Commit d0f209f6 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

ksm: remove unswappable max_kernel_pages 



Now that ksm pages are swappable, and the known holes plugged, remove
mention of unswappable kernel pages from KSM documentation and comments.

Remove the totalram_pages/4 initialization of max_kernel_pages.  In fact,
remove max_kernel_pages altogether - we can reinstate it if removal turns
out to break someone's script; but if we later want to limit KSM's memory
usage, limiting the stable nodes would not be an effective approach.

Signed-off-by: default avatarHugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 62b61f61

Documentation/vm/ksm.txt

+7 −15
Original line number Diff line number Diff line
@@ -16,9 +16,9 @@ by sharing the data common between them. But it can be useful to any 
application which generates many instances of the same data.



KSM only merges anonymous (private) pages, never pagecache (file) pages.

KSM's merged pages are at present locked into kernel memory for as long

as they are shared: so cannot be swapped out like the user pages they

replace (but swapping KSM pages should follow soon in a later release).

KSM's merged pages were originally locked into kernel memory, but can now

be swapped out just like other user pages (but sharing is broken when they

are swapped back in: ksmd must rediscover their identity and merge again).



KSM only operates on those areas of address space which an application

has advised to be likely candidates for merging, by using the madvise(2)
@@ -44,20 +44,12 @@ includes unmapped gaps (though working on the intervening mapped areas), 
and might fail with EAGAIN if not enough memory for internal structures.



Applications should be considerate in their use of MADV_MERGEABLE,

restricting its use to areas likely to benefit.  KSM's scans may use

a lot of processing power, and its kernel-resident pages are a limited

resource.  Some installations will disable KSM for these reasons.

restricting its use to areas likely to benefit.  KSM's scans may use a lot

of processing power: some installations will disable KSM for that reason.



The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,

readable by all but writable only by root:



max_kernel_pages - set to maximum number of kernel pages that KSM may use

                   e.g. "echo 100000 > /sys/kernel/mm/ksm/max_kernel_pages"

                   Value 0 imposes no limit on the kernel pages KSM may use;

                   but note that any process using MADV_MERGEABLE can cause

                   KSM to allocate these pages, unswappable until it exits.

                   Default: quarter of memory (chosen to not pin too much)



pages_to_scan    - how many present pages to scan before ksmd goes to sleep

                   e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"

                   Default: 100 (chosen for demonstration purposes)
@@ -75,7 +67,7 @@ run - set 0 to stop ksmd from running but keep merged pages, 


The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:



pages_shared     - how many shared unswappable kernel pages KSM is using

pages_shared     - how many shared pages are being used

pages_sharing    - how many more sites are sharing them i.e. how much saved

pages_unshared   - how many pages unique but repeatedly checked for merging

pages_volatile   - how many pages changing too fast to be placed in a tree
@@ -87,4 +79,4 @@ pages_volatile embraces several different kinds of activity, but a high 
proportion there would also indicate poor use of madvise MADV_MERGEABLE.



Izik Eidus,

Hugh Dickins, 24 Sept 2009
 
Hugh Dickins, 17 Nov 2009

mm/Kconfig

+1 −1
Original line number Diff line number Diff line
@@ -212,7 +212,7 @@ config KSM 
	  Enable Kernel Samepage Merging: KSM periodically scans those areas

	  of an application's address space that an app has advised may be

	  mergeable.  When it finds pages of identical content, it replaces

	  the many instances by a single resident page with that content, so

	  the many instances by a single page with that content, so

	  saving memory until one or another app needs to modify the content.

	  Recommended for use with KVM, or with other duplicative applications.

	  See Documentation/vm/ksm.txt for more information: KSM is inactive

mm/ksm.c

+2 −39
Original line number Diff line number Diff line
@@ -179,9 +179,6 @@ static unsigned long ksm_pages_unshared; 
/* The number of rmap_items in use: to calculate pages_volatile */

static unsigned long ksm_rmap_items;



/* Limit on the number of unswappable pages used */

static unsigned long ksm_max_kernel_pages;



/* Number of pages ksmd should scan in one batch */

static unsigned int ksm_thread_pages_to_scan = 100;
@@ -943,14 +940,6 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item, 
{

	int err;



	/*

	 * The number of nodes in the stable tree

	 * is the number of kernel pages that we hold.

	 */

	if (ksm_max_kernel_pages &&

	    ksm_max_kernel_pages <= ksm_pages_shared)

		return NULL;



	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);

	if (!err) {

		err = try_to_merge_with_ksm_page(tree_rmap_item,
@@ -1850,8 +1839,8 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr, 
	/*

	 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.

	 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,

	 * breaking COW to free the unswappable pages_shared (but leaves

	 * mm_slots on the list for when ksmd may be set running again).

	 * breaking COW to free the pages_shared (but leaves mm_slots

	 * on the list for when ksmd may be set running again).

	 */



	mutex_lock(&ksm_thread_mutex);
@@ -1876,29 +1865,6 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr, 
}

KSM_ATTR(run);



static ssize_t max_kernel_pages_store(struct kobject *kobj,

				      struct kobj_attribute *attr,

				      const char *buf, size_t count)

{

	int err;

	unsigned long nr_pages;



	err = strict_strtoul(buf, 10, &nr_pages);

	if (err)

		return -EINVAL;



	ksm_max_kernel_pages = nr_pages;



	return count;

}



static ssize_t max_kernel_pages_show(struct kobject *kobj,

				     struct kobj_attribute *attr, char *buf)

{

	return sprintf(buf, "%lu\n", ksm_max_kernel_pages);

}

KSM_ATTR(max_kernel_pages);



static ssize_t pages_shared_show(struct kobject *kobj,

				 struct kobj_attribute *attr, char *buf)

{
@@ -1948,7 +1914,6 @@ static struct attribute *ksm_attrs[] = { 
	&sleep_millisecs_attr.attr,

	&pages_to_scan_attr.attr,

	&run_attr.attr,

	&max_kernel_pages_attr.attr,

	&pages_shared_attr.attr,

	&pages_sharing_attr.attr,

	&pages_unshared_attr.attr,
@@ -1968,8 +1933,6 @@ static int __init ksm_init(void) 
	struct task_struct *ksm_thread;

	int err;



	ksm_max_kernel_pages = totalram_pages / 4;



	err = ksm_slab_init();

	if (err)

		goto out;

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