[PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Youling Tang]

From: Youling Tang <tangyouling@kylinos.cn>

When I created a directory, I found that its hard link count was
1 (unlike other file system phenomena, including the "." directory,
which defaults to an initial count of 2).

By analyzing the code, it is found that the nlink of the directory
in btrfs has always been kept at 1, which is a deliberate design.

Adding its comments can prevent it from being mistakenly regarded
as a BUG.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
---
 include/uapi/linux/btrfs_tree.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index fc29d273845d..b4f7da90fd0e 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -876,6 +876,7 @@ struct btrfs_inode_item {
 	__le64 size;
 	__le64 nbytes;
 	__le64 block_group;
+	/* nlink in directories is fixed at 1 */
 	__le32 nlink;
 	__le32 uid;
 	__le32 gid;
-- 
2.43.0

Re: [PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Qu Wenruo]

在 2025/9/26 17:15, Youling Tang 写道:
> From: Youling Tang <tangyouling@kylinos.cn>
> 
> When I created a directory, I found that its hard link count was
> 1 (unlike other file system phenomena, including the "." directory,
> which defaults to an initial count of 2).
> 
> By analyzing the code, it is found that the nlink of the directory
> in btrfs has always been kept at 1, which is a deliberate design.
> 
> Adding its comments can prevent it from being mistakenly regarded
> as a BUG.
> 
> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
> ---
>   include/uapi/linux/btrfs_tree.h | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
> index fc29d273845d..b4f7da90fd0e 100644
> --- a/include/uapi/linux/btrfs_tree.h
> +++ b/include/uapi/linux/btrfs_tree.h
> @@ -876,6 +876,7 @@ struct btrfs_inode_item {
>   	__le64 size;
>   	__le64 nbytes;
>   	__le64 block_group;
> +	/* nlink in directories is fixed at 1 */

nlink of what?

Shouldn't be "nlink of directories" or "nlink of directory inodes"?


There are better location like 
btrfs-progs/Documentation/dev/On-disk-format.rst for this.

And you're only adding one single comment for a single member?
Even this is a different behavior compared to other fses, why not 
explain what the impact of the change?


If you really want to add proper comments, spend more time and effort 
like commit 9c6b1c4de1c6 ("btrfs: document device locking") to do it 
correctly.

Thanks,
Qu

>   	__le32 nlink;
>   	__le32 uid;
>   	__le32 gid;

Re: [PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Youling Tang]

Hi, Wenruo

On 9/26/25 16:34, Qu Wenruo wrote:
>
>
> 在 2025/9/26 17:15, Youling Tang 写道:
>> From: Youling Tang <tangyouling@kylinos.cn>
>>
>> When I created a directory, I found that its hard link count was
>> 1 (unlike other file system phenomena, including the "." directory,
>> which defaults to an initial count of 2).
>>
>> By analyzing the code, it is found that the nlink of the directory
>> in btrfs has always been kept at 1, which is a deliberate design.
>>
>> Adding its comments can prevent it from being mistakenly regarded
>> as a BUG.
>>
>> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
>> ---
>>   include/uapi/linux/btrfs_tree.h | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/include/uapi/linux/btrfs_tree.h 
>> b/include/uapi/linux/btrfs_tree.h
>> index fc29d273845d..b4f7da90fd0e 100644
>> --- a/include/uapi/linux/btrfs_tree.h
>> +++ b/include/uapi/linux/btrfs_tree.h
>> @@ -876,6 +876,7 @@ struct btrfs_inode_item {
>>       __le64 size;
>>       __le64 nbytes;
>>       __le64 block_group;
>> +    /* nlink in directories is fixed at 1 */
>
> nlink of what?
>
> Shouldn't be "nlink of directories" or "nlink of directory inodes"?
>
>
> There are better location like 
> btrfs-progs/Documentation/dev/On-disk-format.rst for this.
>
> And you're only adding one single comment for a single member?
> Even this is a different behavior compared to other fses, why not 
> explain what the impact of the change?
>
>
> If you really want to add proper comments, spend more time and effort 
> like commit 9c6b1c4de1c6 ("btrfs: document device locking") to do it 
> correctly.

My understanding of nlink is as follows, please correct me if I'm wrong,

/*
  * nlink represents the hard link count (corresponds to inode->i_nlink 
value).
  * For directories, this value is always 1, which differs from other 
filesystems
  * where a newly created directory has an inode->i_nlink value of 2 
(including
  * the "." entry pointing to itself).
  *
  * BTRFS maintains parent-child relationships through explicit back 
references
  * (BTRFS_INODE_REF_KEY items) rather than link count accounting.
  *
  * This design simplifies metadata management in the copy-on-write 
environment
  * and enables more reliable consistency checking. Directory link count
  * verification is performed during tree checking in 
check_inode_item(), where
  * values greater than 1 are treated as corruption.
  *
  * For regular files, nlink behaves traditionally and represents the actual
  * hard link count of the file.
  */

Thanks,
Youling.
>
> Thanks,
> Qu
>
>>       __le32 nlink;
>>       __le32 uid;
>>       __le32 gid;
>

Re: [PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Qu Wenruo]

在 2025/9/28 11:44, Youling Tang 写道:
> Hi, Wenruo
> 
> On 9/26/25 16:34, Qu Wenruo wrote:
>>
>>
>> 在 2025/9/26 17:15, Youling Tang 写道:
>>> From: Youling Tang <tangyouling@kylinos.cn>
>>>
>>> When I created a directory, I found that its hard link count was
>>> 1 (unlike other file system phenomena, including the "." directory,
>>> which defaults to an initial count of 2).
>>>
>>> By analyzing the code, it is found that the nlink of the directory
>>> in btrfs has always been kept at 1, which is a deliberate design.
>>>
>>> Adding its comments can prevent it from being mistakenly regarded
>>> as a BUG.
>>>
>>> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
>>> ---
>>>   include/uapi/linux/btrfs_tree.h | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/ 
>>> btrfs_tree.h
>>> index fc29d273845d..b4f7da90fd0e 100644
>>> --- a/include/uapi/linux/btrfs_tree.h
>>> +++ b/include/uapi/linux/btrfs_tree.h
>>> @@ -876,6 +876,7 @@ struct btrfs_inode_item {
>>>       __le64 size;
>>>       __le64 nbytes;
>>>       __le64 block_group;
>>> +    /* nlink in directories is fixed at 1 */
>>
>> nlink of what?
>>
>> Shouldn't be "nlink of directories" or "nlink of directory inodes"?
>>
>>
>> There are better location like btrfs-progs/Documentation/dev/On-disk- 
>> format.rst for this.
>>
>> And you're only adding one single comment for a single member?
>> Even this is a different behavior compared to other fses, why not 
>> explain what the impact of the change?
>>
>>
>> If you really want to add proper comments, spend more time and effort 
>> like commit 9c6b1c4de1c6 ("btrfs: document device locking") to do it 
>> correctly.
> 
> My understanding of nlink is as follows, please correct me if I'm wrong,
> 
> /*
>   * nlink represents the hard link count (corresponds to inode->i_nlink 
> value).
>   * For directories, this value is always 1, which differs from other 
> filesystems
>   * where a newly created directory has an inode->i_nlink value of 2 
> (including
>   * the "." entry pointing to itself).

Have you checked what's the meaning of the nlink number for other fses 
and why other fses go like that?

Especially the impact to user space tools like find?

>   *
>   * BTRFS maintains parent-child relationships through explicit back 
> references
>   * (BTRFS_INODE_REF_KEY items) rather than link count accounting.
>   *
>   * This design simplifies metadata management in the copy-on-write 
> environment
>   * and enables more reliable consistency checking. Directory link count
>   * verification is performed during tree checking in 
> check_inode_item(), where
>   * values greater than 1 are treated as corruption.
>   *
>   * For regular files, nlink behaves traditionally and represents the 
> actual
>   * hard link count of the file.
>   */
> 
> Thanks,
> Youling.
>>
>> Thanks,
>> Qu
>>
>>>       __le32 nlink;
>>>       __le32 uid;
>>>       __le32 gid;
>>

Re: [PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Youling Tang]

On 9/28/25 13:16, Qu Wenruo wrote:

>
>
> 在 2025/9/28 11:44, Youling Tang 写道:
>> Hi, Wenruo
>>
>> On 9/26/25 16:34, Qu Wenruo wrote:
>>>
>>>
>>> 在 2025/9/26 17:15, Youling Tang 写道:
>>>> From: Youling Tang <tangyouling@kylinos.cn>
>>>>
>>>> When I created a directory, I found that its hard link count was
>>>> 1 (unlike other file system phenomena, including the "." directory,
>>>> which defaults to an initial count of 2).
>>>>
>>>> By analyzing the code, it is found that the nlink of the directory
>>>> in btrfs has always been kept at 1, which is a deliberate design.
>>>>
>>>> Adding its comments can prevent it from being mistakenly regarded
>>>> as a BUG.
>>>>
>>>> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
>>>> ---
>>>>   include/uapi/linux/btrfs_tree.h | 1 +
>>>>   1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/ 
>>>> btrfs_tree.h
>>>> index fc29d273845d..b4f7da90fd0e 100644
>>>> --- a/include/uapi/linux/btrfs_tree.h
>>>> +++ b/include/uapi/linux/btrfs_tree.h
>>>> @@ -876,6 +876,7 @@ struct btrfs_inode_item {
>>>>       __le64 size;
>>>>       __le64 nbytes;
>>>>       __le64 block_group;
>>>> +    /* nlink in directories is fixed at 1 */
>>>
>>> nlink of what?
>>>
>>> Shouldn't be "nlink of directories" or "nlink of directory inodes"?
>>>
>>>
>>> There are better location like 
>>> btrfs-progs/Documentation/dev/On-disk- format.rst for this.
>>>
>>> And you're only adding one single comment for a single member?
>>> Even this is a different behavior compared to other fses, why not 
>>> explain what the impact of the change?
>>>
>>>
>>> If you really want to add proper comments, spend more time and 
>>> effort like commit 9c6b1c4de1c6 ("btrfs: document device locking") 
>>> to do it correctly.
>>
>> My understanding of nlink is as follows, please correct me if I'm wrong,
>>
>> /*
>>   * nlink represents the hard link count (corresponds to 
>> inode->i_nlink value).
>>   * For directories, this value is always 1, which differs from other 
>> filesystems
>>   * where a newly created directory has an inode->i_nlink value of 2 
>> (including
>>   * the "." entry pointing to itself).
>
> Have you checked what's the meaning of the nlink number for other fses 
> and why other fses go like that?
>
I have examined ext4, XFS, and bcachefs. In these filesystems,
when performing the following operations:
```
  # mkdir -p a/b
  # cd a/b
  # ls -la
  drwxr-xr-x 2 root root  6 Sep 28 14:45 .
  drwxr-xr-x 3 root root 15 Sep 28 14:45 ..
```

In btrfs:
```
  # ls -la
  drwxr-xr-x 1 root root 0 Sep 28 14:48 .
  drwxr-xr-x 1 root root 2 Sep 28 14:48 ..
```

In filesystems like ext4, we can see that the link counts for
directory 'a' and 'b' are 3 and 2 respectively:
a: The directory itself + "." pointing to itself + ".." from directory b 
pointing to it
b: The directory itself + "." pointing to itself


nlink changes during directory creation in ext4:
```
ext4_mkdir
     ext4_init_new_dir
         set_nlink(inode, 2) //Initial inode->i_nlink value for new 
directory
     ext4_inc_count(dir) //Increase parent directory's nlink by 1 (for "..")
```

In ext4, when the DIR_NLINK feature is enabled, if a directory's link
count exceeds EXT4_LINK_MAX, it will be permanently set to 1.


nlink changes during directory creation in bcachefs:
```
bch2_mkdir
     bch2_mknod
         __bch2_create
             bch2_create_trans
                 dir_u->bi_nlink++ //If creating a directory, increase 
parent's nlink
             bch2_inode_update_after_write
                 set_nlink(&inode->v, bch2_inode_nlink_get(bi))
                     bch2_inode_nlink_get //If directory, nlink 
increased by 2
```


In XFS, the xfs_create function contains the following comment:
/*
  * A newly created regular or special file just has one directory
  * entry pointing to them, but a directory also the "." entry
  * pointing to itself.
  */

Thanks,
Youling.

> Especially the impact to user space tools like find?
>
>>   *
>>   * BTRFS maintains parent-child relationships through explicit back 
>> references
>>   * (BTRFS_INODE_REF_KEY items) rather than link count accounting.
>>   *
>>   * This design simplifies metadata management in the copy-on-write 
>> environment
>>   * and enables more reliable consistency checking. Directory link count
>>   * verification is performed during tree checking in 
>> check_inode_item(), where
>>   * values greater than 1 are treated as corruption.
>>   *
>>   * For regular files, nlink behaves traditionally and represents the 
>> actual
>>   * hard link count of the file.
>>   */
>>
>> Thanks,
>> Youling.
>>>
>>> Thanks,
>>> Qu
>>>
>>>>       __le32 nlink;
>>>>       __le32 uid;
>>>>       __le32 gid;
>>>
>

Re: [PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Qu Wenruo]

在 2025/9/28 16:39, Youling Tang 写道:
> On 9/28/25 13:16, Qu Wenruo wrote:
> 
>>
>>
>> 在 2025/9/28 11:44, Youling Tang 写道:
>>> Hi, Wenruo
>>>
>>> On 9/26/25 16:34, Qu Wenruo wrote:
>>>>
>>>>
>>>> 在 2025/9/26 17:15, Youling Tang 写道:
>>>>> From: Youling Tang <tangyouling@kylinos.cn>
>>>>>
>>>>> When I created a directory, I found that its hard link count was
>>>>> 1 (unlike other file system phenomena, including the "." directory,
>>>>> which defaults to an initial count of 2).
>>>>>
>>>>> By analyzing the code, it is found that the nlink of the directory
>>>>> in btrfs has always been kept at 1, which is a deliberate design.
>>>>>
>>>>> Adding its comments can prevent it from being mistakenly regarded
>>>>> as a BUG.
>>>>>
>>>>> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
>>>>> ---
>>>>>   include/uapi/linux/btrfs_tree.h | 1 +
>>>>>   1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/ 
>>>>> btrfs_tree.h
>>>>> index fc29d273845d..b4f7da90fd0e 100644
>>>>> --- a/include/uapi/linux/btrfs_tree.h
>>>>> +++ b/include/uapi/linux/btrfs_tree.h
>>>>> @@ -876,6 +876,7 @@ struct btrfs_inode_item {
>>>>>       __le64 size;
>>>>>       __le64 nbytes;
>>>>>       __le64 block_group;
>>>>> +    /* nlink in directories is fixed at 1 */
>>>>
>>>> nlink of what?
>>>>
>>>> Shouldn't be "nlink of directories" or "nlink of directory inodes"?
>>>>
>>>>
>>>> There are better location like btrfs-progs/Documentation/dev/On- 
>>>> disk- format.rst for this.
>>>>
>>>> And you're only adding one single comment for a single member?
>>>> Even this is a different behavior compared to other fses, why not 
>>>> explain what the impact of the change?
>>>>
>>>>
>>>> If you really want to add proper comments, spend more time and 
>>>> effort like commit 9c6b1c4de1c6 ("btrfs: document device locking") 
>>>> to do it correctly.
>>>
>>> My understanding of nlink is as follows, please correct me if I'm wrong,
>>>
>>> /*
>>>   * nlink represents the hard link count (corresponds to inode- 
>>> >i_nlink value).
>>>   * For directories, this value is always 1, which differs from other 
>>> filesystems
>>>   * where a newly created directory has an inode->i_nlink value of 2 
>>> (including
>>>   * the "." entry pointing to itself).
>>
>> Have you checked what's the meaning of the nlink number for other fses 
>> and why other fses go like that?
>>
> I have examined ext4, XFS, and bcachefs. In these filesystems,
> when performing the following operations:
> ```
>   # mkdir -p a/b
>   # cd a/b
>   # ls -la
>   drwxr-xr-x 2 root root  6 Sep 28 14:45 .
>   drwxr-xr-x 3 root root 15 Sep 28 14:45 ..
> ```
> 
> In btrfs:
> ```
>   # ls -la
>   drwxr-xr-x 1 root root 0 Sep 28 14:48 .
>   drwxr-xr-x 1 root root 2 Sep 28 14:48 ..
> ```
> 
> In filesystems like ext4, we can see that the link counts for
> directory 'a' and 'b' are 3 and 2 respectively:
> a: The directory itself + "." pointing to itself + ".." from directory b 
> pointing to it
> b: The directory itself + "." pointing to itself
> 
> 
> nlink changes during directory creation in ext4:
> ```
> ext4_mkdir
>      ext4_init_new_dir
>          set_nlink(inode, 2) //Initial inode->i_nlink value for new 
> directory
>      ext4_inc_count(dir) //Increase parent directory's nlink by 1 (for 
> "..")
> ```
> 
> In ext4, when the DIR_NLINK feature is enabled, if a directory's link
> count exceeds EXT4_LINK_MAX, it will be permanently set to 1.
> 
> 
> nlink changes during directory creation in bcachefs:
> ```
> bch2_mkdir
>      bch2_mknod
>          __bch2_create
>              bch2_create_trans
>                  dir_u->bi_nlink++ //If creating a directory, increase 
> parent's nlink
>              bch2_inode_update_after_write
>                  set_nlink(&inode->v, bch2_inode_nlink_get(bi))
>                      bch2_inode_nlink_get //If directory, nlink 
> increased by 2
> ```
> 
> 
> In XFS, the xfs_create function contains the following comment:
> /*
>   * A newly created regular or special file just has one directory
>   * entry pointing to them, but a directory also the "." entry
>   * pointing to itself.
>   */

You didn't even understand what the nlink represents on these filesystems.

If you even bother to check the code of find, it exactly shows the 
meaning of nlinks for directory:

gl/lib/fts.c:

```
/* Minimum link count of a traditional Unix directory.  When leaf
    optimization is OK and a directory's st_nlink == MIN_DIR_NLINK,
    then the directory has no subdirectories.  */
enum { MIN_DIR_NLINK = 2 };

/* Whether leaf optimization is OK for a directory.  */
enum leaf_optimization
   {
     /* st_nlink is not reliable for this directory's subdirectories.  */
     NO_LEAF_OPTIMIZATION,

     /* st_nlink == 2 means the directory lacks subdirectories.  */
     OK_LEAF_OPTIMIZATION
   };
```


For filesystems returning nlinks >= 2, it means they implemented the 
optimization to indicate the number of sub-directories of it.

If you didn't even get this correct, all your words are just words 
salad, no better than AI slops.

> 
> Thanks,
> Youling.
> 
>> Especially the impact to user space tools like find?
>>
>>>   *
>>>   * BTRFS maintains parent-child relationships through explicit back 
>>> references
>>>   * (BTRFS_INODE_REF_KEY items) rather than link count accounting.

This has nothing to do with the nlink implementation of btrfs.

>>>   *
>>>   * This design simplifies metadata management in the copy-on-write 
>>> environment
>>>   * and enables more reliable consistency checking.

All these make no sense.

>>> Directory link count
>>>   * verification is performed during tree checking in 
>>> check_inode_item(), where
>>>   * values greater than 1 are treated as corruption.
>>>   *
>>>   * For regular files, nlink behaves traditionally and represents the 
>>> actual
>>>   * hard link count of the file.
>>>   */
>>>
>>> Thanks,
>>> Youling.
>>>>
>>>> Thanks,
>>>> Qu
>>>>
>>>>>       __le32 nlink;
>>>>>       __le32 uid;
>>>>>       __le32 gid;
>>>>
>>
> 

Re: [PATCH] btrfs: Add the nlink annotation in btrfs_inode_item

[Youling Tang]

On 9/28/25 15:37, Qu Wenruo wrote:

>
>
> 在 2025/9/28 16:39, Youling Tang 写道:
>> On 9/28/25 13:16, Qu Wenruo wrote:
>>
>>>
>>>
>>> 在 2025/9/28 11:44, Youling Tang 写道:
>>>> Hi, Wenruo
>>>>
>>>> On 9/26/25 16:34, Qu Wenruo wrote:
>>>>>
>>>>>
>>>>> 在 2025/9/26 17:15, Youling Tang 写道:
>>>>>> From: Youling Tang <tangyouling@kylinos.cn>
>>>>>>
>>>>>> When I created a directory, I found that its hard link count was
>>>>>> 1 (unlike other file system phenomena, including the "." directory,
>>>>>> which defaults to an initial count of 2).
>>>>>>
>>>>>> By analyzing the code, it is found that the nlink of the directory
>>>>>> in btrfs has always been kept at 1, which is a deliberate design.
>>>>>>
>>>>>> Adding its comments can prevent it from being mistakenly regarded
>>>>>> as a BUG.
>>>>>>
>>>>>> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
>>>>>> ---
>>>>>>   include/uapi/linux/btrfs_tree.h | 1 +
>>>>>>   1 file changed, 1 insertion(+)
>>>>>>
>>>>>> diff --git a/include/uapi/linux/btrfs_tree.h 
>>>>>> b/include/uapi/linux/ btrfs_tree.h
>>>>>> index fc29d273845d..b4f7da90fd0e 100644
>>>>>> --- a/include/uapi/linux/btrfs_tree.h
>>>>>> +++ b/include/uapi/linux/btrfs_tree.h
>>>>>> @@ -876,6 +876,7 @@ struct btrfs_inode_item {
>>>>>>       __le64 size;
>>>>>>       __le64 nbytes;
>>>>>>       __le64 block_group;
>>>>>> +    /* nlink in directories is fixed at 1 */
>>>>>
>>>>> nlink of what?
>>>>>
>>>>> Shouldn't be "nlink of directories" or "nlink of directory inodes"?
>>>>>
>>>>>
>>>>> There are better location like btrfs-progs/Documentation/dev/On- 
>>>>> disk- format.rst for this.
>>>>>
>>>>> And you're only adding one single comment for a single member?
>>>>> Even this is a different behavior compared to other fses, why not 
>>>>> explain what the impact of the change?
>>>>>
>>>>>
>>>>> If you really want to add proper comments, spend more time and 
>>>>> effort like commit 9c6b1c4de1c6 ("btrfs: document device locking") 
>>>>> to do it correctly.
>>>>
>>>> My understanding of nlink is as follows, please correct me if I'm 
>>>> wrong,
>>>>
>>>> /*
>>>>   * nlink represents the hard link count (corresponds to inode- 
>>>> >i_nlink value).
>>>>   * For directories, this value is always 1, which differs from 
>>>> other filesystems
>>>>   * where a newly created directory has an inode->i_nlink value of 
>>>> 2 (including
>>>>   * the "." entry pointing to itself).
>>>
>>> Have you checked what's the meaning of the nlink number for other 
>>> fses and why other fses go like that?
>>>
>> I have examined ext4, XFS, and bcachefs. In these filesystems,
>> when performing the following operations:
>> ```
>>   # mkdir -p a/b
>>   # cd a/b
>>   # ls -la
>>   drwxr-xr-x 2 root root  6 Sep 28 14:45 .
>>   drwxr-xr-x 3 root root 15 Sep 28 14:45 ..
>> ```
>>
>> In btrfs:
>> ```
>>   # ls -la
>>   drwxr-xr-x 1 root root 0 Sep 28 14:48 .
>>   drwxr-xr-x 1 root root 2 Sep 28 14:48 ..
>> ```
>>
>> In filesystems like ext4, we can see that the link counts for
>> directory 'a' and 'b' are 3 and 2 respectively:
>> a: The directory itself + "." pointing to itself + ".." from 
>> directory b pointing to it
>> b: The directory itself + "." pointing to itself
>>
>>
>> nlink changes during directory creation in ext4:
>> ```
>> ext4_mkdir
>>      ext4_init_new_dir
>>          set_nlink(inode, 2) //Initial inode->i_nlink value for new 
>> directory
>>      ext4_inc_count(dir) //Increase parent directory's nlink by 1 
>> (for "..")
>> ```
>>
>> In ext4, when the DIR_NLINK feature is enabled, if a directory's link
>> count exceeds EXT4_LINK_MAX, it will be permanently set to 1.
>>
>>
>> nlink changes during directory creation in bcachefs:
>> ```
>> bch2_mkdir
>>      bch2_mknod
>>          __bch2_create
>>              bch2_create_trans
>>                  dir_u->bi_nlink++ //If creating a directory, 
>> increase parent's nlink
>>              bch2_inode_update_after_write
>>                  set_nlink(&inode->v, bch2_inode_nlink_get(bi))
>>                      bch2_inode_nlink_get //If directory, nlink 
>> increased by 2
>> ```
>>
>>
>> In XFS, the xfs_create function contains the following comment:
>> /*
>>   * A newly created regular or special file just has one directory
>>   * entry pointing to them, but a directory also the "." entry
>>   * pointing to itself.
>>   */
>
> You didn't even understand what the nlink represents on these 
> filesystems.

I understand that the nlink of a directory represents (1 + '.' + number 
of subdirectories).
This was already reflected in the a/b directory example I mentioned earlier.

However, I was unaware that find uses the nlinks >= 2 scenario for 
optimization purposes.
Thank you for letting me know.

Thanks,
Youling.
>
> If you even bother to check the code of find, it exactly shows the 
> meaning of nlinks for directory:
>
> gl/lib/fts.c:
>
> ```
> /* Minimum link count of a traditional Unix directory.  When leaf
>    optimization is OK and a directory's st_nlink == MIN_DIR_NLINK,
>    then the directory has no subdirectories.  */
> enum { MIN_DIR_NLINK = 2 };
>
> /* Whether leaf optimization is OK for a directory.  */
> enum leaf_optimization
>   {
>     /* st_nlink is not reliable for this directory's subdirectories.  */
>     NO_LEAF_OPTIMIZATION,
>
>     /* st_nlink == 2 means the directory lacks subdirectories.  */
>     OK_LEAF_OPTIMIZATION
>   };
> ```
>
>
> For filesystems returning nlinks >= 2, it means they implemented the 
> optimization to indicate the number of sub-directories of it.
>
> If you didn't even get this correct, all your words are just words 
> salad, no better than AI slops.
>
>>
>> Thanks,
>> Youling.
>>
>>> Especially the impact to user space tools like find?
>>>
>>>>   *
>>>>   * BTRFS maintains parent-child relationships through explicit 
>>>> back references
>>>>   * (BTRFS_INODE_REF_KEY items) rather than link count accounting.
>
> This has nothing to do with the nlink implementation of btrfs.
>
>>>>   *
>>>>   * This design simplifies metadata management in the copy-on-write 
>>>> environment
>>>>   * and enables more reliable consistency checking.
>
> All these make no sense.
>
>>>> Directory link count
>>>>   * verification is performed during tree checking in 
>>>> check_inode_item(), where
>>>>   * values greater than 1 are treated as corruption.
>>>>   *
>>>>   * For regular files, nlink behaves traditionally and represents 
>>>> the actual
>>>>   * hard link count of the file.
>>>>   */
>>>>
>>>> Thanks,
>>>> Youling.
>>>>>
>>>>> Thanks,
>>>>> Qu
>>>>>
>>>>>>       __le32 nlink;
>>>>>>       __le32 uid;
>>>>>>       __le32 gid;
>>>>>
>>>
>>
>