Documentation/filesystems/aufs/design/10dynop.txt

   1
   2 # Copyright (C) 2010-2016 Junjiro R. Okajima
   3 #
   4 # This program is free software; you can redistribute it and/or modify
   5 # it under the terms of the GNU General Public License as published by
   6 # the Free Software Foundation; either version 2 of the License, or
   7 # (at your option) any later version.
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   9 # This program is distributed in the hope that it will be useful,
  10 # but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 # GNU General Public License for more details.
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  14 # You should have received a copy of the GNU General Public License
  15 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
  16
  17 Dynamically customizable FS operations
  18 ----------------------------------------------------------------------
  19 Generally FS operations (struct inode_operations, struct
  20 address_space_operations, struct file_operations, etc.) are defined as
  21 "static const", but it never means that FS have only one set of
  22 operation. Some FS have multiple sets of them. For instance, ext2 has
  23 three sets, one for XIP, for NOBH, and for normal.
  24 Since aufs overrides and redirects these operations, sometimes aufs has
  25 to change its behaviour according to the branch FS type. More importantly
  26 VFS acts differently if a function (member in the struct) is set or
  27 not. It means aufs should have several sets of operations and select one
  28 among them according to the branch FS definition.
  29
  30 In order to solve this problem and not to affect the behaviour of VFS,
  31 aufs defines these operations dynamically. For instance, aufs defines
  32 dummy direct_IO function for struct address_space_operations, but it may
  33 not be set to the address_space_operations actually. When the branch FS
  34 doesn't have it, aufs doesn't set it to its address_space_operations
  35 while the function definition itself is still alive. So the behaviour
  36 itself will not change, and it will return an error when direct_IO is
  37 not set.
  38
  39 The lifetime of these dynamically generated operation object is
  40 maintained by aufs branch object. When the branch is removed from aufs,
  41 the reference counter of the object is decremented. When it reaches
  42 zero, the dynamically generated operation object will be freed.
  43
  44 This approach is designed to support AIO (io_submit), Direct I/O and
  45 XIP (DAX) mainly.
  46 Currently this approach is applied to address_space_operations for
  47 regular files only.