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Commit b74c79e9 authored by Nick Piggin's avatar Nick Piggin
Browse files

fs: provide rcu-walk aware permission i_ops 



Signed-off-by: default avatarNick Piggin <npiggin@kernel.dk>
parent 34286d66

Documentation/filesystems/Locking

+3 −3
Original line number Original line Diff line number Diff line
@@ -47,8 +47,8 @@ ata *); 
	void * (*follow_link) (struct dentry *, struct nameidata *);

	void * (*follow_link) (struct dentry *, struct nameidata *);

	void (*put_link) (struct dentry *, struct nameidata *, void *);

	void (*put_link) (struct dentry *, struct nameidata *, void *);

	void (*truncate) (struct inode *);

	void (*truncate) (struct inode *);

	int (*permission) (struct inode *, int, struct nameidata *);

	int (*permission) (struct inode *, int, unsigned int);

	int (*check_acl)(struct inode *, int);

	int (*check_acl)(struct inode *, int, unsigned int);

	int (*setattr) (struct dentry *, struct iattr *);

	int (*setattr) (struct dentry *, struct iattr *);

	int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);

	int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);

	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);

	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
@@ -76,7 +76,7 @@ follow_link: no 
put_link:	no

put_link:	no

truncate:	yes		(see below)

truncate:	yes		(see below)

setattr:	yes

setattr:	yes

permission:	no

permission:	no (may not block if called in rcu-walk mode)

check_acl:	no

check_acl:	no

getattr:	no

getattr:	no

setxattr:	yes

setxattr:	yes

Documentation/filesystems/path-lookup.txt

+41 −3
Original line number Original line Diff line number Diff line
@@ -316,11 +316,9 @@ The detailed design for rcu-walk is like this: 




The cases where rcu-walk cannot continue are:

The cases where rcu-walk cannot continue are:

* NULL dentry (ie. any uncached path element)

* NULL dentry (ie. any uncached path element)

* parent with d_inode->i_op->permission or ACLs

* Following links

* Following links





In future patches, permission checks become rcu-walk aware. It may be possible

It may be possible eventually to make following links rcu-walk aware.

eventually to make following links rcu-walk aware.





Uncached path elements will always require dropping to ref-walk mode, at the

Uncached path elements will always require dropping to ref-walk mode, at the

very least because i_mutex needs to be grabbed, and objects allocated.

very least because i_mutex needs to be grabbed, and objects allocated.
@@ -336,9 +334,49 @@ or stored into. The result is massive improvements in performance and 
scalability of path resolution.

scalability of path resolution.









Interesting statistics

======================



The following table gives rcu lookup statistics for a few simple workloads

(2s12c24t Westmere, debian non-graphical system). Ungraceful are attempts to

drop rcu that fail due to d_seq failure and requiring the entire path lookup

again. Other cases are successful rcu-drops that are required before the final

element, nodentry for missing dentry, revalidate for filesystem revalidate

routine requiring rcu drop, permission for permission check requiring drop,

and link for symlink traversal requiring drop.



     rcu-lookups     restart  nodentry          link  revalidate  permission

bootup     47121           0      4624          1010       10283        7852

dbench  25386793           0   6778659(26.7%)     55         549        1156

kbuild   2696672          10     64442(2.3%)  108764(4.0%)     1        1590

git diff   39605           0        28             2           0         106

vfstest 24185492        4945    708725(2.9%) 1076136(4.4%)     0        2651



What this shows is that failed rcu-walk lookups, ie. ones that are restarted

entirely with ref-walk, are quite rare. Even the "vfstest" case which

specifically has concurrent renames/mkdir/rmdir/ creat/unlink/etc to excercise

such races is not showing a huge amount of restarts.



Dropping from rcu-walk to ref-walk mean that we have encountered a dentry where

the reference count needs to be taken for some reason. This is either because

we have reached the target of the path walk, or because we have encountered a

condition that can't be resolved in rcu-walk mode.  Ideally, we drop rcu-walk

only when we have reached the target dentry, so the other statistics show where

this does not happen.



Note that a graceful drop from rcu-walk mode due to something such as the

dentry not existing (which can be common) is not necessarily a failure of

rcu-walk scheme, because some elements of the path may have been walked in

rcu-walk mode. The further we get from common path elements (such as cwd or

root), the less contended the dentry is likely to be. The closer we are to

common path elements, the more likely they will exist in dentry cache.





Papers and other documentation on dcache locking

Papers and other documentation on dcache locking

================================================

================================================





1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).

1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).





2. http://lse.sourceforge.net/locking/dcache/dcache.html

2. http://lse.sourceforge.net/locking/dcache/dcache.html

Documentation/filesystems/porting

+5 −0
Original line number Original line Diff line number Diff line
@@ -379,4 +379,9 @@ where possible. 
the filesystem provides it), which requires dropping out of rcu-walk mode. This

the filesystem provides it), which requires dropping out of rcu-walk mode. This

may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be

may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be

returned if the filesystem cannot handle rcu-walk. See

returned if the filesystem cannot handle rcu-walk. See

Documentation/filesystems/vfs.txt for more details.



	permission and check_acl are inode permission checks that are called

on many or all directory inodes on the way down a path walk (to check for

exec permission). These must now be rcu-walk aware (flags & IPERM_RCU). See

Documentation/filesystems/vfs.txt for more details.
 
Documentation/filesystems/vfs.txt for more details.

Documentation/filesystems/vfs.txt

+9 −1
Original line number Original line Diff line number Diff line
@@ -325,7 +325,8 @@ struct inode_operations { 
        void * (*follow_link) (struct dentry *, struct nameidata *);

        void * (*follow_link) (struct dentry *, struct nameidata *);

        void (*put_link) (struct dentry *, struct nameidata *, void *);

        void (*put_link) (struct dentry *, struct nameidata *, void *);

	void (*truncate) (struct inode *);

	void (*truncate) (struct inode *);

	int (*permission) (struct inode *, int, struct nameidata *);

	int (*permission) (struct inode *, int, unsigned int);

	int (*check_acl)(struct inode *, int, unsigned int);

	int (*setattr) (struct dentry *, struct iattr *);

	int (*setattr) (struct dentry *, struct iattr *);

	int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);

	int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);

	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);

	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
@@ -414,6 +415,13 @@ otherwise noted. 
  permission: called by the VFS to check for access rights on a POSIX-like

  permission: called by the VFS to check for access rights on a POSIX-like

  	filesystem.

  	filesystem.





	May be called in rcu-walk mode (flags & IPERM_RCU). If in rcu-walk

	mode, the filesystem must check the permission without blocking or

	storing to the inode.



	If a situation is encountered that rcu-walk cannot handle, return

	-ECHILD and it will be called again in ref-walk mode.



  setattr: called by the VFS to set attributes for a file. This method

  setattr: called by the VFS to set attributes for a file. This method

  	is called by chmod(2) and related system calls.

  	is called by chmod(2) and related system calls.

drivers/staging/smbfs/file.c

+4 −1
Original line number Original line Diff line number Diff line
@@ -407,11 +407,14 @@ smb_file_release(struct inode *inode, struct file * file) 
 * privileges, so we need our own check for this.

 * privileges, so we need our own check for this.

 */

 */

static int

static int

smb_file_permission(struct inode *inode, int mask)

smb_file_permission(struct inode *inode, int mask, unsigned int flags)

{

{

	int mode = inode->i_mode;

	int mode = inode->i_mode;

	int error = 0;

	int error = 0;





	if (flags & IPERM_FLAG_RCU)

		return -ECHILD;



	VERBOSE("mode=%x, mask=%x\n", mode, mask);

	VERBOSE("mode=%x, mask=%x\n", mode, mask);





	/* Look at user permissions */

	/* Look at user permissions */