[PATCH 7/7] khugepaged: Introduce 'max_ptes_shared' tunable

Kirill A. Shutemov
2020-03-27T17:06:27Z

``max_ptes_shared`` speicies how many pages can be shared across multiple
processes. Exeeding the number woul block the collapse::

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared

A higher value may increase memory footprint for some workloads.

By default, at least half of pages has to be not shared.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
Documentation/admin-guide/mm/transhuge.rst | 7 ++
mm/khugepaged.c | 52 ++++++++++++--
tools/testing/selftests/vm/khugepaged.c | 83 ++++++++++++++++++++++
3 files changed, 138 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index bd5714547cee..d16e4f2bb70f 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -220,6 +220,13 @@ memory. A lower value can prevent THPs from being
collapsed, resulting fewer pages being collapsed into
THPs, and lower memory access performance.

+``max_ptes_shared`` speicies how many pages can be shared across multiple
+processes. Exeeding the number woul block the collapse::
+
+ /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
+
+A higher value may increase memory footprint for some workloads.
+
Boot parameter
==============

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c8c2c463095c..8e728a602491 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -28,6 +28,8 @@ enum scan_result {
SCAN_SUCCEED,
SCAN_PMD_NULL,
SCAN_EXCEED_NONE_PTE,
+ SCAN_EXCEED_SWAP_PTE,
+ SCAN_EXCEED_SHARED_PTE,
SCAN_PTE_NON_PRESENT,
SCAN_PAGE_RO,
SCAN_LACK_REFERENCED_PAGE,
@@ -46,7 +48,6 @@ enum scan_result {
SCAN_DEL_PAGE_LRU,
SCAN_ALLOC_HUGE_PAGE_FAIL,
SCAN_CGROUP_CHARGE_FAIL,
- SCAN_EXCEED_SWAP_PTE,
SCAN_TRUNCATED,
SCAN_PAGE_HAS_PRIVATE,
};
@@ -71,6 +72,7 @@ static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
*/
static unsigned int khugepaged_max_ptes_none __read_mostly;
static unsigned int khugepaged_max_ptes_swap __read_mostly;
+static unsigned int khugepaged_max_ptes_shared __read_mostly;

#define MM_SLOTS_HASH_BITS 10
static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
@@ -290,15 +292,43 @@ static struct kobj_attribute khugepaged_max_ptes_swap_attr =
__ATTR(max_ptes_swap, 0644, khugepaged_max_ptes_swap_show,
khugepaged_max_ptes_swap_store);

+static ssize_t khugepaged_max_ptes_shared_show(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ char *buf)
+{
+ return sprintf(buf, "%u\n", khugepaged_max_ptes_shared);
+}
+
+static ssize_t khugepaged_max_ptes_shared_store(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int err;
+ unsigned long max_ptes_shared;
+
+ err = kstrtoul(buf, 10, &max_ptes_shared);
+ if (err || max_ptes_shared > HPAGE_PMD_NR-1)
+ return -EINVAL;
+
+ khugepaged_max_ptes_shared = max_ptes_shared;
+
+ return count;
+}
+
+static struct kobj_attribute khugepaged_max_ptes_shared_attr =
+ __ATTR(max_ptes_shared, 0644, khugepaged_max_ptes_shared_show,
+ khugepaged_max_ptes_shared_store);
+
static struct attribute *khugepaged_attr[] = {
&khugepaged_defrag_attr.attr,
&khugepaged_max_ptes_none_attr.attr,
+ &khugepaged_max_ptes_swap_attr.attr,
+ &khugepaged_max_ptes_shared_attr.attr,
&pages_to_scan_attr.attr,
&pages_collapsed_attr.attr,
&full_scans_attr.attr,
&scan_sleep_millisecs_attr.attr,
&alloc_sleep_millisecs_attr.attr,
- &khugepaged_max_ptes_swap_attr.attr,
NULL,
};

@@ -360,6 +390,7 @@ int __init khugepaged_init(void)
khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;
+ khugepaged_max_ptes_shared = HPAGE_PMD_NR / 2;

return 0;
}
@@ -546,7 +577,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
{
struct page *page = NULL;
pte_t *_pte;
- int none_or_zero = 0, result = 0, referenced = 0;
+ int none_or_zero = 0, shared = 0, result = 0, referenced = 0;
bool writable = false;
LIST_HEAD(compound_pagelist);

@@ -575,6 +606,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,

VM_BUG_ON_PAGE(!PageAnon(page), page);

+ if (page_mapcount(page) > 1 &&
+ ++shared > khugepaged_max_ptes_shared) {
+ result = SCAN_EXCEED_SHARED_PTE;
+ goto out;
+ }
+
if (PageCompound(page)) {
struct page *p;
page = compound_head(page);
@@ -1160,7 +1197,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
{
pmd_t *pmd;
pte_t *pte, *_pte;
- int ret = 0, none_or_zero = 0, result = 0, referenced = 0;
+ int ret = 0, result = 0, referenced = 0;
+ int none_or_zero = 0, shared = 0;
struct page *page = NULL;
unsigned long _address;
spinlock_t *ptl;
@@ -1210,6 +1248,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
goto out_unmap;
}

+ if (page_mapcount(page) > 1 &&
+ ++shared > khugepaged_max_ptes_shared) {
+ result = SCAN_EXCEED_SHARED_PTE;
+ goto out_unmap;
+ }
+
page = compound_head(page);

/*
diff --git a/tools/testing/selftests/vm/khugepaged.c b/tools/testing/selftests/vm/khugepaged.c
index 193bde6a1534..3a98d5b2d6d8 100644
--- a/tools/testing/selftests/vm/khugepaged.c
+++ b/tools/testing/selftests/vm/khugepaged.c
@@ -77,6 +77,7 @@ struct khugepaged_settings {
unsigned int scan_sleep_millisecs;
unsigned int max_ptes_none;
unsigned int max_ptes_swap;
+ unsigned int max_ptes_shared;
unsigned long pages_to_scan;
};

@@ -276,6 +277,7 @@ static void write_settings(struct settings *settings)
khugepaged->scan_sleep_millisecs);
write_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
write_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
+ write_num("khugepaged/max_ptes_shared", khugepaged->max_ptes_shared);
write_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
}

@@ -312,6 +314,7 @@ static void save_settings(void)
read_num("khugepaged/scan_sleep_millisecs"),
.max_ptes_none = read_num("khugepaged/max_ptes_none"),
.max_ptes_swap = read_num("khugepaged/max_ptes_swap"),
+ .max_ptes_shared = read_num("khugepaged/max_ptes_shared"),
.pages_to_scan = read_num("khugepaged/pages_to_scan"),
};
success("OK");
@@ -786,12 +789,90 @@ static void collapse_fork_compound(void)
fail("Fail");
fill_memory(p, 0, page_size);

+ write_num("khugepaged/max_ptes_shared", hpage_pmd_nr - 1);
if (wait_for_scan("Collapse PTE table full of compound pages in child", p))
fail("Timeout");
else if (check_huge(p))
success("OK");
else
fail("Fail");
+ write_num("khugepaged/max_ptes_shared",
+ default_settings.khugepaged.max_ptes_shared);
+
+ validate_memory(p, 0, hpage_pmd_size);
+ munmap(p, hpage_pmd_size);
+ exit(exit_status);
+ }
+
+ wait(&wstatus);
+ exit_status += WEXITSTATUS(wstatus);
+
+ printf("Check if parent still has huge page...");
+ if (check_huge(p))
+ success("OK");
+ else
+ fail("Fail");
+ validate_memory(p, 0, hpage_pmd_size);
+ munmap(p, hpage_pmd_size);
+}
+
+static void collapse_max_ptes_shared()
+{
+ int max_ptes_shared = read_num("khugepaged/max_ptes_shared");
+ int wstatus;
+ void *p;
+
+ p = alloc_mapping();
+
+ printf("Allocate huge page...");
+ madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+ fill_memory(p, 0, hpage_pmd_size);
+ if (check_huge(p))
+ success("OK");
+ else
+ fail("Fail");
+
+ printf("Share huge page over fork()...");
+ if (!fork()) {
+ /* Do not touch settings on child exit */
+ skip_settings_restore = true;
+ exit_status = 0;
+
+ if (check_huge(p))
+ success("OK");
+ else
+ fail("Fail");
+
+ printf("Trigger CoW in %d of %d...",
+ hpage_pmd_nr - max_ptes_shared - 1, hpage_pmd_nr);
+ fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared - 1) * page_size);
+ if (!check_huge(p))
+ success("OK");
+ else
+ fail("Fail");
+
+ if (wait_for_scan("Do not collapse with max_ptes_shared exeeded", p))
+ fail("Timeout");
+ else if (!check_huge(p))
+ success("OK");
+ else
+ fail("Fail");
+
+ printf("Trigger CoW in %d of %d...",
+ hpage_pmd_nr - max_ptes_shared, hpage_pmd_nr);
+ fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared) * page_size);
+ if (!check_huge(p))
+ success("OK");
+ else
+ fail("Fail");
+
+
+ if (wait_for_scan("Collapse with max_ptes_shared PTEs shared", p))
+ fail("Timeout");
+ else if (check_huge(p))
+ success("OK");
+ else
+ fail("Fail");

validate_memory(p, 0, hpage_pmd_size);
munmap(p, hpage_pmd_size);
@@ -820,6 +901,7 @@ int main(void)

default_settings.khugepaged.max_ptes_none = hpage_pmd_nr - 1;
default_settings.khugepaged.max_ptes_swap = hpage_pmd_nr / 8;
+ default_settings.khugepaged.max_ptes_shared = hpage_pmd_nr / 2;
default_settings.khugepaged.pages_to_scan = hpage_pmd_nr * 8;

save_settings();
@@ -836,6 +918,7 @@ int main(void)
collapse_full_of_compound();
collapse_fork();
collapse_fork_compound();
+ collapse_max_ptes_shared();

restore_settings(0);
}
--
2.26.0