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

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

[Victor Cheng-Yen Yang via ltp]

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


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

[linuxtestproject.agent]

Hi Victor,

On Fri, 21 Aug 2026, Victor Cheng-Yen Yang wrote:
> readahead02: Fall back to the parent disk's BDI for partitions

> +		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);
>
> 	if (orig_bdi_limit)
> 		SAFE_FILE_PRINTF(sys_bdi_ra_path, "%d", orig_bdi_limit);

Could restoration track whether the value was read separately from its
numeric value? The kernel accepts zero for read_ahead_kb, so if the parent
BDI starts at zero, setup writes a positive value but cleanup skips the
restore. This leaves the parent and its sibling partitions modified after
the test.

Verdict - Needs revision

---
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Regards,
LTP AI Reviewer

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