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authorAl Viro <viro@zeniv.linux.org.uk>2014-02-27 09:35:45 -0500
committerAl Viro <viro@zeniv.linux.org.uk>2014-04-01 23:19:08 -0400
commitf2ebb3a921c1ca1e2ddd9242e95a1989a50c4c68 (patch)
tree0a14b8ec08d94aff427a5446b8b5c4e68b4e032e /include/linux/mount.h
parent38129a13e6e71f666e0468e99fdd932a687b4d7e (diff)
smarter propagate_mnt()
The current mainline has copies propagated to *all* nodes, then tears down the copies we made for nodes that do not contain counterparts of the desired mountpoint. That sets the right propagation graph for the copies (at teardown time we move the slaves of removed node to a surviving peer or directly to master), but we end up paying a fairly steep price in useless allocations. It's fairly easy to create a situation where N calls of mount(2) create exactly N bindings, with O(N^2) vfsmounts allocated and freed in process. Fortunately, it is possible to avoid those allocations/freeings. The trick is to create copies in the right order and find which one would've eventually become a master with the current algorithm. It turns out to be possible in O(nodes getting propagation) time and with no extra allocations at all. One part is that we need to make sure that eventual master will be created before its slaves, so we need to walk the propagation tree in a different order - by peer groups. And iterate through the peers before dealing with the next group. Another thing is finding the (earlier) copy that will be a master of one we are about to create; to do that we are (temporary) marking the masters of mountpoints we are attaching the copies to. Either we are in a peer of the last mountpoint we'd dealt with, or we have the following situation: we are attaching to mountpoint M, the last copy S_0 had been attached to M_0 and there are sequences S_0...S_n, M_0...M_n such that S_{i+1} is a master of S_{i}, S_{i} mounted on M{i} and we need to create a slave of the first S_{k} such that M is getting propagation from M_{k}. It means that the master of M_{k} will be among the sequence of masters of M. On the other hand, the nearest marked node in that sequence will either be the master of M_{k} or the master of M_{k-1} (the latter - in the case if M_{k-1} is a slave of something M gets propagation from, but in a wrong peer group). So we go through the sequence of masters of M until we find a marked one (P). Let N be the one before it. Then we go through the sequence of masters of S_0 until we find one (say, S) mounted on a node D that has P as master and check if D is a peer of N. If it is, S will be the master of new copy, if not - the master of S will be. That's it for the hard part; the rest is fairly simple. Iterator is in next_group(), handling of one prospective mountpoint is propagate_one(). It seems to survive all tests and gives a noticably better performance than the current mainline for setups that are seriously using shared subtrees. Cc: stable@vger.kernel.org Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'include/linux/mount.h')
-rw-r--r--include/linux/mount.h3
1 files changed, 3 insertions, 0 deletions
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 371d346fa270..839bac270904 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -44,6 +44,8 @@ struct mnt_namespace;
#define MNT_SHARED_MASK (MNT_UNBINDABLE)
#define MNT_PROPAGATION_MASK (MNT_SHARED | MNT_UNBINDABLE)
+#define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
+ MNT_DOOMED | MNT_SYNC_UMOUNT | MNT_MARKED)
#define MNT_INTERNAL 0x4000
@@ -51,6 +53,7 @@ struct mnt_namespace;
#define MNT_LOCKED 0x800000
#define MNT_DOOMED 0x1000000
#define MNT_SYNC_UMOUNT 0x2000000
+#define MNT_MARKED 0x4000000
struct vfsmount {
struct dentry *mnt_root; /* root of the mounted tree */