the page by writing to the file. A value written to the file is OR-ed with the

current bitmap value.

Only accesses to user memory pages are tracked. These are pages mapped to a

process address space, page cache and buffer pages, swap cache pages. For other

page types (e.g. SLAB pages) an attempt to mark a page idle is silently ignored,

and hence such pages are never reported idle.

For huge pages the idle flag is set only on the head page, so one has to read

/proc/kpageflags in order to correctly count idle huge pages.

Reading from or writing to /sys/kernel/mm/page_idle/bitmap will return

-EINVAL if you are not starting the read/write on an 8-byte boundary, or

if the size of the read/write is not a multiple of 8 bytes. Writing to

this file beyond max PFN will return -ENXIO.

That said, in order to estimate the amount of pages that are not used by a

workload one should:

 1. Mark all the workload's pages as idle by setting corresponding bits in

    /sys/kernel/mm/page_idle/bitmap. The pages can be found by reading

    /proc/pid/pagemap if the workload is represented by a process, or by

    filtering out alien pages using /proc/kpagecgroup in case the workload is

    placed in a memory cgroup.

 2. Wait until the workload accesses its working set.

 3. Read /sys/kernel/mm/page_idle/bitmap and count the number of bits set. If

    one wants to ignore certain types of pages, e.g. mlocked pages since they

    are not reclaimable, he or she can filter them out using /proc/kpageflags.

See Documentation/vm/pagemap.txt for more information about /proc/pid/pagemap,

/proc/kpageflags, and /proc/kpagecgroup.

IMPLEMENTATION DETAILS

The kernel internally keeps track of accesses to user memory pages in order to

reclaim unreferenced pages first on memory shortage conditions. A page is

considered referenced if it has been recently accessed via a process address

space, in which case one or more PTEs it is mapped to will have the Accessed bit

set, or marked accessed explicitly by the kernel (see mark_page_accessed()). The

latter happens when:

 - a userspace process reads or writes a page using a system call (e.g. read(2)

   or write(2))

 - a page that is used for storing filesystem buffers is read or written,

   because a process needs filesystem metadata stored in it (e.g. lists a

   directory tree)

 - a page is accessed by a device driver using get_user_pages()

When a dirty page is written to swap or disk as a result of memory reclaim or

exceeding the dirty memory limit, it is not marked referenced.

The idle memory tracking feature adds a new page flag, the Idle flag. This flag

is set manually, by writing to /sys/kernel/mm/page_idle/bitmap (see the USER API

section), and cleared automatically whenever a page is referenced as defined

above.

When a page is marked idle, the Accessed bit must be cleared in all PTEs it is

mapped to, otherwise we will not be able to detect accesses to the page coming

from a process address space. To avoid interference with the reclaimer, which,

as noted above, uses the Accessed bit to promote actively referenced pages, one

more page flag is introduced, the Young flag. When the PTE Accessed bit is

cleared as a result of setting or updating a page's Idle flag, the Young flag

is set on the page. The reclaimer treats the Young flag as an extra PTE

Accessed bit and therefore will consider such a page as referenced.

Since the idle memory tracking feature is based on the memory reclaimer logic,

it only works with pages that are on an LRU list, other pages are silently

ignored. That means it will ignore a user memory page if it is isolated, but

since there are usually not many of them, it should not affect the overall

result noticeably. In order not to stall scanning of the idle page bitmap,

locked pages may be skipped too.

#include <linux/seq_file.h>

#include <linux/hugetlb.h>

#include <linux/memcontrol.h>

#include <linux/mmu_notifier.h>

#include <linux/page_idle.h>

#include <linux/kernel-page-flags.h>

#include <asm/uaccess.h>

#include "internal.h"

#define KPMSIZE sizeof(u64)

#define KPMMASK (KPMSIZE - 1)

#define KPMBITS (KPMSIZE * BITS_PER_BYTE)

/* /proc/kpagecount - an array exposing page counts

 *

#include <linux/swap.h>

#include <linux/swapops.h>

#include <linux/mmu_notifier.h>

#include <linux/page_idle.h>

#include <asm/elf.h>

#include <asm/uaccess.h>

	mss->resident += size;

	/* Accumulate the size in pages that have been accessed. */

	if (young || PageReferenced(page))

	if (young || page_is_young(page) || PageReferenced(page))

		mss->referenced += size;

	mapcount = page_mapcount(page);

	if (mapcount >= 2) {

		/* Clear accessed and referenced bits. */

		pmdp_test_and_clear_young(vma, addr, pmd);

		test_and_clear_page_young(page);

		ClearPageReferenced(page);

out:

		spin_unlock(ptl);

		/* Clear accessed and referenced bits. */

		ptep_test_and_clear_young(vma, addr, pte);

		test_and_clear_page_young(page);

		ClearPageReferenced(page);

	}

	pte_unmap_unlock(pte - 1, ptl);

#define ptep_clear_flush_young_notify ptep_clear_flush_young

#define pmdp_clear_flush_young_notify pmdp_clear_flush_young

#define ptep_clear_young_notify ptep_test_and_clear_young

#define pmdp_clear_young_notify pmdp_test_and_clear_young

#define	ptep_clear_flush_notify ptep_clear_flush

#define pmdp_huge_clear_flush_notify pmdp_huge_clear_flush

#define pmdp_huge_get_and_clear_notify pmdp_huge_get_and_clear