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Diffstat (limited to 'Documentation/filesystems/aufs/design/06mmap.txt')
| -rw-r--r-- | Documentation/filesystems/aufs/design/06mmap.txt | 72 |
1 files changed, 0 insertions, 72 deletions
diff --git a/Documentation/filesystems/aufs/design/06mmap.txt b/Documentation/filesystems/aufs/design/06mmap.txt deleted file mode 100644 index a42364eee..000000000 --- a/Documentation/filesystems/aufs/design/06mmap.txt +++ /dev/null @@ -1,72 +0,0 @@ - -# Copyright (C) 2005-2015 Junjiro R. Okajima -# -# This program is free software; you can redistribute it and/or modify -# it under the terms of the GNU General Public License as published by -# the Free Software Foundation; either version 2 of the License, or -# (at your option) any later version. -# -# This program is distributed in the hope that it will be useful, -# but WITHOUT ANY WARRANTY; without even the implied warranty of -# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the -# GNU General Public License for more details. -# -# You should have received a copy of the GNU General Public License -# along with this program. If not, see <http://www.gnu.org/licenses/>. - -mmap(2) -- File Memory Mapping ----------------------------------------------------------------------- -In aufs, the file-mapped pages are handled by a branch fs directly, no -interaction with aufs. It means aufs_mmap() calls the branch fs's -->mmap(). -This approach is simple and good, but there is one problem. -Under /proc, several entries show the mmapped files by its path (with -device and inode number), and the printed path will be the path on the -branch fs's instead of virtual aufs's. -This is not a problem in most cases, but some utilities lsof(1) (and its -user) may expect the path on aufs. - -To address this issue, aufs adds a new member called vm_prfile in struct -vm_area_struct (and struct vm_region). The original vm_file points to -the file on the branch fs in order to handle everything correctly as -usual. The new vm_prfile points to a virtual file in aufs, and the -show-functions in procfs refers to vm_prfile if it is set. -Also we need to maintain several other places where touching vm_file -such like -- fork()/clone() copies vma and the reference count of vm_file is - incremented. -- merging vma maintains the ref count too. - -This is not a good approach. It just fakes the printed path. But it -leaves all behaviour around f_mapping unchanged. This is surely an -advantage. -Actually aufs had adopted another complicated approach which calls -generic_file_mmap() and handles struct vm_operations_struct. In this -approach, aufs met a hard problem and I could not solve it without -switching the approach. - -There may be one more another approach which is -- bind-mount the branch-root onto the aufs-root internally -- grab the new vfsmount (ie. struct mount) -- lazy-umount the branch-root internally -- in open(2) the aufs-file, open the branch-file with the hidden - vfsmount (instead of the original branch's vfsmount) -- ideally this "bind-mount and lazy-umount" should be done atomically, - but it may be possible from userspace by the mount helper. - -Adding the internal hidden vfsmount and using it in opening a file, the -file path under /proc will be printed correctly. This approach looks -smarter, but is not possible I am afraid. -- aufs-root may be bind-mount later. when it happens, another hidden - vfsmount will be required. -- it is hard to get the chance to bind-mount and lazy-umount - + in kernel-space, FS can have vfsmount in open(2) via - file->f_path, and aufs can know its vfsmount. But several locks are - already acquired, and if aufs tries to bind-mount and lazy-umount - here, then it may cause a deadlock. - + in user-space, bind-mount doesn't invoke the mount helper. -- since /proc shows dev and ino, aufs has to give vma these info. it - means a new member vm_prinode will be necessary. this is essentially - equivalent to vm_prfile described above. - -I have to give up this "looks-smater" approach. |