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>

Documentation/filesystems/proc.rst

@@ -48,6 +48,7 @@ fixes/update part 1.1  Stefani Seibold <stefani@seibold.net>    June 9 2009
   3.11	/proc/<pid>/patch_state - Livepatch patch operation state
   3.12	/proc/<pid>/arch_status - Task architecture specific information
   3.13  /proc/<pid>/fd - List of symlinks to open files
+  3.14  /proc/<pid/ksm_stat - Information about the process' ksm status.
 
   4	Configuring procfs
   4.1	Mount options
@@ -2232,6 +2233,73 @@ The number of open files for the process is stored in 'size' member
 of stat() output for /proc/<pid>/fd for fast access.
 -------------------------------------------------------
 
+3.14 /proc/<pid/ksm_stat - Information about the process' ksm status
+--------------------------------------------------------------------
+When CONFIG_KSM is enabled, each process has this file which displays
+the information of ksm merging status.
+
+Example
+~~~~~~~
+
+::
+
+    / # cat /proc/self/ksm_stat
+    ksm_rmap_items 0
+    ksm_zero_pages 0
+    ksm_merging_pages 0
+    ksm_process_profit 0
+    ksm_merge_any: no
+    ksm_mergeable: no
+
+Description
+~~~~~~~~~~~
+
+ksm_rmap_items
+^^^^^^^^^^^^^^
+
+The number of ksm_rmap_item structures in use.  The structure
+ksm_rmap_item stores the reverse mapping information for virtual
+addresses.  KSM will generate a ksm_rmap_item for each ksm-scanned page of
+the process.
+
+ksm_zero_pages
+^^^^^^^^^^^^^^
+
+When /sys/kernel/mm/ksm/use_zero_pages is enabled, it represent how many
+empty pages are merged with kernel zero pages by KSM.
+
+ksm_merging_pages
+^^^^^^^^^^^^^^^^^
+
+It represents how many pages of this process are involved in KSM merging
+(not including ksm_zero_pages). It is the same with what
+/proc/<pid>/ksm_merging_pages shows.
+
+ksm_process_profit
+^^^^^^^^^^^^^^^^^^
+
+The profit that KSM brings (Saved bytes). KSM can save memory by merging
+identical pages, but also can consume additional memory, because it needs
+to generate a number of rmap_items to save each scanned page's brief rmap
+information. Some of these pages may be merged, but some may not be abled
+to be merged after being checked several times, which are unprofitable
+memory consumed.
+
+ksm_merge_any
+^^^^^^^^^^^^^
+
+It specifies whether the process'mm is added by prctl() into the candidate list
+of KSM or not, and if KSM scanning is fully enabled at process level.
+
+ksm_mergeable
+^^^^^^^^^^^^^
+
+It specifies whether any VMAs of the process'mm are currently applicable
+to KSM.
+
+More information about KSM can be found in
+Documentation/admin-guide/mm/ksm.rst.
+
 
 Chapter 4: Configuring procfs
 =============================

fs/proc/base.c

@@ -3269,6 +3269,7 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
 				struct pid *pid, struct task_struct *task)
 {
 	struct mm_struct *mm;
+	int ret = 0;
 
 	mm = get_task_mm(task);
 	if (mm) {
@@ -3276,6 +3277,16 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
 		seq_printf(m, "ksm_zero_pages %ld\n", mm_ksm_zero_pages(mm));
 		seq_printf(m, "ksm_merging_pages %lu\n", mm->ksm_merging_pages);
 		seq_printf(m, "ksm_process_profit %ld\n", ksm_process_profit(mm));
+		seq_printf(m, "ksm_merge_any: %s\n",
+				test_bit(MMF_VM_MERGE_ANY, &mm->flags) ? "yes" : "no");
+		ret = mmap_read_lock_killable(mm);
+		if (ret) {
+			mmput(mm);
+			return ret;
+		}
+		seq_printf(m, "ksm_mergeable: %s\n",
+				ksm_process_mergeable(mm) ? "yes" : "no");
+		mmap_read_unlock(mm);
 		mmput(mm);
 	}
 

include/linux/ksm.h

@@ -93,6 +93,7 @@ void folio_migrate_ksm(struct folio *newfolio, struct folio *folio);
 void collect_procs_ksm(const struct folio *folio, const struct page *page,
 		struct list_head *to_kill, int force_early);
 long ksm_process_profit(struct mm_struct *);
+bool ksm_process_mergeable(struct mm_struct *mm);
 
 #else  /* !CONFIG_KSM */
 

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 -