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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 <email@example.com>: - minimal remount support - use correct seek function for directories - initialise is_real before use - rename ovl_fill_cache to ovl_dir_read Felix Fietkau <firstname.lastname@example.org>: - fix a deadlock in ovl_dir_read_merged - fix a deadlock in ovl_remove_whiteouts Erez Zadok <email@example.com> - fix cleanup after WARN_ON Sedat Dilek <firstname.lastname@example.org> - fix up permission to confirm to new API Robin Dong <email@example.com> - fix possible leak in ovl_new_inode - create new inode in ovl_link Andy Whitcroft <firstname.lastname@example.org> - 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 <email@example.com> Andy Whitcroft <firstname.lastname@example.org> Michal Suchanek <email@example.com> Felix Fietkau <firstname.lastname@example.org> Erez Zadok <email@example.com> Randy Dunlap <firstname.lastname@example.org> Signed-off-by: Miklos Szeredi <email@example.com>
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|+ tristate "Overlay filesystem support"|
|+ An overlay filesystem combines two filesystems - an 'upper' filesystem|
|+ and a 'lower' filesystem. When a name exists in both filesystems, the|
|+ object in the 'upper' filesystem is visible while the object in the|
|+ 'lower' filesystem is either hidden or, in the case of directories,|
|+ merged with the 'upper' object.|
|+ For more information see Documentation/filesystems/overlayfs.txt|
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