3d733633a6
This is the real meat of the entire series. It actually implements the tracking of the number of writers to a mount. However, it causes scalability problems because there can be hundreds of cpus doing open()/close() on files on the same mnt at the same time. Even an atomic_t in the mnt has massive scalaing problems because the cacheline gets so terribly contended. This uses a statically-allocated percpu variable. All want/drop operations are local to a cpu as long that cpu operates on the same mount, and there are no writer count imbalances. Writer count imbalances happen when a write is taken on one cpu, and released on another, like when an open/close pair is performed on two Upon a remount,ro request, all of the data from the percpu variables is collected (expensive, but very rare) and we determine if there are any outstanding writers to the mount. I've written a little benchmark to sit in a loop for a couple of seconds in several cpus in parallel doing open/write/close loops. http://sr71.net/~dave/linux/openbench.c The code in here is a a worst-possible case for this patch. It does opens on a _pair_ of files in two different mounts in parallel. This should cause my code to lose its "operate on the same mount" optimization completely. This worst-case scenario causes a 3% degredation in the benchmark. I could probably get rid of even this 3%, but it would be more complex than what I have here, and I think this is getting into acceptable territory. In practice, I expect writing more than 3 bytes to a file, as well as disk I/O to mask any effects that this has. (To get rid of that 3%, we could have an #defined number of mounts in the percpu variable. So, instead of a CPU getting operate only on percpu data when it accesses only one mount, it could stay on percpu data when it only accesses N or fewer mounts.) [AV] merged fix for __clear_mnt_mount() stepping on freed vfsmount Acked-by: Al Viro <viro@ZenIV.linux.org.uk> Signed-off-by: Christoph Hellwig <hch@infradead.org> Signed-off-by: Dave Hansen <haveblue@us.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
118 lines
3.5 KiB
C
118 lines
3.5 KiB
C
/*
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*
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* Definitions for mount interface. This describes the in the kernel build
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* linkedlist with mounted filesystems.
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*
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* Author: Marco van Wieringen <mvw@planets.elm.net>
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*
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* Version: $Id: mount.h,v 2.0 1996/11/17 16:48:14 mvw Exp mvw $
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*
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*/
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#ifndef _LINUX_MOUNT_H
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#define _LINUX_MOUNT_H
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#ifdef __KERNEL__
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#include <linux/types.h>
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#include <linux/list.h>
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#include <linux/nodemask.h>
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#include <linux/spinlock.h>
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#include <asm/atomic.h>
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struct super_block;
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struct vfsmount;
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struct dentry;
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struct mnt_namespace;
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#define MNT_NOSUID 0x01
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#define MNT_NODEV 0x02
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#define MNT_NOEXEC 0x04
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#define MNT_NOATIME 0x08
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#define MNT_NODIRATIME 0x10
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#define MNT_RELATIME 0x20
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#define MNT_SHRINKABLE 0x100
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#define MNT_IMBALANCED_WRITE_COUNT 0x200 /* just for debugging */
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#define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */
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#define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */
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#define MNT_PNODE_MASK 0x3000 /* propagation flag mask */
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struct vfsmount {
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struct list_head mnt_hash;
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struct vfsmount *mnt_parent; /* fs we are mounted on */
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struct dentry *mnt_mountpoint; /* dentry of mountpoint */
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struct dentry *mnt_root; /* root of the mounted tree */
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struct super_block *mnt_sb; /* pointer to superblock */
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struct list_head mnt_mounts; /* list of children, anchored here */
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struct list_head mnt_child; /* and going through their mnt_child */
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int mnt_flags;
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/* 4 bytes hole on 64bits arches */
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char *mnt_devname; /* Name of device e.g. /dev/dsk/hda1 */
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struct list_head mnt_list;
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struct list_head mnt_expire; /* link in fs-specific expiry list */
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struct list_head mnt_share; /* circular list of shared mounts */
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struct list_head mnt_slave_list;/* list of slave mounts */
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struct list_head mnt_slave; /* slave list entry */
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struct vfsmount *mnt_master; /* slave is on master->mnt_slave_list */
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struct mnt_namespace *mnt_ns; /* containing namespace */
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/*
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* We put mnt_count & mnt_expiry_mark at the end of struct vfsmount
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* to let these frequently modified fields in a separate cache line
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* (so that reads of mnt_flags wont ping-pong on SMP machines)
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*/
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atomic_t mnt_count;
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int mnt_expiry_mark; /* true if marked for expiry */
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int mnt_pinned;
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int mnt_ghosts;
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/*
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* This value is not stable unless all of the mnt_writers[] spinlocks
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* are held, and all mnt_writer[]s on this mount have 0 as their ->count
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*/
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atomic_t __mnt_writers;
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};
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static inline struct vfsmount *mntget(struct vfsmount *mnt)
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{
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if (mnt)
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atomic_inc(&mnt->mnt_count);
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return mnt;
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}
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extern int mnt_want_write(struct vfsmount *mnt);
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extern void mnt_drop_write(struct vfsmount *mnt);
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extern void mntput_no_expire(struct vfsmount *mnt);
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extern void mnt_pin(struct vfsmount *mnt);
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extern void mnt_unpin(struct vfsmount *mnt);
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extern int __mnt_is_readonly(struct vfsmount *mnt);
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static inline void mntput(struct vfsmount *mnt)
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{
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if (mnt) {
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mnt->mnt_expiry_mark = 0;
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mntput_no_expire(mnt);
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}
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}
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extern void free_vfsmnt(struct vfsmount *mnt);
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extern struct vfsmount *alloc_vfsmnt(const char *name);
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extern struct vfsmount *do_kern_mount(const char *fstype, int flags,
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const char *name, void *data);
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struct file_system_type;
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extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
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int flags, const char *name,
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void *data);
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struct nameidata;
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extern int do_add_mount(struct vfsmount *newmnt, struct nameidata *nd,
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int mnt_flags, struct list_head *fslist);
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extern void mark_mounts_for_expiry(struct list_head *mounts);
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extern spinlock_t vfsmount_lock;
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extern dev_t name_to_dev_t(char *name);
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#endif
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#endif /* _LINUX_MOUNT_H */
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