[PATCH RESEND v4] selftests: mincore: count file-mmap readahead on both sides

[王翊嘉]

From: Yijia Wang <wangyijia.yeah@xxxxxxxxxxxxx>

check_file_mmap() faults a page in the middle of a file mapping and
expects the mmap read-around path to make neighbouring pages resident.
The test currently counts only pages after the faulted page.

That misses valid read-around on systems with large base page sizes. On
arm64 with 64K pages and the default 128K readahead setting, the
read-around window is two pages wide and centred on the faulting page.
Faulting page 32 makes pages 31 and 32 resident, so the forward-only
scan from page 33 reports ra_pages == 0 even though a neighbouring page
was brought in.

Keep the existing readahead assertion, but count resident neighbouring
pages on both sides of the faulted page. This fixes the 64K-page false
failure without teaching the selftest to compute the expected readahead
window from sysfs or other implementation details.

Signed-off-by: Yijia Wang <wangyijia.yeah@xxxxxxxxxxxxx>
---
Changes in v4:
- Drop the sysfs read_ahead_kb helper and skip logic from v3.
- Keep the existing ra_pages assertion and count resident readahead
  pages on both sides of the faulted page.

 tools/testing/selftests/mincore/mincore_selftest.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mincore/mincore_selftest.c b/tools/testing/selftests/mincore/mincore_selftest.c
index 3182043..ebf2b06 100644
--- a/tools/testing/selftests/mincore/mincore_selftest.c
+++ b/tools/testing/selftests/mincore/mincore_selftest.c
@@ -243,8 +243,10 @@ TEST(check_file_mmap)
 	}
 
 	/*
-	 * Touch a page in the middle of the mapping. We expect the next
-	 * few pages (the readahead window) to be populated too.
+	 * Touch a page in the middle of the mapping. We expect some
+	 * surrounding pages (the readahead window) to be populated too.
+	 * Depending on the page size and readahead setting, the pages may
+	 * land before the faulted page rather than after it.
 	 */
 	addr[FILE_SIZE / 2] = 1;
 	retval = mincore(addr, FILE_SIZE, vec);
@@ -253,6 +255,12 @@ TEST(check_file_mmap)
 		TH_LOG("Page not found in memory after use");
 	}
 
+	i = FILE_SIZE / 2 / page_size - 1;
+	while (i >= 0 && vec[i]) {
+		ra_pages++;
+		i--;
+	}
+
 	i = FILE_SIZE / 2 / page_size + 1;
 	while (i < vec_size && vec[i]) {
 		ra_pages++;
-- 
2.43.0