161 lines
7.1 KiB
Plaintext
161 lines
7.1 KiB
Plaintext
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Making Filesystems Exportable
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=============================
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Overview
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--------
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All filesystem operations require a dentry (or two) as a starting
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point. Local applications have a reference-counted hold on suitable
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dentries via open file descriptors or cwd/root. However remote
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applications that access a filesystem via a remote filesystem protocol
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such as NFS may not be able to hold such a reference, and so need a
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different way to refer to a particular dentry. As the alternative
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form of reference needs to be stable across renames, truncates, and
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server-reboot (among other things, though these tend to be the most
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problematic), there is no simple answer like 'filename'.
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The mechanism discussed here allows each filesystem implementation to
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specify how to generate an opaque (outside of the filesystem) byte
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string for any dentry, and how to find an appropriate dentry for any
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given opaque byte string.
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This byte string will be called a "filehandle fragment" as it
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corresponds to part of an NFS filehandle.
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A filesystem which supports the mapping between filehandle fragments
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and dentries will be termed "exportable".
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Dcache Issues
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-------------
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The dcache normally contains a proper prefix of any given filesystem
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tree. This means that if any filesystem object is in the dcache, then
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all of the ancestors of that filesystem object are also in the dcache.
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As normal access is by filename this prefix is created naturally and
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maintained easily (by each object maintaining a reference count on
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its parent).
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However when objects are included into the dcache by interpreting a
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filehandle fragment, there is no automatic creation of a path prefix
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for the object. This leads to two related but distinct features of
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the dcache that are not needed for normal filesystem access.
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1/ The dcache must sometimes contain objects that are not part of the
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proper prefix. i.e that are not connected to the root.
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2/ The dcache must be prepared for a newly found (via ->lookup) directory
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to already have a (non-connected) dentry, and must be able to move
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that dentry into place (based on the parent and name in the
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->lookup). This is particularly needed for directories as
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it is a dcache invariant that directories only have one dentry.
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To implement these features, the dcache has:
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a/ A dentry flag DCACHE_DISCONNECTED which is set on
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any dentry that might not be part of the proper prefix.
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This is set when anonymous dentries are created, and cleared when a
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dentry is noticed to be a child of a dentry which is in the proper
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prefix. If the refcount on a dentry with this flag set
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becomes zero, the dentry is immediately discarded, rather than being
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kept in the dcache. If a dentry that is not already in the dcache
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is repeatedly accessed by filehandle (as NFSD might do), an new dentry
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will be a allocated for each access, and discarded at the end of
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the access.
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Note that such a dentry can acquire children, name, ancestors, etc.
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without losing DCACHE_DISCONNECTED - that flag is only cleared when
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subtree is successfully reconnected to root. Until then dentries
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in such subtree are retained only as long as there are references;
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refcount reaching zero means immediate eviction, same as for unhashed
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dentries. That guarantees that we won't need to hunt them down upon
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umount.
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b/ A primitive for creation of secondary roots - d_obtain_root(inode).
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Those do _not_ bear DCACHE_DISCONNECTED. They are placed on the
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per-superblock list (->s_roots), so they can be located at umount
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time for eviction purposes.
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c/ Helper routines to allocate anonymous dentries, and to help attach
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loose directory dentries at lookup time. They are:
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d_obtain_alias(inode) will return a dentry for the given inode.
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If the inode already has a dentry, one of those is returned.
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If it doesn't, a new anonymous (IS_ROOT and
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DCACHE_DISCONNECTED) dentry is allocated and attached.
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In the case of a directory, care is taken that only one dentry
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can ever be attached.
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d_splice_alias(inode, dentry) will introduce a new dentry into the tree;
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either the passed-in dentry or a preexisting alias for the given inode
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(such as an anonymous one created by d_obtain_alias), if appropriate.
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It returns NULL when the passed-in dentry is used, following the calling
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convention of ->lookup.
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Filesystem Issues
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-----------------
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For a filesystem to be exportable it must:
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1/ provide the filehandle fragment routines described below.
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2/ make sure that d_splice_alias is used rather than d_add
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when ->lookup finds an inode for a given parent and name.
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If inode is NULL, d_splice_alias(inode, dentry) is equivalent to
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d_add(dentry, inode), NULL
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Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
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Typically the ->lookup routine will simply end with a:
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return d_splice_alias(inode, dentry);
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}
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A file system implementation declares that instances of the filesystem
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are exportable by setting the s_export_op field in the struct
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super_block. This field must point to a "struct export_operations"
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struct which has the following members:
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encode_fh (optional)
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Takes a dentry and creates a filehandle fragment which can later be used
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to find or create a dentry for the same object. The default
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implementation creates a filehandle fragment that encodes a 32bit inode
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and generation number for the inode encoded, and if necessary the
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same information for the parent.
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fh_to_dentry (mandatory)
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Given a filehandle fragment, this should find the implied object and
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create a dentry for it (possibly with d_obtain_alias).
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fh_to_parent (optional but strongly recommended)
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Given a filehandle fragment, this should find the parent of the
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implied object and create a dentry for it (possibly with
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d_obtain_alias). May fail if the filehandle fragment is too small.
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get_parent (optional but strongly recommended)
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When given a dentry for a directory, this should return a dentry for
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the parent. Quite possibly the parent dentry will have been allocated
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by d_alloc_anon. The default get_parent function just returns an error
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so any filehandle lookup that requires finding a parent will fail.
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->lookup("..") is *not* used as a default as it can leave ".." entries
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in the dcache which are too messy to work with.
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get_name (optional)
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When given a parent dentry and a child dentry, this should find a name
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in the directory identified by the parent dentry, which leads to the
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object identified by the child dentry. If no get_name function is
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supplied, a default implementation is provided which uses vfs_readdir
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to find potential names, and matches inode numbers to find the correct
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match.
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A filehandle fragment consists of an array of 1 or more 4byte words,
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together with a one byte "type".
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The decode_fh routine should not depend on the stated size that is
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passed to it. This size may be larger than the original filehandle
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generated by encode_fh, in which case it will have been padded with
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nuls. Rather, the encode_fh routine should choose a "type" which
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indicates the decode_fh how much of the filehandle is valid, and how
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it should be interpreted.
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