From ec23eb54fbc7a07405d416d77e8115e575ce3adc Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Fri, 26 Jul 2019 09:51:27 -0300 Subject: docs: fs: convert docs without extension to ReST There are 3 remaining files without an extension inside the fs docs dir. Manually convert them to ReST. In the case of the nfs/exporting.rst file, as the nfs docs aren't ported yet, I opted to convert and add a :orphan: there, with should be removed when it gets added into a nfs-specific part of the fs documentation. Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Jonathan Corbet --- Documentation/filesystems/nfs/Exporting | 160 --------------------------- Documentation/filesystems/nfs/exporting.rst | 165 ++++++++++++++++++++++++++++ 2 files changed, 165 insertions(+), 160 deletions(-) delete mode 100644 Documentation/filesystems/nfs/Exporting create mode 100644 Documentation/filesystems/nfs/exporting.rst (limited to 'Documentation/filesystems/nfs') diff --git a/Documentation/filesystems/nfs/Exporting b/Documentation/filesystems/nfs/Exporting deleted file mode 100644 index 63889149f532..000000000000 --- a/Documentation/filesystems/nfs/Exporting +++ /dev/null @@ -1,160 +0,0 @@ - -Making Filesystems Exportable -============================= - -Overview --------- - -All filesystem operations require a dentry (or two) as a starting -point. Local applications have a reference-counted hold on suitable -dentries via open file descriptors or cwd/root. However remote -applications that access a filesystem via a remote filesystem protocol -such as NFS may not be able to hold such a reference, and so need a -different way to refer to a particular dentry. As the alternative -form of reference needs to be stable across renames, truncates, and -server-reboot (among other things, though these tend to be the most -problematic), there is no simple answer like 'filename'. - -The mechanism discussed here allows each filesystem implementation to -specify how to generate an opaque (outside of the filesystem) byte -string for any dentry, and how to find an appropriate dentry for any -given opaque byte string. -This byte string will be called a "filehandle fragment" as it -corresponds to part of an NFS filehandle. - -A filesystem which supports the mapping between filehandle fragments -and dentries will be termed "exportable". - - - -Dcache Issues -------------- - -The dcache normally contains a proper prefix of any given filesystem -tree. This means that if any filesystem object is in the dcache, then -all of the ancestors of that filesystem object are also in the dcache. -As normal access is by filename this prefix is created naturally and -maintained easily (by each object maintaining a reference count on -its parent). - -However when objects are included into the dcache by interpreting a -filehandle fragment, there is no automatic creation of a path prefix -for the object. This leads to two related but distinct features of -the dcache that are not needed for normal filesystem access. - -1/ The dcache must sometimes contain objects that are not part of the - proper prefix. i.e that are not connected to the root. -2/ The dcache must be prepared for a newly found (via ->lookup) directory - to already have a (non-connected) dentry, and must be able to move - that dentry into place (based on the parent and name in the - ->lookup). This is particularly needed for directories as - it is a dcache invariant that directories only have one dentry. - -To implement these features, the dcache has: - -a/ A dentry flag DCACHE_DISCONNECTED which is set on - any dentry that might not be part of the proper prefix. - This is set when anonymous dentries are created, and cleared when a - dentry is noticed to be a child of a dentry which is in the proper - prefix. If the refcount on a dentry with this flag set - becomes zero, the dentry is immediately discarded, rather than being - kept in the dcache. If a dentry that is not already in the dcache - is repeatedly accessed by filehandle (as NFSD might do), an new dentry - will be a allocated for each access, and discarded at the end of - the access. - - Note that such a dentry can acquire children, name, ancestors, etc. - without losing DCACHE_DISCONNECTED - that flag is only cleared when - subtree is successfully reconnected to root. Until then dentries - in such subtree are retained only as long as there are references; - refcount reaching zero means immediate eviction, same as for unhashed - dentries. That guarantees that we won't need to hunt them down upon - umount. - -b/ A primitive for creation of secondary roots - d_obtain_root(inode). - Those do _not_ bear DCACHE_DISCONNECTED. They are placed on the - per-superblock list (->s_roots), so they can be located at umount - time for eviction purposes. - -c/ Helper routines to allocate anonymous dentries, and to help attach - loose directory dentries at lookup time. They are: - d_obtain_alias(inode) will return a dentry for the given inode. - If the inode already has a dentry, one of those is returned. - If it doesn't, a new anonymous (IS_ROOT and - DCACHE_DISCONNECTED) dentry is allocated and attached. - In the case of a directory, care is taken that only one dentry - can ever be attached. - d_splice_alias(inode, dentry) will introduce a new dentry into the tree; - either the passed-in dentry or a preexisting alias for the given inode - (such as an anonymous one created by d_obtain_alias), if appropriate. - It returns NULL when the passed-in dentry is used, following the calling - convention of ->lookup. - -Filesystem Issues ------------------ - -For a filesystem to be exportable it must: - - 1/ provide the filehandle fragment routines described below. - 2/ make sure that d_splice_alias is used rather than d_add - when ->lookup finds an inode for a given parent and name. - - If inode is NULL, d_splice_alias(inode, dentry) is equivalent to - - d_add(dentry, inode), NULL - - Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err) - - Typically the ->lookup routine will simply end with a: - - return d_splice_alias(inode, dentry); - } - - - - A file system implementation declares that instances of the filesystem -are exportable by setting the s_export_op field in the struct -super_block. This field must point to a "struct export_operations" -struct which has the following members: - - encode_fh (optional) - Takes a dentry and creates a filehandle fragment which can later be used - to find or create a dentry for the same object. The default - implementation creates a filehandle fragment that encodes a 32bit inode - and generation number for the inode encoded, and if necessary the - same information for the parent. - - fh_to_dentry (mandatory) - Given a filehandle fragment, this should find the implied object and - create a dentry for it (possibly with d_obtain_alias). - - fh_to_parent (optional but strongly recommended) - Given a filehandle fragment, this should find the parent of the - implied object and create a dentry for it (possibly with - d_obtain_alias). May fail if the filehandle fragment is too small. - - get_parent (optional but strongly recommended) - When given a dentry for a directory, this should return a dentry for - the parent. Quite possibly the parent dentry will have been allocated - by d_alloc_anon. The default get_parent function just returns an error - so any filehandle lookup that requires finding a parent will fail. - ->lookup("..") is *not* used as a default as it can leave ".." entries - in the dcache which are too messy to work with. - - get_name (optional) - When given a parent dentry and a child dentry, this should find a name - in the directory identified by the parent dentry, which leads to the - object identified by the child dentry. If no get_name function is - supplied, a default implementation is provided which uses vfs_readdir - to find potential names, and matches inode numbers to find the correct - match. - - -A filehandle fragment consists of an array of 1 or more 4byte words, -together with a one byte "type". -The decode_fh routine should not depend on the stated size that is -passed to it. This size may be larger than the original filehandle -generated by encode_fh, in which case it will have been padded with -nuls. Rather, the encode_fh routine should choose a "type" which -indicates the decode_fh how much of the filehandle is valid, and how -it should be interpreted. diff --git a/Documentation/filesystems/nfs/exporting.rst b/Documentation/filesystems/nfs/exporting.rst new file mode 100644 index 000000000000..33d588a01ace --- /dev/null +++ b/Documentation/filesystems/nfs/exporting.rst @@ -0,0 +1,165 @@ +:orphan: + +Making Filesystems Exportable +============================= + +Overview +-------- + +All filesystem operations require a dentry (or two) as a starting +point. Local applications have a reference-counted hold on suitable +dentries via open file descriptors or cwd/root. However remote +applications that access a filesystem via a remote filesystem protocol +such as NFS may not be able to hold such a reference, and so need a +different way to refer to a particular dentry. As the alternative +form of reference needs to be stable across renames, truncates, and +server-reboot (among other things, though these tend to be the most +problematic), there is no simple answer like 'filename'. + +The mechanism discussed here allows each filesystem implementation to +specify how to generate an opaque (outside of the filesystem) byte +string for any dentry, and how to find an appropriate dentry for any +given opaque byte string. +This byte string will be called a "filehandle fragment" as it +corresponds to part of an NFS filehandle. + +A filesystem which supports the mapping between filehandle fragments +and dentries will be termed "exportable". + + + +Dcache Issues +------------- + +The dcache normally contains a proper prefix of any given filesystem +tree. This means that if any filesystem object is in the dcache, then +all of the ancestors of that filesystem object are also in the dcache. +As normal access is by filename this prefix is created naturally and +maintained easily (by each object maintaining a reference count on +its parent). + +However when objects are included into the dcache by interpreting a +filehandle fragment, there is no automatic creation of a path prefix +for the object. This leads to two related but distinct features of +the dcache that are not needed for normal filesystem access. + +1. The dcache must sometimes contain objects that are not part of the + proper prefix. i.e that are not connected to the root. +2. The dcache must be prepared for a newly found (via ->lookup) directory + to already have a (non-connected) dentry, and must be able to move + that dentry into place (based on the parent and name in the + ->lookup). This is particularly needed for directories as + it is a dcache invariant that directories only have one dentry. + +To implement these features, the dcache has: + +a. A dentry flag DCACHE_DISCONNECTED which is set on + any dentry that might not be part of the proper prefix. + This is set when anonymous dentries are created, and cleared when a + dentry is noticed to be a child of a dentry which is in the proper + prefix. If the refcount on a dentry with this flag set + becomes zero, the dentry is immediately discarded, rather than being + kept in the dcache. If a dentry that is not already in the dcache + is repeatedly accessed by filehandle (as NFSD might do), an new dentry + will be a allocated for each access, and discarded at the end of + the access. + + Note that such a dentry can acquire children, name, ancestors, etc. + without losing DCACHE_DISCONNECTED - that flag is only cleared when + subtree is successfully reconnected to root. Until then dentries + in such subtree are retained only as long as there are references; + refcount reaching zero means immediate eviction, same as for unhashed + dentries. That guarantees that we won't need to hunt them down upon + umount. + +b. A primitive for creation of secondary roots - d_obtain_root(inode). + Those do _not_ bear DCACHE_DISCONNECTED. They are placed on the + per-superblock list (->s_roots), so they can be located at umount + time for eviction purposes. + +c. Helper routines to allocate anonymous dentries, and to help attach + loose directory dentries at lookup time. They are: + + d_obtain_alias(inode) will return a dentry for the given inode. + If the inode already has a dentry, one of those is returned. + + If it doesn't, a new anonymous (IS_ROOT and + DCACHE_DISCONNECTED) dentry is allocated and attached. + + In the case of a directory, care is taken that only one dentry + can ever be attached. + + d_splice_alias(inode, dentry) will introduce a new dentry into the tree; + either the passed-in dentry or a preexisting alias for the given inode + (such as an anonymous one created by d_obtain_alias), if appropriate. + It returns NULL when the passed-in dentry is used, following the calling + convention of ->lookup. + +Filesystem Issues +----------------- + +For a filesystem to be exportable it must: + + 1. provide the filehandle fragment routines described below. + 2. make sure that d_splice_alias is used rather than d_add + when ->lookup finds an inode for a given parent and name. + + If inode is NULL, d_splice_alias(inode, dentry) is equivalent to:: + + d_add(dentry, inode), NULL + + Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err) + + Typically the ->lookup routine will simply end with a:: + + return d_splice_alias(inode, dentry); + } + + + +A file system implementation declares that instances of the filesystem +are exportable by setting the s_export_op field in the struct +super_block. This field must point to a "struct export_operations" +struct which has the following members: + + encode_fh (optional) + Takes a dentry and creates a filehandle fragment which can later be used + to find or create a dentry for the same object. The default + implementation creates a filehandle fragment that encodes a 32bit inode + and generation number for the inode encoded, and if necessary the + same information for the parent. + + fh_to_dentry (mandatory) + Given a filehandle fragment, this should find the implied object and + create a dentry for it (possibly with d_obtain_alias). + + fh_to_parent (optional but strongly recommended) + Given a filehandle fragment, this should find the parent of the + implied object and create a dentry for it (possibly with + d_obtain_alias). May fail if the filehandle fragment is too small. + + get_parent (optional but strongly recommended) + When given a dentry for a directory, this should return a dentry for + the parent. Quite possibly the parent dentry will have been allocated + by d_alloc_anon. The default get_parent function just returns an error + so any filehandle lookup that requires finding a parent will fail. + ->lookup("..") is *not* used as a default as it can leave ".." entries + in the dcache which are too messy to work with. + + get_name (optional) + When given a parent dentry and a child dentry, this should find a name + in the directory identified by the parent dentry, which leads to the + object identified by the child dentry. If no get_name function is + supplied, a default implementation is provided which uses vfs_readdir + to find potential names, and matches inode numbers to find the correct + match. + + +A filehandle fragment consists of an array of 1 or more 4byte words, +together with a one byte "type". +The decode_fh routine should not depend on the stated size that is +passed to it. This size may be larger than the original filehandle +generated by encode_fh, in which case it will have been padded with +nuls. Rather, the encode_fh routine should choose a "type" which +indicates the decode_fh how much of the filehandle is valid, and how +it should be interpreted. -- cgit