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SubjectRe: [RFC][PATCH] ext2: Resolve i_nlink race in ext2_rename
On Thu 24-02-11 12:18:32, Josh Hunt wrote:
> On 02/24/2011 03:20 AM, Jan Kara wrote:
> > On Thu 24-02-11 06:37:49, Al Viro wrote:
> >> On Wed, Feb 23, 2011 at 10:21:41PM -0800, Josh Hunt wrote:
> >>> [resending: left Jan off the original mail by accident]
> >>>
> >>> We have a multi-threaded workload which is currently "losing" files in the form
> >>> of unattached inodes. The workload is link, rename, unlink intensive. This is
> >>> happening on an ext2 filesystem and have reproduced the issue in kernel
> >>> 2.6.37. Here's a sample strace:
> >>>
> >>> open("/a/tmp/tmpfile.1296184058", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 9
> >>> link("/a/tmp/tmpfile.1296184058", "/a/tmp/tmpfile.28117.1296184059") = 0
> >>> rename("/a/tmp/tmpfile.28117.1296184059", "/a/tmp/tmpfile") = 0
> >>> stat64("/a/tmp/tmpfile", {st_mode=S_IFREG|0644, st_size=24248267, ...}) = 0
> >>> link("/a/tmp/tmpfile", "/a/tmp/submit/tmpfile") = 0
> >>> open("/a/tmp/tmpfile.1296184058", O_RDONLY) = 13
> >>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDWR|O_CREAT|O_EXCL, 0600) = 824
> >>> rename("/a/tmp/submit/tmpfile", "/a/tmp/submit/tmpfile.send.q9SNoL") = 0
> >>> unlink("/a/tmp/tmpfile.1296184058") = 0
> >>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDONLY) = 827
> >>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDONLY) = 828
> >>> open("/a/tmp/submit/tmpfile.send.q9SNoL", O_RDONLY) = 829
> >>> unlink("/a/tmp/submit/tmpfile.send.q9SNoL") = 0
> >>>
> >>> The application behavior shown above repeats indefinitely with most filenames
> >>> changing during each iteration except for 'tmpfile'. Looking into this issue I
> >>> see that vfs_rename_other() only takes i_mutex for the new inode and the new
> >>> inode's directory as well as the old directory's mutex. This works for
> >>> modifying the dir entry and appears to be fine for most filesystems, but
> >>> ext2 and a few others (exofs, minix, nilfs2, omfs, sysv, ufs) modify i_nlink
> >>> inside of their respective rename functions without grabbing the i_mutex. The
> >>> modifications are done through calls to inode_inc_link_count(old_inode) and
> >>> inode_dec_link_count(old_inode), etc.
> >>>
> >>> Taking the mutex for the old inode appears to resolve the issue of the
> >>> lost files/unattached inodes that I am seeing with this workload. I've attached
> >>> a patch below doing what I've described above. If this is an accepted solution
> >>> I believe other filesystems may also be affected by this and I could provide
> >>> a patch for those as well.
> >>
> >> I don't know... The thing is, we mostly do that to make life easier for
> >> fsck in case of crash. Other than that, there's no reason to play with
> >> link count of that sucker at all. The question is, do we really want
> >> such rename() interrupted by dirty shutdown to result in what looks like two
> >> legitimate links to that inode without any indications of what had happened?
> >> Note that fsck (at least on ext2) will correct link counts anyway and if
> >> nothing else, we probably want some noise pointing to the inode in question...
> > Yeah, I agree that playing with the link count is not worth it. It is
> > even more disputable because it would have some reasonable effect only if
> > we happened to write out the moved inode after it is linked to the new
> > directory and before it is unlinked from the old one. Moreover we'd need
> > to writeout the new directory and not the old directory before crash
> > happens. All this is highly unlikely and even if that happens, it is
> > questionable whether the result is worth it. So I'll just do away with
> > those games with link count...
> > The patch is attached. Josh, can you test it as well? Thanks.
> >
> > Honza
> Jan
>
> I'm not seeing the problem with your patch as was expected since we're
> not messing with i_nlink anymore. Al suggested marking the inode as
> dirty where we were previously doing the old_inode dec. I believe this
> is needed as well since we are updating it's ctime. I've attached a
> version marking the inode dirty and it also fixes the comment making
> reference to calling inode_dec_link_count().
Yeah, good catch. Thanks.

> I'm not completely clear on the historical reasons for messing with the
> link count of old_inode in the first place. It was just to simulate the
> linking and unlinking of the old_inode?
Yes.

So I took your patch and used a changelog from mine as I find it more
descriptive. The resulting patch is in my tree (and attached).

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR
From 56c78bf2068b00bb6e7726bd8a94a34a1e206f03 Mon Sep 17 00:00:00 2001
From: Josh Hunt <johunt@akamai.com>
Date: Thu, 24 Feb 2011 11:48:22 +0100
Subject: [PATCH] ext2: Fix link count corruption under heavy link+rename load

vfs_rename_other() does not lock renamed inode with i_mutex. Thus changing
i_nlink in a non-atomic manner (which happens in ext2_rename()) can corrupt
it as reported and analyzed by Josh.

In fact, there is no good reason to mess with i_nlink of the moved file.
We did it presumably to simulate linking into the new directory and unlinking
from an old one. But the practical effect of this is disputable because fsck
can possibly treat file as being properly linked into both directories without
writing any error which is confusing. So we just stop increment-decrement
games with i_nlink which also fixes the corruption.

CC: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
fs/ext2/namei.c | 9 ++-------
1 files changed, 2 insertions(+), 7 deletions(-)
diff --git a/fs/ext2/namei.c b/fs/ext2/namei.c
index 2e1d834..adb9185 100644
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -344,7 +344,6 @@ static int ext2_rename (struct inode * old_dir, struct dentry * old_dentry,
new_de = ext2_find_entry (new_dir, &new_dentry->d_name, &new_page);
if (!new_de)
goto out_dir;
- inode_inc_link_count(old_inode);
ext2_set_link(new_dir, new_de, new_page, old_inode, 1);
new_inode->i_ctime = CURRENT_TIME_SEC;
if (dir_de)
@@ -356,12 +355,9 @@ static int ext2_rename (struct inode * old_dir, struct dentry * old_dentry,
if (new_dir->i_nlink >= EXT2_LINK_MAX)
goto out_dir;
}
- inode_inc_link_count(old_inode);
err = ext2_add_link(new_dentry, old_inode);
- if (err) {
- inode_dec_link_count(old_inode);
+ if (err)
goto out_dir;
- }
if (dir_de)
inode_inc_link_count(new_dir);
}
@@ -369,12 +365,11 @@ static int ext2_rename (struct inode * old_dir, struct dentry * old_dentry,
/*
* Like most other Unix systems, set the ctime for inodes on a
* rename.
- * inode_dec_link_count() will mark the inode dirty.
*/
old_inode->i_ctime = CURRENT_TIME_SEC;
+ mark_inode_dirty(old_inode);

ext2_delete_entry (old_de, old_page);
- inode_dec_link_count(old_inode);

if (dir_de) {
if (old_dir != new_dir)
--
1.7.1
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