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mm/ksm: refactor out try_to_merge_with_zero_page()

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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@linux.dev
Link: https://lkml.kernel.org/r/20240621-b4-ksm-scan-optimize-v2-1-1c328aa9e30b@linux.dev
Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Chengming Zhou 2024-06-21 15:54:29 +08:00 committed by Andrew Morton
parent 003af997c8
commit ac90c56bbd
2 changed files with 40 additions and 31 deletions
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1
mm/hugetlb.c
View File

@ -2666,7 +2666,6 @@ static int gather_surplus_pages(struct hstate *h, long delta)
retry:
spin_unlock_irq(&hugetlb_lock);
for (i = 0; i < needed; i++) {
folio = NULL;
for_each_node_mask(node, cpuset_current_mems_allowed) {
if (!mbind_nodemask || node_isset(node, *mbind_nodemask)) {
folio = alloc_surplus_hugetlb_folio(h, htlb_alloc_mask(h),

70
mm/ksm.c
View File

@ -1527,6 +1527,44 @@ out:
return err;
}
/*
* 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 ksm_rmap_item *rmap_item,
*/
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 page *page, struct ksm_rmap_item *rmap_ite
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) {