[LTP] [PATCH] fs_fill: Fix test when running on a 256 CPU+ machine

[Andrei Gherzan]

The fs_fill test runs a fill test on all the supported filesystems. One
of them being vfat. This filesystem is configured dynamically or through
flags/arguments for its file allocation table type (12/16/32).

The size of the test device (which is a loop-mounted fs) is 300MB. When
not instructed, mkfs will "automatically select between 12, 16 and 32
bit, whatever fits better for the filesystem size"[1]. In the case of
a 300Mb that would end up as FAT16.

This is linked with another configuration that is the actual impact on
this issue: the maximum number of directories in the root of the
filesystem. FAT12 and FAT16 use a special system region: "Root Directory
Region". And by default (there is also an argument to configure this at
mkfs-time) this ends up being 256 when no custom arguments are provided.

On the other hand, the test sets up the filesystem with a
"tst_ncpus_conf + 2" number of directories in the test filesystem's root
which can break the limit explained above on systems with a number of
CPUs that would amount above the limit.

This change addresses this situation by using a subdirectory in the test
filesystem which is not subject to this limit.

Signed-off-by: Andrei Gherzan <andrei.gherzan@canonical.com>
---
 testcases/kernel/fs/fs_fill/fs_fill.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/testcases/kernel/fs/fs_fill/fs_fill.c b/testcases/kernel/fs/fs_fill/fs_fill.c
index 2027b6df1..dd8ad2935 100644
--- a/testcases/kernel/fs/fs_fill/fs_fill.c
+++ b/testcases/kernel/fs/fs_fill/fs_fill.c
@@ -18,6 +18,7 @@
 #include "tst_test.h"
 
 #define MNTPOINT "mntpoint"
+#define THREADS_SUBDIR "subdir"
 
 static volatile int run;
 static unsigned int nthreads;
@@ -99,9 +100,13 @@ static void setup(void)
 	nthreads = tst_ncpus_conf() + 2;
 	workers = SAFE_MALLOC(sizeof(struct worker) * nthreads);
 
+	// Avoid creating the thread directories in the root of the filesystem
+	// to not hit the root entries limit on a  FAT16 filesystem.
+	SAFE_MKDIR(MNTPOINT "/" THREADS_SUBDIR, 0700);
+
 	for (i = 0; i < nthreads; i++) {
 		snprintf(workers[i].dir, sizeof(workers[i].dir),
-			 MNTPOINT "/thread%i", i + 1);
+			 MNTPOINT "/" THREADS_SUBDIR "/thread%i", i + 1);
 		SAFE_MKDIR(workers[i].dir, 0700);
 	}
 
-- 
2.34.1