Re: [rfc] new stat*fs-like syscall?

From: J. R. Okajima
Date: Sat Jun 26 2010 - 01:54:28 EST

Nick Piggin:
> Is there anything more we should add here? Samba wants a capabilities
> field, with things like sparse files, quotas, compression, encryption,
> case preserving/sensitive.

How about the max link count?
There was a post in last December.
See <> and its
thread in detail.

J. R. Okajima

The pathconf(_PC_LINK_MAX) cannot get the correct value, since linux
kernel doesn't provide such interface. And the current implementation in
GLibc issues statfs(2) first and then returns the predefined value
(EXT2_LINK_MAX, etc) based upoin the filesystem type. But GLibc doesn't
support all filesystem types. ie. when the target filesystem is unknown
to pathconf(3), it will return LINUX_LINK_MAX (127).
For GLibc, there is no way except implementing this poor method.

This patch makes statfs(2) return the correct value via struct

- Can we use f_spare for this purpose?
- Does pathconf(_PC_LINK_MAX) distinguish a dir and a non-dir?
If a filesystem sets different limit for a dir as a link count from a
non-dir, then should the filesystem checks the type of the specified
dentry->d_inode->i_mode and return the different value?
This patch series doesn't distinguish them and return a single value.
- Here I tried supporting only ext[23], nfs and tmpfs. Since I can test
them by myself. I left other FSs as it is, which means if FS doesn't
support _PC_LINK_MAX by modifying its s_op->statfs(), the default
value will be returned. The default value is taken from GLibc trying
to keep the compatibility. But it may not be important.
- Some FS such as ms-dos based one which doesn't support hardlink, will
return LINK_MAX_UNSUPPORTED which is defined as 1.
- Other FS such as tmpfs which doesn't check the link count in link(2),
will return LINK_MAX_UNLIMITED which is defined as -1. This value
doesn't mean an error. The negative return value of pathconf(3) is

Even if linux kernel return a correct value via statfs(2) (or anything
else), users will not get the value at once since the support in libc is
necessary too.

J. R. Okajima (5):
vfs, support pathconf(3) with _PC_LINK_MAX
ext2, support pathconf(3) with _PC_LINK_MAX
ext3, support pathconf(3) with _PC_LINK_MAX
nfs, support pathconf(3) with _PC_LINK_MAX
tmpfs, support pathconf(3) with _PC_LINK_MAX

fs/compat.c | 5 +++--
fs/ext2/super.c | 1 +
fs/ext3/super.c | 1 +
fs/libfs.c | 1 +
fs/nfs/client.c | 10 +++++++---
fs/nfs/super.c | 1 +
fs/open.c | 9 +++++++--
include/linux/nfs_fs_sb.h | 1 +
include/linux/statfs.h | 6 ++++++
mm/shmem.c | 1 +
10 files changed, 29 insertions(+), 7 deletions(-)
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