[PATCH] tmpfs: Don't use 64-bit inodes by defulat with 32-bit ino_t

[Seth Forshee <seth.forshee@canonical.com>]

Currently there seems to be an assumption in tmpfs that 64-bit
architectures also have a 64-bit ino_t. This is not true; s390 at
least has a 32-bit ino_t. With CONFIG_TMPFS_INODE64=y tmpfs
mounts will get 64-bit inode numbers and display "inode64" in the
mount options, but passing the "inode64" mount option will fail.
This leads to the following behavior:

 # mkdir mnt
 # mount -t tmpfs nodev mnt
 # mount -o remount,rw mnt
 mount: /home/ubuntu/mnt: mount point not mounted or bad option.

As mount sees "inode64" in the mount options and thus passes it
in the options for the remount.

Ideally CONFIG_TMPFS_INODE64 would depend on sizeof(ino_t) < 8,
but I don't think it's possible to test for this (potentially
CONFIG_ARCH_HAS_64BIT_INO_T or similar could be added, but I'm
not sure whether or not that is wanted). So fix this by simply
refusing to honor the CONFIG_TMPFS_INODE64 setting when
sizeof(ino_t) < 8.

Fixes: ea3271f7196c ("tmpfs: support 64-bit inums per-sb")
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 7c6b6d8f6c39..efde42acdc7a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3733,7 +3733,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 			ctx->blocks = shmem_default_max_blocks();
 		if (!(ctx->seen & SHMEM_SEEN_INODES))
 			ctx->inodes = shmem_default_max_inodes();
-		if (!(ctx->seen & SHMEM_SEEN_INUMS))
+		if (!(ctx->seen & SHMEM_SEEN_INUMS) && sizeof(ino_t) >= 8)
 			ctx->full_inums = IS_ENABLED(CONFIG_TMPFS_INODE64);
 	} else {
 		sb->s_flags |= SB_NOUSER;
-- 
2.29.2