[PATCH v3 05/18] selftests/mm: keep khugepaged out of the swapout the swap cases set up

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

collapse_swapin_single_pte() and collapse_max_ptes_swap() page a range out
and then require smaps to report exactly the count they asked for.  Two
things keep it from arriving.  MADV_PAGEOUT is best effort, so the count
often turns up a moment late.  And wait_for_scan() leaves MADV_HUGEPAGE
behind, so khugepaged is still working on the range: collapsing one with up
to max_ptes_swap pages swapped out means reading them back in, and the
daemon empties the swap as fast as the case fills it.  On arm64 with 64K
pages, where max_ptes_swap is 1024 pages, that is 64M a step and the case
loses:

  # Swapout 1024 of 8192 pages... Fail
  not ok 10 collapse_max_ptes_swap

Ask again for up to two seconds, with the range held out of the daemon's
reach while asking.  The collapse each case runs next puts MADV_HUGEPAGE
back, so only the setup is affected.

If the pages still will not go, skip.  is_swap_enabled() covers a machine
with no swap; what is left -- swap too small, full, capped by a memcg, busy
with writeback -- is not the kernel under test refusing.  An error from
madvise() itself still ends the run.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 tools/testing/selftests/mm/khugepaged.c | 53 +++++++++++++++++++------
 1 file changed, 41 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index c049ac997def..8458cd2ff0df 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -241,6 +241,41 @@ static bool check_swap(void *addr, unsigned long size)
 	return swap;
 }
 
+/*
+ * Page the range out and wait for the swap count to say so.
+ *
+ * Two things get in the way.  MADV_PAGEOUT is best effort:
+ * shrink_folio_list() leaves a folio alone when it cannot reclaim it right
+ * away, and one still under writeback from an earlier pageout is the common
+ * case, so the count the caller asks for arrives a moment later.  And a range
+ * an earlier collapse left MADV_HUGEPAGE is one khugepaged is still working
+ * on: collapsing a range with up to max_ptes_swap pages swapped out means
+ * reading those pages back in, so the daemon undoes the pageout as fast as it
+ * is asked for.  Keep the range out of its reach; the collapse the caller runs
+ * next puts MADV_HUGEPAGE back.
+ *
+ * Failing to get the pages out is the machine's answer, not the kernel's --
+ * swap too small, swap full, a memcg cap, a folio still under writeback -- so
+ * callers skip rather than fail.  An error from madvise() is different, and
+ * ends the run here.
+ */
+static bool swapout_range(void *p, unsigned long size)
+{
+	int i;
+
+	if (madvise(p, size, MADV_NOHUGEPAGE))
+		ksft_exit_fail_perror("madvise(MADV_NOHUGEPAGE)");
+
+	for (i = 0; i < 40; i++) {
+		if (madvise(p, size, MADV_PAGEOUT))
+			ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
+		if (check_swap(p, size))
+			return true;
+		usleep(50 * 1000);
+	}
+	return false;
+}
+
 static void *alloc_mapping(int nr)
 {
 	void *p;
@@ -855,12 +890,10 @@ static void collapse_swapin_single_pte(struct collapse_context *c, struct mem_op
 	p = ops->setup_area(1);
 	ops->fault(p, 0, hpage_pmd_size);
 
-	if (madvise(p, page_size, MADV_PAGEOUT))
-		ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
-	if (check_swap(p, page_size)) {
+	if (swapout_range(p, page_size)) {
 		success("OK");
 	} else {
-		fail("Fail");
+		skip("Could not swap out");
 		goto out;
 	}
 
@@ -887,12 +920,10 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
 	p = ops->setup_area(1);
 	ops->fault(p, 0, hpage_pmd_size);
 
-	if (madvise(p, (max_ptes_swap + 1) * page_size, MADV_PAGEOUT))
-		ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
-	if (check_swap(p, (max_ptes_swap + 1) * page_size)) {
+	if (swapout_range(p, (max_ptes_swap + 1) * page_size)) {
 		success("OK");
 	} else {
-		fail("Fail");
+		skip("Could not swap out");
 		goto out;
 	}
 
@@ -904,12 +935,10 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
 		ops->fault(p, 0, hpage_pmd_size);
 		ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap,
 		       hpage_pmd_nr);
-		if (madvise(p, max_ptes_swap * page_size, MADV_PAGEOUT))
-			ksft_exit_fail_perror("madvise(MADV_PAGEOUT)");
-		if (check_swap(p, max_ptes_swap * page_size)) {
+		if (swapout_range(p, max_ptes_swap * page_size)) {
 			success("OK");
 		} else {
-			fail("Fail");
+			skip("Could not swap out");
 			goto out;
 		}
 
-- 
2.54.0