ksm: add ksm involvement information for each process

In /proc/<pid>/ksm_stat, add two extra ksm involvement items including
KSM_mergeable and KSM_merge_any.  It helps administrators to better know
the system's KSM behavior at process level.

ksm_merge_any: yes/no
	whether the process'mm is added by prctl() into the candidate list
	of KSM or not, and fully enabled at process level.

ksm_mergeable: yes/no
    whether any VMAs of the process'mm are currently applicable to KSM.

Purpose
=======
These two items are just to improve the observability of KSM at process
level, so that users can know if a certain process has enabled KSM.

For example, if without these two items, when we look at
/proc/<pid>/ksm_stat and there's no merging pages found, We are not sure
whether it is because KSM was not enabled or because KSM did not
successfully merge any pages.

Although "mg" in /proc/<pid>/smaps indicate VM_MERGEABLE, it's opaque
and not very obvious for non professionals.

[akpm@linux-foundation.org: wording tweaks, per David and akpm]
Link: https://lkml.kernel.org/r/20250110174034304QOb8eDoqtFkp3_t8mqnqc@zte.com.cn
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Acked-by: David Hildenbrand <david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Cc: Wang Yaxin <wang.yaxin@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index 31a9bc365437..8be2b144fefd 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -3262,6 +3262,25 @@ static void wait_while_offlining(void)
 #endif /* CONFIG_MEMORY_HOTREMOVE */
 
 #ifdef CONFIG_PROC_FS
+/*
+ * The process is mergeable only if any VMA is currently
+ * applicable to KSM.
+ *
+ * The mmap lock must be held in read mode.
+ */
+bool ksm_process_mergeable(struct mm_struct *mm)
+{
+	struct vm_area_struct *vma;
+
+	mmap_assert_locked(mm);
+	VMA_ITERATOR(vmi, mm, 0);
+	for_each_vma(vmi, vma)
+		if (vma->vm_flags & VM_MERGEABLE)
+			return true;
+
+	return false;
+}
+
 long ksm_process_profit(struct mm_struct *mm)
 {
 	return (long)(mm->ksm_merging_pages + mm_ksm_zero_pages(mm)) * PAGE_SIZE -