selftests: mincore: count file-mmap readahead on both sides

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.

Link: https://lore.kernel.org/20260713094319.771550-1-wangyijia.yeah@bytedance.com
Signed-off-by: Yijia Wang <wangyijia.yeah@bytedance.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/tools/testing/selftests/mincore/mincore_selftest.c b/tools/testing/selftests/mincore/mincore_selftest.c
index cdd022c1c497..18dfbf318b1f 100644
--- a/tools/testing/selftests/mincore/mincore_selftest.c
+++ b/tools/testing/selftests/mincore/mincore_selftest.c
@@ -242,8 +242,10 @@ 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);
@@ -252,6 +254,12 @@ 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++;