Oops due to VFS mount 'shm_mnt' not marked as a long-term internal mount

Issue

• Oops due to VFS mount shm_mnt not marked as a long-term internal mount

• The real mount of the VFS mount shm_mnt was de-allocated (via mntfree()) due to a final call to mntput(); subsequently in the free mount code path, the corresponding super_block object i.e. m->mnt_sb was subsequently de-allocated too

• According to the Oops, the system crash occurred in shmem_reserve_inode(), since the s_fs_info field of the super_block object was unexpectedly NULL:

• The following is an excerpt of dmesg

[5777285.223382] <26>Unable to handle kernel paging request for data at
                     address 0x00000030
[5777285.223395] <26>Faulting instruction address: 0xc000000000280ed8
[5777285.223399] <82>Oops: Kernel access of bad area, sig: 11 [#1]

• The Faulting instruction address (0xc000000000280ed8) translates to mm/shmem.c:203

• The following is an excerpt of shmem_reserve_inode()

             200 static int shmem_reserve_inode(struct super_block *sb)
             201 {
             202         struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
  (1) ---->  203         if (sbinfo->max_inodes) {

• The max_inodes field of struct shmem_sb_info is at offset +0x30

crash> p &((struct shmem_sb_info *)0x0)->max_inodes
$1 = (unsigned long *) 0x30

As seen from the excerpt of shmem_reserve_inode() above, it is not expected for sb->s_fs_info to be NULL.