On Tue, Apr 09, 2019 at 06:22:49PM -0700, Eric Biggers wrote: > > Non-NULL ->get_link() => DCACHE_SYMLINK_TYPE in ->d_flags => > > d_is_symlink() true => step_into() progresses to pick_link(). > > > > IOW, non-NULL ->get_link() is what tells you that we have > > a symlink there. > > I think that's pretty unintuitive. The fact that multiple filesystems including > ext4 set ->i_link on fast symlinks, then set ->get_link() to a function that > returns ->i_link, made me assume that's the mechanism by which such symlink > targets are returned to the VFS. When in fact fs/namei.c just uses ->i_link, > and never calls ->get_link(). > > Is there any reason why d_flags_for_inode() doesn't check S_ISLNK() instead, and > then fs/namei.c would call ->get_link() if non-NULL, otherwise use ->i_link? Extra check and dereference on hot path with no visible benefits of doing it that way, for starters. Really, what _is_ the benefit of pessimizing that? Most of the symlinks we run into will have ->i_link set; checking ->i_op->get_link first is extra work for no good reason... What's more, ->get_link is visible in inode_operations; ->i_link (let alone ->i_mode) isn't. As it is, we can easily tell symlink inode_operations from everything else on the source level. With your scheme we won't.
