On Sun, Sep 01, 2013 at 01:59:22PM -0700, Linus Torvalds wrote:
> [ Side note: Al, that lg_local_lock really is annoying: it's
> br_read_lock(mntput_no_expire), with two thirds of the calls coming
> from mntput_no_expire, and the rest from path_init -> lock_rcu_walk.
How much of that is due to br_write_lock() taken in mntput_no_expire()
for no good reason? IOW, could you try shmem.c patch I've sent yesterday
and see how much effect does it have?[1] Basically, we get it grabbed
exclusive on each final fput() of a struct file created by shmem_file_setup(),
which is _not_ a rare event. And the only reason for that is not having
shm_mnt marked long-living, even though its refcount never hits 0...
> I really really wonder if we could get rid of the
> br_read_lock(&vfsmount_lock) for rcu_walk_init(), and use just the RCU
> read accesses for the mount-namespaces too. What is that lock really
> protecting against during lookup anyway? ]
A lot of things, I'm afraid. It's not as simple as just the access
to vfsmount hash... ;-/ I'll need to do some digging to put together
a full analysis, but there had been quite a few subtle issues where
it played...
[1] sits in the local queue, will push tonight:
commit e7db6c4c1d01032f53262f03b5f38899f9db8add
Author: Al Viro <v...@zeniv.linux.org.uk>
Date: Sat Aug 31 12:57:10 2013 -0400
shm_mnt is as longterm as it gets, TYVM...
Signed-off-by: Al Viro <v...@zeniv.linux.org.uk>
diff --git a/mm/shmem.c b/mm/shmem.c
index e43dc55..5261498 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2615,13 +2615,15 @@ int shmem_fill_super(struct super_block *sb, void
*data, int silent)
* tmpfs instance, limiting inodes to one per page of lowmem;
* but the internal instance is left unlimited.
*/
- if (!(sb->s_flags & MS_NOUSER)) {
+ if (!(sb->s_flags & MS_KERNMOUNT)) {
sbinfo->max_blocks = shmem_default_max_blocks();
sbinfo->max_inodes = shmem_default_max_inodes();
if (shmem_parse_options(data, sbinfo, false)) {
err = -EINVAL;
goto failed;
}
+ } else {
+ sb->s_flags |= MS_NOUSER;
}
sb->s_export_op = &shmem_export_ops;
sb->s_flags |= MS_NOSEC;
@@ -2831,8 +2833,7 @@ int __init shmem_init(void)
goto out2;
}
- shm_mnt = vfs_kern_mount(&shmem_fs_type, MS_NOUSER,
- shmem_fs_type.name, NULL);
+ shm_mnt = kern_mount(&shmem_fs_type);
if (IS_ERR(shm_mnt)) {
error = PTR_ERR(shm_mnt);
printk(KERN_ERR "Could not kern_mount tmpfs\n");
--
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