diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/filesystems/Locking | 6 | ||||
| -rw-r--r-- | Documentation/filesystems/path-lookup.txt | 44 | ||||
| -rw-r--r-- | Documentation/filesystems/porting | 5 | ||||
| -rw-r--r-- | Documentation/filesystems/vfs.txt | 10 |
4 files changed, 58 insertions, 7 deletions
diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking index e90ffe61eb65..977d8919cc69 100644 --- a/Documentation/filesystems/Locking +++ b/Documentation/filesystems/Locking @@ -47,8 +47,8 @@ ata *); void * (*follow_link) (struct dentry *, struct nameidata *); void (*put_link) (struct dentry *, struct nameidata *, void *); void (*truncate) (struct inode *); - int (*permission) (struct inode *, int, struct nameidata *); - int (*check_acl)(struct inode *, int); + int (*permission) (struct inode *, int, unsigned int); + int (*check_acl)(struct inode *, int, unsigned int); int (*setattr) (struct dentry *, struct iattr *); int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *); int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); @@ -76,7 +76,7 @@ follow_link: no put_link: no truncate: yes (see below) setattr: yes -permission: no +permission: no (may not block if called in rcu-walk mode) check_acl: no getattr: no setxattr: yes diff --git a/Documentation/filesystems/path-lookup.txt b/Documentation/filesystems/path-lookup.txt index 8789d1810bed..eb59c8b44be9 100644 --- a/Documentation/filesystems/path-lookup.txt +++ b/Documentation/filesystems/path-lookup.txt @@ -316,11 +316,9 @@ The detailed design for rcu-walk is like this: The cases where rcu-walk cannot continue are: * NULL dentry (ie. any uncached path element) -* parent with d_inode->i_op->permission or ACLs * Following links -In future patches, permission checks become rcu-walk aware. It may be possible -eventually to make following links rcu-walk aware. +It may be possible eventually to make following links rcu-walk aware. 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. @@ -336,9 +334,49 @@ or stored into. The result is massive improvements in performance and 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 ================================================ 1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124). 2. http://lse.sourceforge.net/locking/dcache/dcache.html + + diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting index cd9756a2709d..07a32b42cf9c 100644 --- a/Documentation/filesystems/porting +++ b/Documentation/filesystems/porting @@ -380,3 +380,8 @@ 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 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. diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index c936b4912383..fbb324e2bd43 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -325,7 +325,8 @@ struct inode_operations { void * (*follow_link) (struct dentry *, struct nameidata *); void (*put_link) (struct dentry *, struct nameidata *, void *); 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 (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); 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 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 is called by chmod(2) and related system calls. |