On Wed, Apr 10, 2019 at 02:04:25AM +0100, Al Viro wrote: > On Tue, Apr 09, 2019 at 05:45:54PM -0700, Eric Biggers wrote: > > On Wed, Apr 10, 2019 at 01:33:46AM +0100, Al Viro wrote: > > > On Tue, Apr 09, 2019 at 04:35:44PM -0700, Eric Biggers wrote: > > > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > > > > > Path lookups that traverse encrypted symlink(s) are very slow because > > > > each encrypted symlink needs to be decrypted each time it's followed. > > > > > > > > Make encrypted symlinks faster by caching the decrypted symlink target > > > > in ->i_link. The first call to ->get_link() sets it; later calls simply > > > > return it. ->symlink() also sets it when the symlink is created. > > > > > > > > When the inode's ->i_crypt_info is freed, ->i_link is freed too. > > > > > > > > Note: RCU-delayed freeing of ->i_link is not yet implemented. > > > > Therefore, for now even when ->i_link is set, path lookups must continue > > > > to drop out of RCU-walk mode when following an encrypted symlink. > > > > > > And how the devil would they continue to do that, if I might ask? > > > ->get_link() is *NOT* called if ->i_link is non-NULL, period. > > > > You're right, I didn't notice that ->get_link() isn't called when > > ->i_link is non-NULL. > > > > But that being the case, what's the point of simple_get_link()? > > 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? - Eric
