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

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Date:	Sat, 26 Jun 2010 14:53:32 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [rfc] new stat*fs-like syscall?


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 <http://marc.info/?l=linux-kernel&m=126008640210762&w=2> 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
statfs.f_spare[0].

RFC:
- 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
  valid.

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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