lib: adjust the tmpfs size according to .dev_min_size and MemAvailable

Since commit c305a53 (lib: limit the size of tmpfs in LTP, Jul 9)
Ltp set tmpfs mount size according to the tdev.size. This cause a
new problem on small RAM system, which consume too much memory and
finally trigger OOM.

To fix this, let's adjust the tmpfs-size according to both free memory
and .dev_min_size:

 - if .dev_min_size is defined and system has enough free memory,
   set tmpfs-size to tdev.size

 - if .dev_min_size is defined and there is not enough free
   memory -> TCONF

 - if the test not define .dev_min_size, set the size for tmpfs to
   be really small 32MB

 - if .dev_min_size is not define and there is not even 64MB of free
   memory (for 32MB limit) -> TCONF

Reported-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Fixes: c305a53 ("lib: limit the size of tmpfs in LTP")
Suggested-by: Cyril Hrubis <chrubis@suse.cz>
Signed-off-by: Li Wang <liwang@redhat.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Petr Vorel <pvorel@suse.cz>

include/tst_test.h

@@ -42,6 +42,7 @@
 #include "tst_lockdown.h"
 #include "tst_fips.h"
 #include "tst_taint.h"
+#include "tst_memutils.h"
 
 /*
  * Reports testcase result.

lib/tst_test.c

@@ -892,15 +892,25 @@ static void prepare_and_mount_dev_fs(const char *mntpoint)
 static const char *limit_tmpfs_mount_size(const char *mnt_data,
 		char *buf, size_t buf_size, const char *fs_type)
 {
+	unsigned int tmpfs_size;
+
 	if (strcmp(fs_type, "tmpfs"))
 		return mnt_data;
 
+	if (!tst_test->dev_min_size)
+		tmpfs_size = 32;
+	else
+		tmpfs_size = tdev.size;
+
+	if ((tst_available_mem() / 1024) < (tmpfs_size * 2))
+		tst_brk(TCONF, "No enough memory for tmpfs use");
+
 	if (mnt_data)
-		snprintf(buf, buf_size, "%s,size=%luM", mnt_data, tdev.size);
+		snprintf(buf, buf_size, "%s,size=%luM", mnt_data, tmpfs_size);
 	else
-		snprintf(buf, buf_size, "size=%luM", tdev.size);
+		snprintf(buf, buf_size, "size=%luM", tmpfs_size);
 
-	tst_res(TINFO, "Limiting tmpfs size to %luMB", tdev.size);
+	tst_res(TINFO, "Limiting tmpfs size to %luMB", tmpfs_size);
 
 	return buf;
 }

testcases/kernel/syscalls/fallocate/fallocate05.c

@@ -147,7 +147,6 @@ static void cleanup(void)
 static struct tst_test test = {
 	.needs_root = 1,
 	.mount_device = 1,
-	.dev_min_size = 512,
 	.mntpoint = MNTPOINT,
 	.all_filesystems = 1,
 	.setup = setup,