Merge master.kernel.org:/pub/scm/linux/kernel/git/davem/sparc-2.6

[powerpc.git] / Documentation / filesystems / fuse.txt

diff --git a/Documentation/filesystems/fuse.txt b/Documentation/filesystems/fuse.txt
index 33f7431..a584f05 100644 (file)
--- a/Documentation/filesystems/fuse.txt
+++ b/Documentation/filesystems/fuse.txt
@@ -18,6 +18,14 @@ Non-privileged mount (or user mount):
   user.  NOTE: this is not the same as mounts allowed with the "user"
   option in /etc/fstab, which is not discussed here.
 
   user.  NOTE: this is not the same as mounts allowed with the "user"
   option in /etc/fstab, which is not discussed here.
 

+Filesystem connection:
+
+  A connection between the filesystem daemon and the kernel.  The
+  connection exists until either the daemon dies, or the filesystem is
+  umounted.  Note that detaching (or lazy umounting) the filesystem
+  does _not_ break the connection, in this case it will exist until
+  the last reference to the filesystem is released.
+

 Mount owner:
 
   The user who does the mounting.
 Mount owner:
 
   The user who does the mounting.


@@ -86,16 +94,20 @@ Mount options
   The default is infinite.  Note that the size of read requests is
   limited anyway to 32 pages (which is 128kbyte on i386).
 
   The default is infinite.  Note that the size of read requests is
   limited anyway to 32 pages (which is 128kbyte on i386).
 

-Sysfs
-~~~~~
+Control filesystem
+~~~~~~~~~~~~~~~~~~
+
+There's a control filesystem for FUSE, which can be mounted by:

 
 

-FUSE sets up the following hierarchy in sysfs:
+  mount -t fusectl none /sys/fs/fuse/connections

 
 

-  /sys/fs/fuse/connections/N/
+Mounting it under the '/sys/fs/fuse/connections' directory makes it
+backwards compatible with earlier versions.

 
 

-where N is an increasing number allocated to each new connection.
+Under the fuse control filesystem each connection has a directory
+named by a unique number.

 
 

-For each connection the following attributes are defined:
+For each connection the following files exist within this directory:

 
  'waiting'
 
 
  'waiting'
 


@@ -110,7 +122,47 @@ For each connection the following attributes are defined:
   connection.  This means that all waiting requests will be aborted an
   error returned for all aborted and new requests.
 
   connection.  This means that all waiting requests will be aborted an
   error returned for all aborted and new requests.
 

-Only a privileged user may read or write these attributes.
+Only the owner of the mount may read or write these files.
+
+Interrupting filesystem operations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If a process issuing a FUSE filesystem request is interrupted, the
+following will happen:
+
+  1) If the request is not yet sent to userspace AND the signal is
+     fatal (SIGKILL or unhandled fatal signal), then the request is
+     dequeued and returns immediately.
+
+  2) If the request is not yet sent to userspace AND the signal is not
+     fatal, then an 'interrupted' flag is set for the request.  When
+     the request has been successfully transfered to userspace and
+     this flag is set, an INTERRUPT request is queued.
+
+  3) If the request is already sent to userspace, then an INTERRUPT
+     request is queued.
+
+INTERRUPT requests take precedence over other requests, so the
+userspace filesystem will receive queued INTERRUPTs before any others.
+
+The userspace filesystem may ignore the INTERRUPT requests entirely,
+or may honor them by sending a reply to the _original_ request, with
+the error set to EINTR.
+
+It is also possible that there's a race between processing the
+original request and it's INTERRUPT request.  There are two possibilities:
+
+  1) The INTERRUPT request is processed before the original request is
+     processed
+
+  2) The INTERRUPT request is processed after the original request has
+     been answered
+
+If the filesystem cannot find the original request, it should wait for
+some timeout and/or a number of new requests to arrive, after which it
+should reply to the INTERRUPT request with an EAGAIN error.  In case
+1) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
+reply will be ignored.

 
 Aborting a filesystem connection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Aborting a filesystem connection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


@@ -139,8 +191,8 @@ the filesystem.  There are several ways to do this:
   - Use forced umount (umount -f).  Works in all cases but only if
     filesystem is still attached (it hasn't been lazy unmounted)
 
   - Use forced umount (umount -f).  Works in all cases but only if
     filesystem is still attached (it hasn't been lazy unmounted)
 

-  - Abort filesystem through the sysfs interface.  Most powerful
-    method, always works.
+  - Abort filesystem through the FUSE control filesystem.  Most
+    powerful method, always works.

 
 How do non-privileged mounts work?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 How do non-privileged mounts work?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


@@ -304,25 +356,7 @@ Scenario 1 -  Simple deadlock
  |                                    |     for "file"]
  |                                    |    *DEADLOCK*
 
  |                                    |     for "file"]
  |                                    |    *DEADLOCK*
 

-The solution for this is to allow requests to be interrupted while
-they are in userspace:
-
- |      [interrupted by signal]       |
- |    <fuse_unlink()                  |
- |    [release semaphore]             |    [semaphore acquired]
- |  <sys_unlink()                     |
- |                                    |    >fuse_unlink()
- |                                    |      [queue req on fc->pending]
- |                                    |      [wake up fc->waitq]
- |                                    |      [sleep on req->waitq]
-
-If the filesystem daemon was single threaded, this will stop here,
-since there's no other thread to dequeue and execute the request.
-In this case the solution is to kill the FUSE daemon as well.  If
-there are multiple serving threads, you just have to kill them as
-long as any remain.
-
-Moral: a filesystem which deadlocks, can soon find itself dead.
+The solution for this is to allow the filesystem to be aborted.

 
 Scenario 2 - Tricky deadlock
 ----------------------------
 
 Scenario 2 - Tricky deadlock
 ----------------------------


@@ -355,24 +389,14 @@ but is caused by a pagefault.
  |                                    |           [lock page]
  |                                    |           * DEADLOCK *
 
  |                                    |           [lock page]
  |                                    |           * DEADLOCK *
 

-Solution is again to let the the request be interrupted (not
-elaborated further).
-
-An additional problem is that while the write buffer is being
-copied to the request, the request must not be interrupted.  This
-is because the destination address of the copy may not be valid
-after the request is interrupted.
-
-This is solved with doing the copy atomically, and allowing
-interruption while the page(s) belonging to the write buffer are
-faulted with get_user_pages().  The 'req->locked' flag indicates
-when the copy is taking place, and interruption is delayed until
-this flag is unset.
+Solution is basically the same as above.

 
 

-Scenario 3 - Tricky deadlock with asynchronous read
----------------------------------------------------
+An additional problem is that while the write buffer is being copied
+to the request, the request must not be interrupted/aborted.  This is
+because the destination address of the copy may not be valid after the
+request has returned.

 
 

-The same situation as above, except thread-1 will wait on page lock
-and hence it will be uninterruptible as well.  The solution is to
-abort the connection with forced umount (if mount is attached) or
-through the abort attribute in sysfs.
+This is solved with doing the copy atomically, and allowing abort
+while the page(s) belonging to the write buffer are faulted with
+get_user_pages().  The 'req->locked' flag indicates when the copy is
+taking place, and abort is delayed until this flag is unset.