allowed to root, but this restriction can be removed with a
(userspace) configuration option.
-'kernel_cache'
+'max_read=N'
- This option disables flushing the cache of the file contents on
- every open(). This should only be enabled on filesystems, where the
- file data is never changed externally (not through the mounted FUSE
- filesystem). Thus it is not suitable for network filesystems and
- other "intermediate" filesystems.
+ With this option the maximum size of read operations can be set.
+ The default is infinite. Note that the size of read requests is
+ limited anyway to 32 pages (which is 128kbyte on i386).
- NOTE: if this option is not specified (and neither 'direct_io') data
- is still cached after the open(), so a read() system call will not
- always initiate a read operation.
+Sysfs
+~~~~~
-'direct_io'
+FUSE sets up the following hierarchy in sysfs:
- This option disables the use of page cache (file content cache) in
- the kernel for this filesystem. This has several affects:
+ /sys/fs/fuse/connections/N/
- - Each read() or write() system call will initiate one or more
- read or write operations, data will not be cached in the
- kernel.
+where N is an increasing number allocated to each new connection.
- - The return value of the read() and write() system calls will
- correspond to the return values of the read and write
- operations. This is useful for example if the file size is not
- known in advance (before reading it).
+For each connection the following attributes are defined:
-'max_read=N'
+ 'waiting'
- With this option the maximum size of read operations can be set.
- The default is infinite. Note that the size of read requests is
- limited anyway to 32 pages (which is 128kbyte on i386).
+ The number of requests which are waiting to be transfered to
+ userspace or being processed by the filesystem daemon. If there is
+ no filesystem activity and 'waiting' is non-zero, then the
+ filesystem is hung or deadlocked.
+
+ 'abort'
+
+ Writing anything into this file will abort the filesystem
+ 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.
+
+Aborting a filesystem connection
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is possible to get into certain situations where the filesystem is
+not responding. Reasons for this may be:
+
+ a) Broken userspace filesystem implementation
+
+ b) Network connection down
+
+ c) Accidental deadlock
+
+ d) Malicious deadlock
+
+(For more on c) and d) see later sections)
+
+In either of these cases it may be useful to abort the connection to
+the filesystem. There are several ways to do this:
+
+ - Kill the filesystem daemon. Works in case of a) and b)
+
+ - Kill the filesystem daemon and all users of the filesystem. Works
+ in all cases except some malicious deadlocks
+
+ - 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.
How do non-privileged mounts work?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
when the copy is taking place, and interruption is delayed until
this flag is unset.
+Scenario 3 - Tricky deadlock with asynchronous read
+---------------------------------------------------
+
+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.