• Nick Piggin's avatar
    fs: rcu-walk for path lookup · 31e6b01f
    Nick Piggin authored
    Perform common cases of path lookups without any stores or locking in the
    ancestor dentry elements. This is called rcu-walk, as opposed to the current
    algorithm which is a refcount based walk, or ref-walk.
    This results in far fewer atomic operations on every path element,
    significantly improving path lookup performance. It also avoids cacheline
    bouncing on common dentries, significantly improving scalability.
    The overall design is like this:
    * LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk.
    * Take the RCU lock for the entire path walk, starting with the acquiring
      of the starting path (eg. root/cwd/fd-path). So now dentry refcounts are
      not required for dentry persistence.
    * synchronize_rcu is called when unregistering a filesystem, so we can
      access d_ops and i_ops during rcu-walk.
    * Similarly take the vfsmount lock for the entire path walk. So now mnt
      refcounts are not required for persistence. Also we are free to perform mount
      lookups, and to assume dentry mount points and mount roots are stable up and
      down the path.
    * Have a per-dentry seqlock to protect the dentry name, parent, and inode,
      so we can load this tuple atomically, and also check whether any of its
      members have changed.
    * Dentry lookups (based on parent, candidate string tuple) recheck the parent
      sequence after the child is found in case anything changed in the parent
      during the path walk.
    * inode is also RCU protected so we can load d_inode and use the inode for
      limited things.
    * i_mode, i_uid, i_gid can be tested for exec permissions during path walk.
    * i_op can be loaded.
    When we reach the destination dentry, we lock it, recheck lookup sequence,
    and increment its refcount and mountpoint refcount. RCU and vfsmount locks
    are dropped. This is termed "dropping rcu-walk". If the dentry refcount does
    not match, we can not drop rcu-walk gracefully at the current point in the
    lokup, so instead return -ECHILD (for want of a better errno). This signals the
    path walking code to re-do the entire lookup with a ref-walk.
    Aside from the final dentry, there are other situations that may be encounted
    where we cannot continue rcu-walk. In that case, we drop rcu-walk (ie. take
    a reference on the last good dentry) and continue with a ref-walk. Again, if
    we can drop rcu-walk gracefully, we return -ECHILD and do the whole lookup
    using ref-walk. But it is very important that we can continue with ref-walk
    for most cases, particularly to avoid the overhead of double lookups, and to
    gain the scalability advantages on common path elements (like cwd and root).
    The cases where rcu-walk cannot continue are:
    * NULL dentry (ie. any uncached path element)
    * parent with d_inode->i_op->permission or ACLs
    * dentries with d_revalidate
    * Following links
    In future patches, permission checks and d_revalidate become rcu-walk aware. It
    may be possible eventually to make following links rcu-walk aware.
    Uncached path elements will always require dropping to ref-walk mode, at the
    very least because i_mutex needs to be grabbed, and objects allocated.
    Signed-off-by: default avatarNick Piggin <npiggin@kernel.dk>
security.c 32.2 KB