Hi, Valerie Aurora: > Writable overlays (formerly union mounts) > ========================================= > > In this document: > - Overview of writable overlays > - Terminology > - VFS implementation ::: While I don't remember exactly when I first read the source files of UnionMount, I think it is promising. And I have written to Val and Jan some of my comments or reviews about UnionMount. Recently I noticed another issue about stat(2) and mountpoint(1). The latter is a part of 'initscripts' package. For example, - you have a union-ed directory, /u = /rw + /ro - /ro/usr dir exists - /rw/usr dir does NOT exist - of course, /u/usr exists As far as I know, UnionMount is expected to handle /u/usr directory as if it exists under /u dir. (I may be wrong since it totally depends upon the design of UnionMount) In this case, stat(2) for /u and /u/usr will return different st_dev from each other. eg. stat(/u/usr) returns the st_dev value of /ro, stat(/u) returns the one for /rw. This behaviour may make /bin/mountpoint confused, particulary in the chroot/switch_root-ed environment. /bin/mountpoint issues stat(2) for the specified dir and its parent, and compares their st_dev. If they differ from each other, the utility handles the specified dir as a "mountpoint". I am afraid it will make some init-scripts crazy because /u/usr is NOT a mountpoint actually. One possible solution will be setting a hook to vfs_stat(), which handles the vfsmount set UNION flag differently and returns the pseudo st_dev for the entires in UnionMount. But it may lead to the duplicated inode number situation which may make applications crazy. For instance, - /ro/fileA is hardlinked to /ro/fileB. - the inode number of them is i100. - /rw/fileC is handlinked to /rw/fileD. - the inode number of them is i100 too. Since /ro and /rw are different, the same inode number is not a problem natively. But if UnionMount takes an approach above, they all have the same st_dev value. And I am afraid some applications may handle them as a single hardlink unexpectedly. So UnionMount should maintain its inode numbers by itself? No, it goes to the filesystem-type implementation. It should not be the way of UnionMount. Are there any ideas to solve this problem? J. R. Okajima -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
