summaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
-rw-r--r--Documentation/vm/ksm.txt14
-rw-r--r--mm/ksm.c68
2 files changed, 79 insertions, 3 deletions
diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
index f34a8ee6f860..0c64a81d6808 100644
--- a/Documentation/vm/ksm.txt
+++ b/Documentation/vm/ksm.txt
@@ -80,6 +80,20 @@ run - set 0 to stop ksmd from running but keep merged pages,
Default: 0 (must be changed to 1 to activate KSM,
except if CONFIG_SYSFS is disabled)
+use_zero_pages - specifies whether empty pages (i.e. allocated pages
+ that only contain zeroes) should be treated specially.
+ When set to 1, empty pages are merged with the kernel
+ zero page(s) instead of with each other as it would
+ happen normally. This can improve the performance on
+ architectures with coloured zero pages, depending on
+ the workload. Care should be taken when enabling this
+ setting, as it can potentially degrade the performance
+ of KSM for some workloads, for example if the checksums
+ of pages candidate for merging match the checksum of
+ an empty page. This setting can be changed at any time,
+ it is only effective for pages merged after the change.
+ Default: 0 (normal KSM behaviour as in earlier releases)
+
The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
pages_shared - how many shared pages are being used
diff --git a/mm/ksm.c b/mm/ksm.c
index 9ae6011a41f8..28bc5ab2b979 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -223,6 +223,12 @@ static unsigned int ksm_thread_pages_to_scan = 100;
/* Milliseconds ksmd should sleep between batches */
static unsigned int ksm_thread_sleep_millisecs = 20;
+/* Checksum of an empty (zeroed) page */
+static unsigned int zero_checksum __read_mostly;
+
+/* Whether to merge empty (zeroed) pages with actual zero pages */
+static bool ksm_use_zero_pages __read_mostly;
+
#ifdef CONFIG_NUMA
/* Zeroed when merging across nodes is not allowed */
static unsigned int ksm_merge_across_nodes = 1;
@@ -926,6 +932,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
struct mm_struct *mm = vma->vm_mm;
pmd_t *pmd;
pte_t *ptep;
+ pte_t newpte;
spinlock_t *ptl;
unsigned long addr;
int err = -EFAULT;
@@ -950,12 +957,22 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
goto out_mn;
}
- get_page(kpage);
- page_add_anon_rmap(kpage, vma, addr, false);
+ /*
+ * No need to check ksm_use_zero_pages here: we can only have a
+ * zero_page here if ksm_use_zero_pages was enabled alreaady.
+ */
+ if (!is_zero_pfn(page_to_pfn(kpage))) {
+ get_page(kpage);
+ page_add_anon_rmap(kpage, vma, addr, false);
+ newpte = mk_pte(kpage, vma->vm_page_prot);
+ } else {
+ newpte = pte_mkspecial(pfn_pte(page_to_pfn(kpage),
+ vma->vm_page_prot));
+ }
flush_cache_page(vma, addr, pte_pfn(*ptep));
ptep_clear_flush_notify(vma, addr, ptep);
- set_pte_at_notify(mm, addr, ptep, mk_pte(kpage, vma->vm_page_prot));
+ set_pte_at_notify(mm, addr, ptep, newpte);
page_remove_rmap(page, false);
if (!page_mapped(page))
@@ -1467,6 +1484,23 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
return;
}
+ /*
+ * Same checksum as an empty page. We attempt to merge it with the
+ * appropriate zero page if the user enabled this via sysfs.
+ */
+ if (ksm_use_zero_pages && (checksum == zero_checksum)) {
+ struct vm_area_struct *vma;
+
+ vma = find_mergeable_vma(rmap_item->mm, rmap_item->address);
+ err = try_to_merge_one_page(vma, page,
+ ZERO_PAGE(rmap_item->address));
+ /*
+ * In case of failure, the page was not really empty, so we
+ * need to continue. Otherwise we're done.
+ */
+ if (!err)
+ return;
+ }
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {
@@ -2233,6 +2267,28 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
KSM_ATTR(merge_across_nodes);
#endif
+static ssize_t use_zero_pages_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ return sprintf(buf, "%u\n", ksm_use_zero_pages);
+}
+static ssize_t use_zero_pages_store(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int err;
+ bool value;
+
+ err = kstrtobool(buf, &value);
+ if (err)
+ return -EINVAL;
+
+ ksm_use_zero_pages = value;
+
+ return count;
+}
+KSM_ATTR(use_zero_pages);
+
static ssize_t pages_shared_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf)
{
@@ -2290,6 +2346,7 @@ static struct attribute *ksm_attrs[] = {
#ifdef CONFIG_NUMA
&merge_across_nodes_attr.attr,
#endif
+ &use_zero_pages_attr.attr,
NULL,
};
@@ -2304,6 +2361,11 @@ static int __init ksm_init(void)
struct task_struct *ksm_thread;
int err;
+ /* The correct value depends on page size and endianness */
+ zero_checksum = calc_checksum(ZERO_PAGE(0));
+ /* Default to false for backwards compatibility */
+ ksm_use_zero_pages = false;
+
err = ksm_slab_init();
if (err)
goto out;