ksm: count all zero pages placed by KSM

As pages_sharing and pages_shared don't include the number of zero pages
merged by KSM, we cannot know how many pages are zero pages placed by KSM
when enabling use_zero_pages, which leads to KSM not being transparent
with all actual merged pages by KSM.  In the early days of use_zero_pages,
zero-pages was unable to get unshared by the ways like MADV_UNMERGEABLE so
it's hard to count how many times one of those zeropages was then
unmerged.

But now, unsharing KSM-placed zero page accurately has been achieved, so
we can easily count both how many times a page full of zeroes was merged
with zero-page and how many times one of those pages was then unmerged.
and so, it helps to estimate memory demands when each and every shared
page could get unshared.

So we add ksm_zero_pages under /sys/kernel/mm/ksm/ to show the number
of all zero pages placed by KSM. Meanwhile, we update the Documentation.

Link: https://lkml.kernel.org/r/20230613030934.185944-1-yang.yang29@zte.com.cn
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Xuexin Jiang <jiang.xuexin@zte.com.cn>
Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index 7626392fe82cb5d02e9ecd57fb6d7ed80fdc1053..6cc919dbfd55b39226e0469d67ea80f37a10da25 100644 (file)
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -173,6 +173,13 @@ stable_node_chains
         the number of KSM pages that hit the ``max_page_sharing`` limit
 stable_node_dups
         number of duplicated KSM pages
+ksm_zero_pages
+        how many zero pages that are still mapped into processes were mapped by
+        KSM when deduplicating.
+
+When ``use_zero_pages`` is/was enabled, the sum of ``pages_sharing`` +
+``ksm_zero_pages`` represents the actual number of pages saved by KSM.
+if ``use_zero_pages`` has never been enabled, ``ksm_zero_pages`` is 0.
 
 A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
 sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 98878107244fde2a89a83c2ed9c3b21e1d0bd764..e80aa49009b262411ad197c5db05b1739da60639 100644 (file)
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -33,6 +33,14 @@ void __ksm_exit(struct mm_struct *mm);
  */
 #define is_ksm_zero_pte(pte)   (is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
 
+extern unsigned long ksm_zero_pages;
+
+static inline void ksm_might_unmap_zero_page(pte_t pte)
+{
+       if (is_ksm_zero_pte(pte))
+               ksm_zero_pages--;
+}
+
 static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {
        int ret;
@@ -101,6 +109,10 @@ static inline void ksm_exit(struct mm_struct *mm)
 {
 }
 
+static inline void ksm_might_unmap_zero_page(pte_t pte)
+{
+}
+
 #ifdef CONFIG_MEMORY_FAILURE
 static inline void collect_procs_ksm(struct page *page,
                                     struct list_head *to_kill, int force_early)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 78c8d5d8b628419c998c2ccb95dbf4ca508852f7..419981dcc889db5c7583fbb2421d0f37d9c5d605 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -19,6 +19,7 @@
 #include <linux/page_table_check.h>
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
+#include <linux/ksm.h>
 
 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -709,6 +710,7 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
                                spin_lock(ptl);
                                ptep_clear(vma->vm_mm, address, _pte);
                                spin_unlock(ptl);
+                               ksm_might_unmap_zero_page(pteval);
                        }
                } else {
                        src_page = pte_page(pteval);

diff --git a/mm/ksm.c b/mm/ksm.c
index 99519e22a7616a634c5bd9c91ae8dd90d4bcbfdc..e037d9aad691a0700a28e23296f9c8d831a97fe5 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -278,6 +278,9 @@ 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;
 
+/* The number of zero pages which is placed by KSM */
+unsigned long ksm_zero_pages;
+
 #ifdef CONFIG_NUMA
 /* Zeroed when merging across nodes is not allowed */
 static unsigned int ksm_merge_across_nodes = 1;
@@ -1229,6 +1232,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
                 * the dirty bit in zero page's PTE is set.
                 */
                newpte = pte_mkdirty(pte_mkspecial(pfn_pte(page_to_pfn(kpage), vma->vm_page_prot)));
+               ksm_zero_pages++;
                /*
                 * We're replacing an anonymous page with a zero page, which is
                 * not anonymous. We need to do proper accounting otherwise we
@@ -3356,6 +3360,13 @@ static ssize_t pages_volatile_show(struct kobject *kobj,
 }
 KSM_ATTR_RO(pages_volatile);
 
+static ssize_t ksm_zero_pages_show(struct kobject *kobj,
+                               struct kobj_attribute *attr, char *buf)
+{
+       return sysfs_emit(buf, "%ld\n", ksm_zero_pages);
+}
+KSM_ATTR_RO(ksm_zero_pages);
+
 static ssize_t general_profit_show(struct kobject *kobj,
                                   struct kobj_attribute *attr, char *buf)
 {
@@ -3423,6 +3434,7 @@ static struct attribute *ksm_attrs[] = {
        &pages_sharing_attr.attr,
        &pages_unshared_attr.attr,
        &pages_volatile_attr.attr,
+       &ksm_zero_pages_attr.attr,
        &full_scans_attr.attr,
 #ifdef CONFIG_NUMA
        &merge_across_nodes_attr.attr,

diff --git a/mm/memory.c b/mm/memory.c
index e9f9944c7370cc0584f30d355bb41cd5b12b607c..c256da05bb5e41ae07597bfeda487c225aad2449 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1433,8 +1433,10 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
                        tlb_remove_tlb_entry(tlb, pte, addr);
                        zap_install_uffd_wp_if_needed(vma, addr, pte, details,
                                                      ptent);
-                       if (unlikely(!page))
+                       if (unlikely(!page)) {
+                               ksm_might_unmap_zero_page(ptent);
                                continue;
+                       }
 
                        delay_rmap = 0;
                        if (!PageAnon(page)) {
@@ -3128,6 +3130,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
                                inc_mm_counter(mm, MM_ANONPAGES);
                        }
                } else {
+                       ksm_might_unmap_zero_page(vmf->orig_pte);
                        inc_mm_counter(mm, MM_ANONPAGES);
                }
                flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));