[LTP] [PATCH] readahead02: Fall back to the parent disk's BDI for partitions

[Victor Cheng-Yen Yang]

Partitions (e.g. mmcblk0p3) do not have their own BDI sysfs entry;
the BDI belongs to the parent disk.

In sysfs a partition is a subdirectory of its disk, so retry one
level up. That resolves for every partition naming scheme without
parsing the device name.

Without this fix, setup_readahead_length() silently returns the
default readahead_length, causing lots of readahead() calls per
test case and a multi-hour runtime on MMC-backed FPGA boards.

Signed-off-by: Victor Cheng-Yen Yang <cyyang772@andestech.com>
---
 testcases/kernel/syscalls/readahead/readahead02.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/testcases/kernel/syscalls/readahead/readahead02.c b/testcases/kernel/syscalls/readahead/readahead02.c
index b65213ad0..6707a1c63 100644
--- a/testcases/kernel/syscalls/readahead/readahead02.c
+++ b/testcases/kernel/syscalls/readahead/readahead02.c
@@ -399,8 +399,14 @@ static void setup_readahead_length(void)
 	backing_dev = basename(tmp);
 	sprintf(sys_bdi_ra_path, "/sys/class/block/%s/bdi/read_ahead_kb",
 		backing_dev);
-	if (access(sys_bdi_ra_path, F_OK))
-		return;
+
+	if (access(sys_bdi_ra_path, F_OK)) {
+		/* Partitions use the parent disk's BDI sysfs entry */
+		snprintf(sys_bdi_ra_path, sizeof(sys_bdi_ra_path),
+			"/sys/class/block/%s/../bdi/read_ahead_kb", backing_dev);
+		if (access(sys_bdi_ra_path, F_OK))
+			return;
+	}
 
 	SAFE_FILE_SCANF(sys_bdi_ra_path, "%d", &orig_bdi_limit);
 
-- 
2.34.1