The patch titled
Subject: mm/ksm: refactor out try_to_merge_with_zero_page()
has been added to the -mm mm-unstable branch. Its filename is
mm-ksm-refactor-out-try_to_merge_with_zero_page.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-refactor-out-try_to_merge_with_zero_page.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Chengming Zhou <chengming.zhou@xxxxxxxxx>
Subject: mm/ksm: refactor out try_to_merge_with_zero_page()
Date: Fri, 21 Jun 2024 15:54:29 +0800
Patch series "mm/ksm: cmp_and_merge_page() optimizations and cleanup", v2.
This series mainly optimizes cmp_and_merge_page() to have more efficient
separate code flow for ksm page and non-ksm anon page.
- ksm page: don't need to calculate the checksum obviously.
- anon page: don't need to search stable tree if changing fast and try
to merge with zero page before searching ksm page on stable tree.
Please see the patch-2 for details.
Patch-3 is cleanup also a little optimization for the chain()/chain_prune
interfaces, which made the stable_tree_search()/stable_tree_insert() over
complex.
I have done simple testing using "hackbench -g 1 -l 300000" (maybe I need
to use a better workload) on my machine, have seen a little CPU usage
decrease of ksmd and some improvements of cmp_and_merge_page() latency:
We can see the latency of cmp_and_merge_page() when handling non-ksm anon
pages has been improved.
This patch (of 3):
In preparation for later changes, refactor out a new function called
try_to_merge_with_zero_page(), which tries to merge with zero page.
Link: https://lkml.kernel.org/r/20240621-b4-ksm-scan-optimize-v2-0-1c328aa9e30b@xxxxxxxxx
Link: https://lkml.kernel.org/r/20240621-b4-ksm-scan-optimize-v2-1-1c328aa9e30b@xxxxxxxxx
Signed-off-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Stefan Roesch <shr@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/ksm.c | 70 ++++++++++++++++++++++++++++++-----------------------
1 file changed, 40 insertions(+), 30 deletions(-)
--- a/mm/ksm.c~mm-ksm-refactor-out-try_to_merge_with_zero_page
+++ a/mm/ksm.c
@@ -1528,6 +1528,44 @@ out:
}
/*
+ * This function returns 0 if the pages were merged or if they are
+ * no longer merging candidates (e.g., VMA stale), -EFAULT otherwise.
+ */
+static int try_to_merge_with_zero_page(struct ksm_rmap_item *rmap_item,
+ struct page *page)
+{
+ struct mm_struct *mm = rmap_item->mm;
+ int err = -EFAULT;
+
+ /*
+ * 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 && (rmap_item->oldchecksum == zero_checksum)) {
+ struct vm_area_struct *vma;
+
+ mmap_read_lock(mm);
+ vma = find_mergeable_vma(mm, rmap_item->address);
+ if (vma) {
+ err = try_to_merge_one_page(vma, page,
+ ZERO_PAGE(rmap_item->address));
+ trace_ksm_merge_one_page(
+ page_to_pfn(ZERO_PAGE(rmap_item->address)),
+ rmap_item, mm, err);
+ } else {
+ /*
+ * If the vma is out of date, we do not need to
+ * continue.
+ */
+ err = 0;
+ }
+ mmap_read_unlock(mm);
+ }
+
+ return err;
+}
+
+/*
* try_to_merge_with_ksm_page - like try_to_merge_two_pages,
* but no new kernel page is allocated: kpage must already be a ksm page.
*
@@ -2302,7 +2340,6 @@ static void stable_tree_append(struct ks
*/
static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_item)
{
- struct mm_struct *mm = rmap_item->mm;
struct ksm_rmap_item *tree_rmap_item;
struct page *tree_page = NULL;
struct ksm_stable_node *stable_node;
@@ -2371,36 +2408,9 @@ static void cmp_and_merge_page(struct pa
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;
+ if (!try_to_merge_with_zero_page(rmap_item, page))
+ return;
- mmap_read_lock(mm);
- vma = find_mergeable_vma(mm, rmap_item->address);
- if (vma) {
- err = try_to_merge_one_page(vma, page,
- ZERO_PAGE(rmap_item->address));
- trace_ksm_merge_one_page(
- page_to_pfn(ZERO_PAGE(rmap_item->address)),
- rmap_item, mm, err);
- } else {
- /*
- * If the vma is out of date, we do not need to
- * continue.
- */
- err = 0;
- }
- mmap_read_unlock(mm);
- /*
- * 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) {
_
Patches currently in -mm which might be from chengming.zhou@xxxxxxxxx are
mm-zswap-use-only-one-pool-in-zswap.patch
mm-ksm-refactor-out-try_to_merge_with_zero_page.patch
mm-ksm-dont-waste-time-searching-stable-tree-for-fast-changing-page.patch
mm-ksm-optimize-the-chain-chain_prune-interfaces.patch
