linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Date:	Sat, 31 Aug 2013 03:35:16 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Waiman Long <waiman.long@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Fri, Aug 30, 2013 at 10:02:36PM -0400, Waiman Long wrote:

> I slapped in the code segment, and the following was logged:
> 
> [  340.871590] type = tmpfs
> [  340.871712]  [<ffffffff8116c5bd>] __fput+0x23d/0x270
> [  340.871715]  [<ffffffff8116c699>] ____fput+0x9/0x10
> [  340.871719]  [<ffffffff81068ae1>] task_work_run+0xb1/0xe0
> [  340.871724]  [<ffffffff81002990>] do_notify_resume+0x80/0x1b0
> [  340.871728]  [<ffffffff811ed120>] ? ipc_lock+0x30/0x50
> [  340.871732]  [<ffffffff8113a356>] ? remove_vma+0x56/0x60
> [  340.871736]  [<ffffffff8113c1bf>] ? do_munmap+0x34f/0x380
> [  340.871741]  [<ffffffff814a5fda>] int_signal+0x12/0x17
> [  340.871744] ---[ end trace aafa6c45f3388d65 ]---

Aha...  OK, I see what's going on.  We end up with shm_mnt *not* marked
as long-living vfsmount, even though it lives forever.  See if the
following helps; if it does (and I very much expect it to), we want to
put it in -stable.  As it is, you get slow path in mntput() each time
a file created by shmem_file_setup() gets closed.  For no reason whatsoever...

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/mm/shmem.c b/mm/shmem.c
index e43dc55..445162c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2615,7 +2615,7 @@ 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)) {
@@ -2831,8 +2831,7 @@ 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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