Hi Al, Here are a set of patches that adds a syscall, fsinfo(), that allows attributes of a filesystem/superblock to be queried. Attributes are of two basic types: fixed-length structure and variable-length string. Attributes can also have multiple values in up to two dimensions. Note that the attributes *are* allowed to vary depending on the specific dentry that you're looking at. I've tried to make the interface as light as possible, so integer/enum attribute selector rather than string and the core does all the allocation and extensibility support work rather than leaving that to the filesystems. That means sb->s_op->get_fsinfo() may assume that the provided buffer is always present and always big enough. Further, this removes the possibility of the filesystem gaining access to the userspace buffer. General superblock attributes include: - The amount of space/free space in a filesystem (as statfs()). - Filesystem identifiers (UUID, volume label, device numbers, ...) - The limits on a filesystem's capabilities - A variety single-bit flags indicating supported capabilities. - Information on supported statx fields and attributes and IOC flags. - Timestamp resolution and range. - Sources (as per mount(2), but fsconfig() allows multiple sources). - In-filesystem filename format information. - I/O parameters. - Filesystem parameters ("mount -o xxx"-type things). Specific network superblock attributes include: - Cell and domain names. - Kerberos realm name. - Server names and addresses. Filesystem configuration metadata attributes include: - Filesystem parameter type descriptions. - Name -> parameter mappings. - Simple enumeration name -> value mappings. fsinfo() also permits you to retrieve information about what the fsinfo() syscall itself supports, including the number of attibutes supported and the number of capability bits supported. The system is extensible: (1) New attributes can be added. There is no requirement that a filesystem implement every attribute. Note that the core VFS keeps a table of types and sizes so it can handle future extensibility rather than delegating this to the filesystems. (2) Structure-typed attributes can be made larger and have more information tacked on the end, provided it keeps the layout of the existing fields. If an older process asks for a shorter structure, it will only be given the bits it asks for. If a newer process asks for a longer structure on an older kernel, the extra space will be set to 0. In all cases, the size of the data available is returned. In essence, the size of a structure is that structure's version: a smaller size is an earlier version and a later version includes everything that the earlier version did. (3) New single-bit capability flags can be added. This is a structure-typed attribute and, as such, (2) applies. Any bits you wanted but the kernel doesn't support are automatically set to 0. If a filesystem-specific attribute is added, it should just take up the next number in the enumeration. Currently, I do not intend that the number space should be subdivided between interested parties. fsinfo() may be called like the following, for example: struct fsinfo_params params = { .at_flags = AT_SYMLINK_NOFOLLOW, .request = FSINFO_ATTR_SERVER_ADDRESS; .Nth = 2; .Mth = 1; }; struct fsinfo_server_address address; len = fsinfo(AT_FDCWD, "/kafs/grand.central.org/doc", ¶ms, &address, sizeof(address)); The above example would query a network filesystem, such as AFS or NFS, and ask what the 2nd address (Mth) of the 3rd server (Nth) that the superblock is using is. Whereas: struct fsinfo_params params = { .at_flags = AT_SYMLINK_NOFOLLOW, .request = FSINFO_ATTR_CELL_NAME; }; char cell_name[256]; len = fsinfo(AT_FDCWD, "/kafs/grand.central.org/doc", ¶ms, &cell_name, sizeof(cell_name)); would retrieve the name of an AFS cell as a string. fsinfo() can also be used to query a context from fsopen() or fspick(): fd = fsopen("ext4", 0); struct fsinfo_params params = { .request = FSINFO_ATTR_PARAM_DESCRIPTION; }; struct fsinfo_param_description desc; fsinfo(fd, NULL, ¶ms, &desc, sizeof(desc)); even if that context doesn't currently have a superblock attached (though if there's no superblock attached, only filesystem-specific things like parameter descriptions can be accessed). The patches can be found here also: https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git on branch: fsinfo David --- David Howells (5): vfs: syscall: Add fsinfo() to query filesystem information afs: Add fsinfo support vfs: Allow fsinfo() to query what's in an fs_context vfs: Allow fsinfo() to be used to query an fs parameter description vfs: Implement parameter value retrieval with fsinfo() arch/x86/entry/syscalls/syscall_32.tbl | 1 arch/x86/entry/syscalls/syscall_64.tbl | 1 fs/afs/internal.h | 1 fs/afs/super.c | 167 +++++++++ fs/hugetlbfs/inode.c | 65 ++++ fs/kernfs/mount.c | 16 + fs/statfs.c | 572 ++++++++++++++++++++++++++++++++ include/linux/fs.h | 4 include/linux/fsinfo.h | 41 ++ include/linux/kernfs.h | 2 include/linux/syscalls.h | 4 include/uapi/linux/fsinfo.h | 302 +++++++++++++++++ kernel/cgroup/cgroup-v1.c | 72 ++++ kernel/cgroup/cgroup.c | 31 ++ samples/Kconfig | 2 samples/statx/Makefile | 7 samples/statx/test-fs-query.c | 137 ++++++++ samples/statx/test-fsinfo.c | 554 +++++++++++++++++++++++++++++++ 18 files changed, 1975 insertions(+), 4 deletions(-) create mode 100644 include/linux/fsinfo.h create mode 100644 include/uapi/linux/fsinfo.h create mode 100644 samples/statx/test-fs-query.c create mode 100644 samples/statx/test-fsinfo.c -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html