tmpfs: fix kernel BUG in shmem_delete_inode

SuSE's insserve initscript ordering program hits kernel BUG at mm/shmem.c:814
on 2.6.26.  It's using posix_fadvise on directories, and the shmem_readpage
method added in 2.6.23 is letting POSIX_FADV_WILLNEED allocate useless pages
to a tmpfs directory, incrementing i_blocks count but never decrementing it.

Fix this by assigning shmem_aops (pointing to readpage and writepage and
set_page_dirty) only when it's needed, on a regular file or a long symlink.

Many thanks to Kel for outstanding bugreport and steps to reproduce it.

Reported-by: Kel Modderman <kel@otaku42.de>
Tested-by: Kel Modderman <kel@otaku42.de>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: <stable@kernel.org>		[2.6.25.x, 2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 952d361774bb..c1e5a3b4f758 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1513,7 +1513,6 @@ shmem_get_inode(struct super_block *sb, int mode, dev_t dev)
                 inode->i_uid = current->fsuid;
                 inode->i_gid = current->fsgid;
                 inode->i_blocks = 0;
-                inode->i_mapping->a_ops = &shmem_aops;
                 inode->i_mapping->backing_dev_info = &shmem_backing_dev_info;
                 inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
                 inode->i_generation = get_seconds();
@@ -1528,6 +1527,7 @@ shmem_get_inode(struct super_block *sb, int mode, dev_t dev)
                         init_special_inode(inode, mode, dev);
                         break;
                 case S_IFREG:
+                        inode->i_mapping->a_ops = &shmem_aops;
                         inode->i_op = &shmem_inode_operations;
                         inode->i_fop = &shmem_file_operations;
                         mpol_shared_policy_init(&info->policy,
@@ -1929,6 +1929,7 @@ static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *s
                         return error;
                 }
                 unlock_page(page);
+                inode->i_mapping->a_ops = &shmem_aops;
                 inode->i_op = &shmem_symlink_inode_operations;
                 kaddr = kmap_atomic(page, KM_USER0);
                 memcpy(kaddr, symname, len);