On Tue, Apr 29, bsn.0007@xxxxxxxxx wrote: > The RFC discussed about the information glibc readdir needs to get about > union mounted directories and I have assumed the following information > to be available from the kernel for this implementation. > > - Kernel would return all the dirents (including duplicates and whiteouts) > starting from the topmost directory of the union. > > - Indication that this directory is a union mounted directory > I have assumed that kernel would return a "." whiteout as the first > directory entry of the union. This would tell glibc readdir(3) that it is > working with a union mounted directory and it needs to do duplicate > elimination and whiteout suppression. It starts building a dirent cache > for this purpose. IIRC the intention was to emit a "." whiteout when "changing" from one directory to the next. That means when the first directory is completely read the whiteout is emitted. After that glibc knows to start duplicate removal. > Ulrich had suggested that we could use the fstat call to recognize union > mounts. But looking at the stat structure from stat(2), it was not obvious > as to which field in there could be used for this purpose. Hence for this > prototype implementation, I decided to go with what Al Viro suggested, which > is about using a "." whiteout. > > - Indication that kernel is done with returning entries from the topmost > directory. > I have assumed that kernel would return a "." whiteout at the beginning > of each directory of the union. So when glibc gets a 2nd "." whiteout, it > will start performing duplicate elimination. See above > - Whiteout indication > glibc will depend on dirent->d_type to be set to DT_WHT on a whiteout > file. Which makes the new filetype very much visible to the userspace but maybe that is the price we have to pay. > Compatibility issues > -------------------- > There are many versions of dirent structure in glibc and I have tried my > best to take care of compatibility issues. But I have not really tested > readdir64 or old_readdir64. Also atleast one version of dirent structure > doesn't have d_type field and my whiteout suppression logic depends on it > and uses it in the generic __READDIR routine which gets used by various > version of readdir and I think this would break that readdir version which > uses dirent structure w/o d_type. I will be taking care of such compatibility > issues more cleanly/thoroughly in subsequent posts. We don't support union mounts on older kernels. Newer kernels return d_type. So I think we don't have a problem. Regards, Jan -- Jan Blunck <jblunck@xxxxxxx> -- 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
