[PATCH 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

[<xu.xin16@zte.com.cn>]

From: xu xin <xu.xin16@zte.com.cn>

Problem
=======
When available memory is extremely tight, causing KSM pages to be swapped
out, or when there is significant memory fragmentation and THP triggers
memory compaction, the system will invoke the rmap_walk_ksm function to
perform reverse mapping. However, we observed that this function becomes
particularly time-consuming when a large number of VMAs (e.g., 20,000)
share the same anon_vma. Through debug trace analysis, we found that most
of the latency occurs within anon_vma_interval_tree_foreach, leading to an
excessively long hold time on the anon_vma lock (even reaching 500ms or
more), which in turn causes upper-layer applications (waiting for the
anon_vma lock) to be blocked for extended periods.

Root Reaon
==========
Further investigation revealed that 99.9% of iterations inside the
anon_vma_interval_tree_foreach loop are skipped due to the first check
"if (addr < vma->vm_start || addr >= vma->vm_end)), indicating that a large
number of loop iterations are ineffective. This inefficiency arises because
the pgoff_start and pgoff_end parameters passed to
anon_vma_interval_tree_foreach span the entire address space from 0 to
ULONG_MAX, resulting in very poor loop efficiency.

Solution
========
In fact, we can significantly improve performance by passing a more precise
range based on the given addr. Since the original pages merged by KSM
correspond to anonymous VMAs, the page offset can be calculated as
pgoff = address >> PAGE_SHIFT. Therefore, we can optimize the call by
defining:

	pgoff_start = rmap_item->address >> PAGE_SHIFT;
	pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;

Performance
===========
In our real embedded Linux environment, the measured metrcis were as follows:

1) Time_ms: Max time for holding anon_vma lock in a single rmap_walk_ksm.
2) Nr_iteration_total: The max times of iterations in a loop of anon_vma_interval_tree_foreach
3) Skip_addr_out_of_range: The max times of skipping due to the first check (vma->vm_start
            and vma->vm_end) in a loop of anon_vma_interval_tree_foreach.
4) Skip_mm_mismatch: The max times of skipping due to the second check (rmap_item->mm == vma->vm_mm)
            in a loop of anon_vma_interval_tree_foreach.

The result is as follows:

                 Time_ms      Nr_iteration_total    Skip_addr_out_of_range   Skip_mm_mismatch
Before patched:  228.65       22169                 22168                    0
After pacthed:   0.396        3                     0                        2

Co-developed-by: Wang Yaxin <wang.yaxin@zte.com.cn>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
---
 mm/ksm.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 335e7151e4a1..0a074ad8e867 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -3172,6 +3172,7 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
 		struct anon_vma_chain *vmac;
 		struct vm_area_struct *vma;
 		unsigned long addr;
+		pgoff_t pgoff_start, pgoff_end;

 		cond_resched();
 		if (!anon_vma_trylock_read(anon_vma)) {
@@ -3185,8 +3186,11 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
 		/* Ignore the stable/unstable/sqnr flags */
 		addr = rmap_item->address & PAGE_MASK;

+		pgoff_start = rmap_item->address >> PAGE_SHIFT;
+		pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;
+
 		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
-					       0, ULONG_MAX) {
+					       pgoff_start, pgoff_end) {

 			cond_resched();
 			vma = vmac->vma;
-- 
2.25.