ext4: avoid deadlock in fs reclaim with page writeback

In the Linux kernel, the following vulnerability has been resolved:

ext4: avoid deadlock in fs reclaim with page writeback

Ext4 has a filesystem wide lock protecting ext4_writepages() calls to

avoid races with switching of journalled data flag or inode format. This

lock can however cause a deadlock like:

CPU0 CPU1

ext4_writepages()

percpu_down_read(sbi->s_writepages_rwsem);

ext4_change_inode_journal_flag()

percpu_down_write(sbi->s_writepages_rwsem);

- blocks, all readers block from now on

ext4_do_writepages()

ext4_init_io_end()

kmem_cache_zalloc(io_end_cachep, GFP_KERNEL)

fs_reclaim frees dentry...

dentry_unlink_inode()

iput() - last ref =>

iput_final() - inode dirty =>

write_inode_now()...

ext4_writepages() tries to acquire sbi->s_writepages_rwsem

and blocks forever

Make sure we cannot recurse into filesystem reclaim from writeback code

to avoid the deadlock.