On Thu, Mar 28, 2019 at 10:00:22PM +0800, Qu Wenruo wrote:
>
>
> On 2019/3/28 下午9:57, David Sterba wrote:
> > On Thu, Mar 28, 2019 at 09:42:59PM +0800, Qu Wenruo wrote:
> >>
> >>
> >> On 2019/3/28 下午9:38, David Sterba wrote:
> >>> On Wed, Mar 20, 2019 at 02:37:16PM +0800, Qu Wenruo wrote:
> >>>> + if (S_ISDIR(mode) && btrfs_inode_nlink(leaf, iitem) > 1) {
> >>>> + inode_item_err(fs_info, leaf, slot,
> >>>> + "invalid nlink: has %u expect no more than 1 for dir",
> >>>> + btrfs_inode_nlink(leaf, iitem));
> >>>> + goto error;
> >>>> + }
> >>>
> >>> I'm not sure about this check, the number of links for a directory is 1,
> >>> but the exact count could be implemented (there's a project idea for
> >>> that). I don't know if this will require an incompat bit or not.
> >>
> >> That means we have hard link for directories.
> >
> > Yes, hard links of directories are forbidden by VFS but that's not the
> > point here:
> >
> > https://btrfs.wiki.kernel.org/index.php/Project_ideas#Track_link_count_for_directories
> >
> > "The link count for directories is traditionally used to count the number
> > of subdirectores iff the link count is >= 2."
>
> Oh, got it.
>
> But for that case, it would be much better to go for things like nbytes
> or size, as by all means, those two members are less meaningful than
> nlinks for directory.
size of a directory is currently 2 * sum of all names in the directory,
ie. both files and directories. The nlink is used as an optimization
during traversal by existing tools (find), we can't simply change that
but would still like to update btrfs to provide support for that.
On the wiki there's note that it's backward compatible, I don't recall
the actual details why it's safe though.
