Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.
1 2 Making Filesystems Exportable 3 ============================= 4 5 Overview 6 -------- 7 8 All filesystem operations require a dentry (or two) as a starting 9 point. Local applications have a reference-counted hold on suitable 10 dentries via open file descriptors or cwd/root. However remote 11 applications that access a filesystem via a remote filesystem protocol 12 such as NFS may not be able to hold such a reference, and so need a 13 different way to refer to a particular dentry. As the alternative 14 form of reference needs to be stable across renames, truncates, and 15 server-reboot (among other things, though these tend to be the most 16 problematic), there is no simple answer like 'filename'. 17 18 The mechanism discussed here allows each filesystem implementation to 19 specify how to generate an opaque (outside of the filesystem) byte 20 string for any dentry, and how to find an appropriate dentry for any 21 given opaque byte string. 22 This byte string will be called a "filehandle fragment" as it 23 corresponds to part of an NFS filehandle. 24 25 A filesystem which supports the mapping between filehandle fragments 26 and dentries will be termed "exportable". 27 28 29 30 Dcache Issues 31 ------------- 32 33 The dcache normally contains a proper prefix of any given filesystem 34 tree. This means that if any filesystem object is in the dcache, then 35 all of the ancestors of that filesystem object are also in the dcache. 36 As normal access is by filename this prefix is created naturally and 37 maintained easily (by each object maintaining a reference count on 38 its parent). 39 40 However when objects are included into the dcache by interpreting a 41 filehandle fragment, there is no automatic creation of a path prefix 42 for the object. This leads to two related but distinct features of 43 the dcache that are not needed for normal filesystem access. 44 45 1/ The dcache must sometimes contain objects that are not part of the 46 proper prefix. i.e that are not connected to the root. 47 2/ The dcache must be prepared for a newly found (via ->lookup) directory 48 to already have a (non-connected) dentry, and must be able to move 49 that dentry into place (based on the parent and name in the 50 ->lookup). This is particularly needed for directories as 51 it is a dcache invariant that directories only have one dentry. 52 53 To implement these features, the dcache has: 54 55 a/ A dentry flag DCACHE_DISCONNECTED which is set on 56 any dentry that might not be part of the proper prefix. 57 This is set when anonymous dentries are created, and cleared when a 58 dentry is noticed to be a child of a dentry which is in the proper 59 prefix. 60 61 b/ A per-superblock list "s_anon" of dentries which are the roots of 62 subtrees that are not in the proper prefix. These dentries, as 63 well as the proper prefix, need to be released at unmount time. As 64 these dentries will not be hashed, they are linked together on the 65 d_hash list_head. 66 67 c/ Helper routines to allocate anonymous dentries, and to help attach 68 loose directory dentries at lookup time. They are: 69 d_obtain_alias(inode) will return a dentry for the given inode. 70 If the inode already has a dentry, one of those is returned. 71 If it doesn't, a new anonymous (IS_ROOT and 72 DCACHE_DISCONNECTED) dentry is allocated and attached. 73 In the case of a directory, care is taken that only one dentry 74 can ever be attached. 75 d_splice_alias(inode, dentry) will introduce a new dentry into the tree; 76 either the passed-in dentry or a preexisting alias for the given inode 77 (such as an anonymous one created by d_obtain_alias), if appropriate. 78 It returns NULL when the passed-in dentry is used, following the calling 79 convention of ->lookup. 80 81 82 Filesystem Issues 83 ----------------- 84 85 For a filesystem to be exportable it must: 86 87 1/ provide the filehandle fragment routines described below. 88 2/ make sure that d_splice_alias is used rather than d_add 89 when ->lookup finds an inode for a given parent and name. 90 91 If inode is NULL, d_splice_alias(inode, dentry) is equivalent to 92 93 d_add(dentry, inode), NULL 94 95 Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err) 96 97 Typically the ->lookup routine will simply end with a: 98 99 return d_splice_alias(inode, dentry); 100 } 101 102 103 104 A file system implementation declares that instances of the filesystem 105 are exportable by setting the s_export_op field in the struct 106 super_block. This field must point to a "struct export_operations" 107 struct which has the following members: 108 109 encode_fh (optional) 110 Takes a dentry and creates a filehandle fragment which can later be used 111 to find or create a dentry for the same object. The default 112 implementation creates a filehandle fragment that encodes a 32bit inode 113 and generation number for the inode encoded, and if necessary the 114 same information for the parent. 115 116 fh_to_dentry (mandatory) 117 Given a filehandle fragment, this should find the implied object and 118 create a dentry for it (possibly with d_obtain_alias). 119 120 fh_to_parent (optional but strongly recommended) 121 Given a filehandle fragment, this should find the parent of the 122 implied object and create a dentry for it (possibly with 123 d_obtain_alias). May fail if the filehandle fragment is too small. 124 125 get_parent (optional but strongly recommended) 126 When given a dentry for a directory, this should return a dentry for 127 the parent. Quite possibly the parent dentry will have been allocated 128 by d_alloc_anon. The default get_parent function just returns an error 129 so any filehandle lookup that requires finding a parent will fail. 130 ->lookup("..") is *not* used as a default as it can leave ".." entries 131 in the dcache which are too messy to work with. 132 133 get_name (optional) 134 When given a parent dentry and a child dentry, this should find a name 135 in the directory identified by the parent dentry, which leads to the 136 object identified by the child dentry. If no get_name function is 137 supplied, a default implementation is provided which uses vfs_readdir 138 to find potential names, and matches inode numbers to find the correct 139 match. 140 141 142 A filehandle fragment consists of an array of 1 or more 4byte words, 143 together with a one byte "type". 144 The decode_fh routine should not depend on the stated size that is 145 passed to it. This size may be larger than the original filehandle 146 generated by encode_fh, in which case it will have been padded with 147 nuls. Rather, the encode_fh routine should choose a "type" which 148 indicates the decode_fh how much of the filehandle is valid, and how 149 it should be interpreted.