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  • Miklos Szeredi's avatar
    overlay filesystem · e9be9d5e
    Miklos Szeredi authored
    
    
    Overlayfs allows one, usually read-write, directory tree to be
    overlaid onto another, read-only directory tree.  All modifications
    go to the upper, writable layer.
    
    This type of mechanism is most often used for live CDs but there's a
    wide variety of other uses.
    
    The implementation differs from other "union filesystem"
    implementations in that after a file is opened all operations go
    directly to the underlying, lower or upper, filesystems.  This
    simplifies the implementation and allows native performance in these
    cases.
    
    The dentry tree is duplicated from the underlying filesystems, this
    enables fast cached lookups without adding special support into the
    VFS.  This uses slightly more memory than union mounts, but dentries
    are relatively small.
    
    Currently inodes are duplicated as well, but it is a possible
    optimization to share inodes for non-directories.
    
    Opening non directories results in the open forwarded to the
    underlying filesystem.  This makes the behavior very similar to union
    mounts (with the same limitations vs. fchmod/fchown on O_RDONLY file
    descriptors).
    
    Usage:
    
      mount -t overlayfs overlayfs -olowerdir=/lower,upperdir=/upper/upper,workdir=/upper/work /overlay
    
    The following cotributions have been folded into this patch:
    
    Neil Brown <neilb@suse.de>:
     - minimal remount support
     - use correct seek function for directories
     - initialise is_real before use
     - rename ovl_fill_cache to ovl_dir_read
    
    Felix Fietkau <nbd@openwrt.org>:
     - fix a deadlock in ovl_dir_read_merged
     - fix a deadlock in ovl_remove_whiteouts
    
    Erez Zadok <ezk@fsl.cs.sunysb.edu>
     - fix cleanup after WARN_ON
    
    Sedat Dilek <sedat.dilek@googlemail.com>
     - fix up permission to confirm to new API
    
    Robin Dong <hao.bigrat@gmail.com>
     - fix possible leak in ovl_new_inode
     - create new inode in ovl_link
    
    Andy Whitcroft <apw@canonical.com>
     - switch to __inode_permission()
     - copy up i_uid/i_gid from the underlying inode
    
    AV:
     - ovl_copy_up_locked() - dput(ERR_PTR(...)) on two failure exits
     - ovl_clear_empty() - one failure exit forgetting to do unlock_rename(),
       lack of check for udir being the parent of upper, dropping and regaining
       the lock on udir (which would require _another_ check for parent being
       right).
     - bogus d_drop() in copyup and rename [fix from your mail]
     - copyup/remove and copyup/rename races [fix from your mail]
     - ovl_dir_fsync() leaving ERR_PTR() in ->realfile
     - ovl_entry_free() is pointless - it's just a kfree_rcu()
     - fold ovl_do_lookup() into ovl_lookup()
     - manually assigning ->d_op is wrong.  Just use ->s_d_op.
     [patches picked from Miklos]:
     * copyup/remove and copyup/rename races
     * bogus d_drop() in copyup and rename
    
    Also thanks to the following people for testing and reporting bugs:
    
      Jordi Pujol <jordipujolp@gmail.com>
      Andy Whitcroft <apw@canonical.com>
      Michal Suchanek <hramrach@centrum.cz>
      Felix Fietkau <nbd@openwrt.org>
      Erez Zadok <ezk@fsl.cs.sunysb.edu>
      Randy Dunlap <rdunlap@xenotime.net>
    
    Signed-off-by: default avatarMiklos Szeredi <mszeredi@suse.cz>
    e9be9d5e