1. 11 Jun, 2009 1 commit
    • Eric Paris's avatar
      fsnotify: unified filesystem notification backend · 90586523
      Eric Paris authored
      fsnotify is a backend for filesystem notification.  fsnotify does
      not provide any userspace interface but does provide the basis
      needed for other notification schemes such as dnotify.  fsnotify
      can be extended to be the backend for inotify or the upcoming
      fanotify.  fsnotify provides a mechanism for "groups" to register for
      some set of filesystem events and to then deliver those events to
      those groups for processing.
      fsnotify has a number of benefits, the first being actually shrinking the size
      of an inode.  Before fsnotify to support both dnotify and inotify an inode had
              unsigned long           i_dnotify_mask; /* Directory notify events */
              struct dnotify_struct   *i_dnotify; /* for directory notifications */
              struct list_head        inotify_watches; /* watches on this inode */
              struct mutex            inotify_mutex;  /* protects the watches list
      But with fsnotify this same functionallity (and more) is done with just
              __u32                   i_fsnotify_mask; /* all events for this inode */
              struct hlist_head       i_fsnotify_mark_entries; /* marks on this inode */
      That's right, inotify, dnotify, and fanotify all in 64 bits.  We used that
      much space just in inotify_watches alone, before this patch set.
      fsnotify object lifetime and locking is MUCH better than what we have today.
      inotify locking is incredibly complex.  See 8f7b0ba1 as an example of
      what's been busted since inception.  inotify needs to know internal semantics
      of superblock destruction and unmounting to function.  The inode pinning and
      vfs contortions are horrible.
      no fsnotify implementers do allocation under locks.  This means things like
       which (due to an overabundance of caution) changes GFP_KERNEL to
      GFP_NOFS can be reverted.  There are no longer any allocation rules when using
      or implementing your own fsnotify listener.
      fsnotify paves the way for fanotify.  In brief fanotify is a notification
      mechanism that delivers the lisener both an 'event' and an open file descriptor
      to the object in question.  This means that fanotify is pathname agnostic.
      Some on lkml may not care for the original companies or users that pushed for
      TALPA, but fanotify was designed with flexibility and input for other users in
      mind.  The readahead group expressed interest in fanotify as it could be used
      to profile disk access on boot without breaking the audit system.  The desktop
      search groups have also expressed interest in fanotify as it solves a number
      of the race conditions and problems present with managing inotify when more
      than a limited number of specific files are of interest.  fanotify can provide
      for a userspace access control system which makes it a clean interface for AV
      vendors to hook without trying to do binary patching on the syscall table,
      LSM, and everywhere else they do their things today.  With this patch series
      fanotify can be implemented in less than 1200 lines of easy to review code.
      Almost all of which is the socket based user interface.
      This patch series builds fsnotify to the point that it can implement
      dnotify and inotify_user.  Patches exist and will be sent soon after
      acceptance to finish the in kernel inotify conversion (audit) and implement
      Signed-off-by: default avatarEric Paris <eparis@redhat.com>
      Acked-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
      Cc: Christoph Hellwig <hch@lst.de>