linux-kernel - Re: [PATCH 3/3] bcachefs: Move the link counting check to the VFS layer

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Message-ID: <b41b9b8e-6623-4e07-a132-515a6e88f1cc@linux.dev>
Date: Fri, 26 Sep 2025 12:22:37 +0800
From: Youling Tang <youling.tang@...ux.dev>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
 Youling Tang <tangyouling@...inos.cn>
Subject: Re: [PATCH 3/3] bcachefs: Move the link counting check to the VFS
 layer

Hi, Kent
On 9/26/25 11:42, Kent Overstreet wrote:
> On Fri, Sep 26, 2025 at 10:21:50AM +0800, Youling Tang wrote:
>> From: Youling Tang <tangyouling@...inos.cn>
>>
>> Currently bcachefs only performs link count checks during link operations,
>> during rename and mkdir operations, the link count should also be checked.
>>
>> This patch moves the checks to the vfs_{link,rename,mkdir} functions when
>> sb->s_max_links is set, eliminating the need for filesystem-specific checks.
>>
>> Signed-off-by: Youling Tang <tangyouling@...inos.cn>
> I applied the other two patches, but not this one. We can't rely on the
> VFS for checks like this, there's lots of routes to modifying the
> filesystem that don't go through the VFS.
```
vfs_link
     bch2_link
         __bch2_link
             bch2_link_trans
                 bch2_inode_nlink_inc

bch2_symlink
     __bch2_link
```
I have traversed the bcachefs code and found that bch2_inode_nlink_inc is
called only once by bch2_link_trans, which in turn is called only once
by __bch2_link.

Although __bch2_link is also called by bch2_symlink, symlink operations
don't involve i_nlink values. Therefore, all effective calls to
bch2_inode_nlink_incoriginate from vfs_link. Please let me know if
there are any calls from other sources.

Even without moving to the VFS layer, do we still need to add link count
validation in bch2_{mkdir, rename}?

Thanks,
Youling.