[Devel] [PATCH 5/5][v5][cr]: Document design of C/R of file-locks

[Sukadev Bhattiprolu]

Summarize the file-system consistency requirements and the design of
the C/R of file-locks and leases.

Changelog[v5]:
	- This version of the patchset only checkpoints/restores file-locks.
	  C/R of file-owner information requires additional work with struct
	  pids and will be addressed in a follow-on patch. C/R of file-leases,
	  depends on C/R of file-owner info Removed the design information of
	  C/R of file leases from the Documenation for now.

Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
---
 Documentation/checkpoint/file-locks |   52 +++++++++++++++++++++++++++++++++++
 1 files changed, 52 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/checkpoint/file-locks

diff --git a/Documentation/checkpoint/file-locks b/Documentation/checkpoint/file-locks
new file mode 100644
index 0000000..ccffdef
--- /dev/null
+++ b/Documentation/checkpoint/file-locks
@@ -0,0 +1,52 @@
+
+Filesystem consistency across C/R.
+==================================
+
+To checkpoint/restart a process that is using any filesystem resource, the
+kernel assumes that the file system state at the time of restart is consistent
+with its state at the time of checkpoint. In general, this consistency can be
+achieved by:
+
+	a. running the application inside a container (to ensure no process
+	   outside the container modifies the filesystem/IPC or other states)
+
+	b. freezing the application before checkpoint
+	c. taking a snapshot of the file system while application is frozen
+	d. checkpointing the application while it is frozen
+
+	e. restoring the file system state to its snapshot
+	f. restart the application inside a container
+
+i.e the kernel assumes that file system state is consistent but it does/can
+NOT verify that it is. The administrator must provide this consistency taking
+into account the file system type including whether it is local or remote,
+and the tools available in the file system (snapshot tools in btrfs or rsync
+etc).
+
+For distributed applications operating on distributed filesystems, it is
+expected that an external mechanism will coordinate the freeze/checkpoint/
+snapshot/restart across the nodes. IOW, the current semantics in the kernel
+provide for C/R on a single node.
+
+Checkpoint/restart of file-locks.
+================================
+
+To checkpoint file-locks in an application, we start with each file-descriptor
+and count the number of file-locks on that file-descriptor. We save this count
+in the checkpoint image, and then information about each file-lock on the
+file-descriptor.
+
+When restarting the application from the checkpoint, we read the file-lock
+count for each file-descriptor and then read the information about each
+file-lock. For each file-lock, we call flock_set() to set a new file-lock.
+
+No special handling is necessary for a process P2 in the checkpointed container
+that is blocked on a file-lock, L1 held by another process P1. Processes in the
+restarted container begin execution only after all processes have restored.
+If the blocked process P2 is restored first, it will prepare to return an
+-ERESTARTSYS from the fcntl() system call, but wait for P1 to be restored.
+When P1 is restored, it will re-acquire the file-lock L1 before P1 and P2 begin
+actual execution.
+
+This ensures that even if P2 is scheduled to run before P1, P2 will go
+back to waiting for the file-lock L1.
-- 
1.6.0.4

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