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9p: don't maintain dir i_nlink if the exported fs doesn't either

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If the exported filesystem dir on 9p server doesn't maintain accurate
i_nlink count, e.g.  always reports i_nlink as 1, then 9p should not
maintain nlink count either, otherwise drop_link would report warning
with i_nlink being zero.

For example:

 - overlayfs sets nlink to 1 for merged dir

 - ext4 (with dir_nlink feature enabled) sets nlink to 1 if a dir has
   more than EXT4_LINK_MAX (65000) links.

In this case, everytime a stat(2) call (getattr) on such exported dirs
on 9p client side, the i_nlink gets reset to 1, then operations like
rmdir(2), unlink(2) and rename(2) would cause the dir nlink to go to
zero (then negative), which results in warnings in drop_nlink() and/or
inc_nlink() calls.

This can be reproduced easily as the following steps:

 - export a merged overlayfs dir via qemu virtfs to guest

 - mount the exported virtfs in guest

 - create two sub-directories in the root dir of the mounted 9pfs

 - stat the root dir of 9pfs, this resets nlink to 1

 - remove all subdirs, the second unlink/rmdir would trigger warning

  ------------[ cut here ]------------
  WARNING: CPU: 3 PID: 1284 at fs/inode.c:282 drop_nlink+0x3e/0x50
  ...
  Call Trace:
    dump_stack+0x63/0x81
    __warn+0xcb/0xf0
    warn_slowpath_null+0x1d/0x20
    drop_nlink+0x3e/0x50
    v9fs_remove+0xaa/0x130 [9p]
    v9fs_vfs_rmdir+0x13/0x20 [9p]
    vfs_rmdir+0xb7/0x130
    do_rmdir+0x1b8/0x230
    SyS_unlinkat+0x22/0x30
    do_syscall_64+0x67/0x180
  ---[ end trace 43758d8ba91e603b ]---

Fix it by leaving i_nlink to be 1 and don't drop nlink if a directory
has nlink <= 2, which indicates that the underlying exported fs doesn't
maintain nlink count accurately.  This follows what ext4 does in
ext4_dec_count().

Link: http://lkml.kernel.org/r/20180312053829.4367-1-eguan@linux.alibaba.com
Signed-off-by: Eryu Guan <eguan@linux.alibaba.com>
Reviewed-by: Yiwen Jiang <jiangyiwen@huawei.com>
Tested-by: Roman Kapl <code@rkapl.cz>
Cc: Caspar Zhang <caspar@linux.alibaba.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: <v9fs-developer@lists.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Eryu Guan 2018-04-05 16:19:49 -07:00 committed by Linus Torvalds
parent a85222435b
commit ac89b2ef9b
1 changed files with 22 additions and 4 deletions
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26
fs/9p/vfs_inode.c
View file

@ -578,6 +578,24 @@ static int v9fs_at_to_dotl_flags(int flags)
return rflags;
}
/**
* v9fs_dec_count - helper functon to drop i_nlink.
*
* If a directory had nlink <= 2 (including . and ..), then we should not drop
* the link count, which indicates the underlying exported fs doesn't maintain
* nlink accurately. e.g.
* - overlayfs sets nlink to 1 for merged dir
* - ext4 (with dir_nlink feature enabled) sets nlink to 1 if a dir has more
* than EXT4_LINK_MAX (65000) links.
*
* @inode: inode whose nlink is being dropped
*/
static void v9fs_dec_count(struct inode *inode)
{
if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2)
drop_nlink(inode);
}
/**
* v9fs_remove - helper function to remove files and directories
* @dir: directory inode that is being deleted
@ -621,9 +639,9 @@ static int v9fs_remove(struct inode *dir, struct dentry *dentry, int flags)
*/
if (flags & AT_REMOVEDIR) {
clear_nlink(inode);
drop_nlink(dir);
v9fs_dec_count(dir);
} else
drop_nlink(inode);
v9fs_dec_count(inode);
v9fs_invalidate_inode_attr(inode);
v9fs_invalidate_inode_attr(dir);
@ -1024,12 +1042,12 @@ v9fs_vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
if (S_ISDIR(new_inode->i_mode))
clear_nlink(new_inode);
else
drop_nlink(new_inode);
v9fs_dec_count(new_inode);
}
if (S_ISDIR(old_inode->i_mode)) {
if (!new_inode)
inc_nlink(new_dir);
drop_nlink(old_dir);
v9fs_dec_count(old_dir);
}
v9fs_invalidate_inode_attr(old_inode);
v9fs_invalidate_inode_attr(old_dir);