1:orphan: 2 3Making Filesystems Exportable 4============================= 5 6Overview 7-------- 8 9All filesystem operations require a dentry (or two) as a starting 10point. Local applications have a reference-counted hold on suitable 11dentries via open file descriptors or cwd/root. However remote 12applications that access a filesystem via a remote filesystem protocol 13such as NFS may not be able to hold such a reference, and so need a 14different way to refer to a particular dentry. As the alternative 15form of reference needs to be stable across renames, truncates, and 16server-reboot (among other things, though these tend to be the most 17problematic), there is no simple answer like 'filename'. 18 19The mechanism discussed here allows each filesystem implementation to 20specify how to generate an opaque (outside of the filesystem) byte 21string for any dentry, and how to find an appropriate dentry for any 22given opaque byte string. 23This byte string will be called a "filehandle fragment" as it 24corresponds to part of an NFS filehandle. 25 26A filesystem which supports the mapping between filehandle fragments 27and dentries will be termed "exportable". 28 29 30 31Dcache Issues 32------------- 33 34The dcache normally contains a proper prefix of any given filesystem 35tree. This means that if any filesystem object is in the dcache, then 36all of the ancestors of that filesystem object are also in the dcache. 37As normal access is by filename this prefix is created naturally and 38maintained easily (by each object maintaining a reference count on 39its parent). 40 41However when objects are included into the dcache by interpreting a 42filehandle fragment, there is no automatic creation of a path prefix 43for the object. This leads to two related but distinct features of 44the dcache that are not needed for normal filesystem access. 45 461. The dcache must sometimes contain objects that are not part of the 47 proper prefix. i.e that are not connected to the root. 482. The dcache must be prepared for a newly found (via ->lookup) directory 49 to already have a (non-connected) dentry, and must be able to move 50 that dentry into place (based on the parent and name in the 51 ->lookup). This is particularly needed for directories as 52 it is a dcache invariant that directories only have one dentry. 53 54To implement these features, the dcache has: 55 56a. A dentry flag DCACHE_DISCONNECTED which is set on 57 any dentry that might not be part of the proper prefix. 58 This is set when anonymous dentries are created, and cleared when a 59 dentry is noticed to be a child of a dentry which is in the proper 60 prefix. If the refcount on a dentry with this flag set 61 becomes zero, the dentry is immediately discarded, rather than being 62 kept in the dcache. If a dentry that is not already in the dcache 63 is repeatedly accessed by filehandle (as NFSD might do), an new dentry 64 will be a allocated for each access, and discarded at the end of 65 the access. 66 67 Note that such a dentry can acquire children, name, ancestors, etc. 68 without losing DCACHE_DISCONNECTED - that flag is only cleared when 69 subtree is successfully reconnected to root. Until then dentries 70 in such subtree are retained only as long as there are references; 71 refcount reaching zero means immediate eviction, same as for unhashed 72 dentries. That guarantees that we won't need to hunt them down upon 73 umount. 74 75b. A primitive for creation of secondary roots - d_obtain_root(inode). 76 Those do _not_ bear DCACHE_DISCONNECTED. They are placed on the 77 per-superblock list (->s_roots), so they can be located at umount 78 time for eviction purposes. 79 80c. Helper routines to allocate anonymous dentries, and to help attach 81 loose directory dentries at lookup time. They are: 82 83 d_obtain_alias(inode) will return a dentry for the given inode. 84 If the inode already has a dentry, one of those is returned. 85 86 If it doesn't, a new anonymous (IS_ROOT and 87 DCACHE_DISCONNECTED) dentry is allocated and attached. 88 89 In the case of a directory, care is taken that only one dentry 90 can ever be attached. 91 92 d_splice_alias(inode, dentry) will introduce a new dentry into the tree; 93 either the passed-in dentry or a preexisting alias for the given inode 94 (such as an anonymous one created by d_obtain_alias), if appropriate. 95 It returns NULL when the passed-in dentry is used, following the calling 96 convention of ->lookup. 97 98Filesystem Issues 99----------------- 100 101For a filesystem to be exportable it must: 102 103 1. provide the filehandle fragment routines described below. 104 2. make sure that d_splice_alias is used rather than d_add 105 when ->lookup finds an inode for a given parent and name. 106 107 If inode is NULL, d_splice_alias(inode, dentry) is equivalent to:: 108 109 d_add(dentry, inode), NULL 110 111 Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err) 112 113 Typically the ->lookup routine will simply end with a:: 114 115 return d_splice_alias(inode, dentry); 116 } 117 118 119 120A file system implementation declares that instances of the filesystem 121are exportable by setting the s_export_op field in the struct 122super_block. This field must point to a struct export_operations 123which has the following members: 124 125.. kernel-doc:: include/linux/exportfs.h 126 :identifiers: struct export_operations 127 128A filehandle fragment consists of an array of 1 or more 4byte words, 129together with a one byte "type". 130The decode_fh routine should not depend on the stated size that is 131passed to it. This size may be larger than the original filehandle 132generated by encode_fh, in which case it will have been padded with 133nuls. Rather, the encode_fh routine should choose a "type" which 134indicates the decode_fh how much of the filehandle is valid, and how 135it should be interpreted. 136 137Export Operations Flags 138----------------------- 139In addition to the operation vector pointers, struct export_operations also 140contains a "flags" field that allows the filesystem to communicate to nfsd 141that it may want to do things differently when dealing with it. The 142following flags are defined: 143 144 EXPORT_OP_NOWCC - disable NFSv3 WCC attributes on this filesystem 145 RFC 1813 recommends that servers always send weak cache consistency 146 (WCC) data to the client after each operation. The server should 147 atomically collect attributes about the inode, do an operation on it, 148 and then collect the attributes afterward. This allows the client to 149 skip issuing GETATTRs in some situations but means that the server 150 is calling vfs_getattr for almost all RPCs. On some filesystems 151 (particularly those that are clustered or networked) this is expensive 152 and atomicity is difficult to guarantee. This flag indicates to nfsd 153 that it should skip providing WCC attributes to the client in NFSv3 154 replies when doing operations on this filesystem. Consider enabling 155 this on filesystems that have an expensive ->getattr inode operation, 156 or when atomicity between pre and post operation attribute collection 157 is impossible to guarantee. 158 159 EXPORT_OP_NOSUBTREECHK - disallow subtree checking on this fs 160 Many NFS operations deal with filehandles, which the server must then 161 vet to ensure that they live inside of an exported tree. When the 162 export consists of an entire filesystem, this is trivial. nfsd can just 163 ensure that the filehandle live on the filesystem. When only part of a 164 filesystem is exported however, then nfsd must walk the ancestors of the 165 inode to ensure that it's within an exported subtree. This is an 166 expensive operation and not all filesystems can support it properly. 167 This flag exempts the filesystem from subtree checking and causes 168 exportfs to get back an error if it tries to enable subtree checking 169 on it. 170 171 EXPORT_OP_CLOSE_BEFORE_UNLINK - always close cached files before unlinking 172 On some exportable filesystems (such as NFS) unlinking a file that 173 is still open can cause a fair bit of extra work. For instance, 174 the NFS client will do a "sillyrename" to ensure that the file 175 sticks around while it's still open. When reexporting, that open 176 file is held by nfsd so we usually end up doing a sillyrename, and 177 then immediately deleting the sillyrenamed file just afterward when 178 the link count actually goes to zero. Sometimes this delete can race 179 with other operations (for instance an rmdir of the parent directory). 180 This flag causes nfsd to close any open files for this inode _before_ 181 calling into the vfs to do an unlink or a rename that would replace 182 an existing file. 183 184 EXPORT_OP_REMOTE_FS - Backing storage for this filesystem is remote 185 PF_LOCAL_THROTTLE exists for loopback NFSD, where a thread needs to 186 write to one bdi (the final bdi) in order to free up writes queued 187 to another bdi (the client bdi). Such threads get a private balance 188 of dirty pages so that dirty pages for the client bdi do not imact 189 the daemon writing to the final bdi. For filesystems whose durable 190 storage is not local (such as exported NFS filesystems), this 191 constraint has negative consequences. EXPORT_OP_REMOTE_FS enables 192 an export to disable writeback throttling. 193 194 EXPORT_OP_NOATOMIC_ATTR - Filesystem does not update attributes atomically 195 EXPORT_OP_NOATOMIC_ATTR indicates that the exported filesystem 196 cannot provide the semantics required by the "atomic" boolean in 197 NFSv4's change_info4. This boolean indicates to a client whether the 198 returned before and after change attributes were obtained atomically 199 with the respect to the requested metadata operation (UNLINK, 200 OPEN/CREATE, MKDIR, etc). 201 202 EXPORT_OP_FLUSH_ON_CLOSE - Filesystem flushes file data on close(2) 203 On most filesystems, inodes can remain under writeback after the 204 file is closed. NFSD relies on client activity or local flusher 205 threads to handle writeback. Certain filesystems, such as NFS, flush 206 all of an inode's dirty data on last close. Exports that behave this 207 way should set EXPORT_OP_FLUSH_ON_CLOSE so that NFSD knows to skip 208 waiting for writeback when closing such files. 209