[PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse

[Lance Yang]

The collapsing behavior of khugepaged with pages
marked using MADV_FREE might cause confusion
among users.

For instance, allocate a 2MB chunk using mmap and
later release it by MADV_FREE. Khugepaged will not
collapse this chunk. From the user's perspective,
it treats lazyfree pages as pte_none. However,
for some pages marked as lazyfree with MADV_FREE,
khugepaged might collapse this chunk and copy
these pages to a new huge page. This inconsistency
in behavior could be confusing for users.

After a successful MADV_FREE operation, if there is
no subsequent write, the kernel can free the pages
at any time. Therefore, in my opinion, counting
lazyfree pages in max_pte_none seems reasonable.

Perhaps treating MADV_FREE like MADV_DONTNEED, not
copying lazyfree pages when khugepaged collapses
huge pages in the background better aligns with
user expectations.

Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
---
 mm/khugepaged.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 2b219acb528e..6cbf46d42c6a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -777,6 +777,7 @@ static int __collapse_huge_page_copy(pte_t *pte,
 				     pmd_t orig_pmd,
 				     struct vm_area_struct *vma,
 				     unsigned long address,
+				     struct collapse_control *cc,
 				     spinlock_t *ptl,
 				     struct list_head *compound_pagelist)
 {
@@ -797,6 +798,13 @@ static int __collapse_huge_page_copy(pte_t *pte,
 			continue;
 		}
 		src_page = pte_page(pteval);
+
+		if (cc->is_khugepaged
+				&& !folio_test_swapbacked(page_folio(src_page))) {
+			clear_user_highpage(page, _address);
+			continue;
+		}
+
 		if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
 			result = SCAN_COPY_MC;
 			break;
@@ -1205,7 +1213,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	anon_vma_unlock_write(vma->anon_vma);
 
 	result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
-					   vma, address, pte_ptl,
+					   vma, address, cc, pte_ptl,
 					   &compound_pagelist);
 	pte_unmap(pte);
 	if (unlikely(result != SCAN_SUCCEED))
-- 
2.33.1