Latest attempt at unifying the various constraints for force umounting
NFS servers that have disappeared in a timely manner. Namely:
* umount2(..., MNT_FORCE) should not be made stronger, unless we know
this is the final umount()
* The "failed server" state should be reversible
* The mechanism should be able to "unstick" a sync(2) that is stuck on
an unresponsive server
I believe the proposal satisfies all of these concerns. There are a few
major components to this proposal:
1) The umount_begin superblock operation now has a corresponding
umount_end operation. This is invoked by umount() when MNT_FORCE is
specified (like umount_begin()), but the actual unmount failed (i.e.
the mount is busy).
2) Instead of killing all the RPC queued at a single point in time, the
NFS mount now kills all queue RPCs and all RPCs that get queued
between nfs_umount_begin() and nfs_umount_end(). I believe this is
not a significant change in behavior because there were always races
between queuing RPCs and killing them in nfs_umount_begin().
3) nfs_umount_end() is *not* called when MNT_DETACH is specified. This
is the indication that the user is done with this mount and all
RPCs will be killed until the mount finally gets removed.
4) The new "transient" mount option prevents sharing nfs_clients
between multiple superblocks. The only exception to this is when the
kernel does an automatic mount to cross a device boundary ("xdev"
NFS mount). In this case, the existing code always shares the
existing nfs_client from parent superblock. The "transient" mount
option implies "nosharecache", as it doesn't make sense to share
superblocks if clients aren't shared.
5) If the "transient" mount option is specified (and hence the
nfs_client is not shared), MNT_FORCE kills all RPCs for the entire
nfs_client (and all its nfs_servers). This effectively enables the
"burn down the forest" option when combined with MNT_DETACH.
The choice to use MNT_FORCE as the mechanism for triggering this
behavior stems from the desire to unstick sync(2) calls that might be
blocked on a non-responsive NFS server. While it was previously
discussed to remount with a new mount option to enable this behavior,
this cannot release the blocked sync(2) call because both
sync_fileystem() and do_remount() lock the struct superblock->s_umount
reader-writer lock. As such, a remount will block until the sync(2)
finishes, which is undesirable. umount2() doesn't have this restriction
and can unblock the sync(2) call.
For the most part, all existing behavior is unchanged if the "transient"
option is not specified. umount -f continues to behave as is has, but
umount -fl (see note below) now does a more aggressive kill to get
everything out of there and allow unmounting in a more timely manner.
Note that there will probably be some communication with the server
still, as destruction of the NFS client ID and such will occur when the
last reference is removed. If the server is truly gone, this can result
in long blocks at that time.
If it is known at mount time that the server might be disappearing, it
should be mounted with "transient". Doing this will allow mount -fl to
do a more complete cleanup and prevent all communication with the
server, which should allow a timely cleanup in all cases.
Notes:
Until recently, libmount did not allow a detached and lazy mount at the
same time. This was recently fixed (see
https://marc.info/?l=util-linux-ng&m=151000714401929&w=2). If you want
to test this, you may need to write a simple test program that directly
calls umount2() with MNT_FORCE | MNT_DETACH.
Thank you all again for your time and comments,
Joshua Watt
Joshua Watt (9):
SUNRPC: Add flag to kill new tasks
SUNRPC: Expose kill_new_tasks in debugfs
SUNRPC: Simplify client shutdown
namespace: Add umount_end superblock operation
NFS: Kill RPCs for the duration of umount
NFS: Add debugfs for nfs_server and nfs_client
NFS: Add transient mount option
NFS: Don't shared transient clients
NFS: Kill all client RPCs if transient
fs/namespace.c | 22 ++++++-
fs/nfs/Makefile | 2 +-
fs/nfs/client.c | 96 +++++++++++++++++++++++++--
fs/nfs/debugfs.c | 143 +++++++++++++++++++++++++++++++++++++++++
fs/nfs/inode.c | 5 ++
fs/nfs/internal.h | 11 ++++
fs/nfs/nfs3client.c | 2 +
fs/nfs/nfs4client.c | 5 ++
fs/nfs/nfs4super.c | 1 +
fs/nfs/super.c | 81 ++++++++++++++++++++---
include/linux/fs.h | 1 +
include/linux/nfs_fs_sb.h | 6 ++
include/linux/sunrpc/clnt.h | 1 +
include/uapi/linux/nfs_mount.h | 1 +
net/sunrpc/clnt.c | 13 ++--
net/sunrpc/debugfs.c | 5 ++
net/sunrpc/sched.c | 3 +
17 files changed, 372 insertions(+), 26 deletions(-)
create mode 100644 fs/nfs/debugfs.c
--
2.13.6
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