Directory is not persisted after removing another directory and creating a hard linked symlink with the same name if system crashes

Directory is not persisted after removing another directory and creating a hard linked symlink with the same name if system crashes

[Vyacheslav Kovalevsky]

Detailed description
====================

Hello, there seems to be an issue with btrfs crash behavior:

1. Create new directory `foo`, sync, then remove the directory.
2. Create new directories `dir1` and `dir2`.
3. Create new symlink named `foo` (NOTE: must have the same name as the 
removed directory).
4. Create new (hard) link to the symlink inside `dir2` (NOTE: this is 
also important).
5. Sync directory `dir2`.
6. Sync root directory.

Directory `dir1` will be missing even though it was synced in the last step.


System info
===========

Linux version 7.0-rc2, also tested on 6.19.2


How to reproduce
================

```
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
   int status;
   int dir_fd;
   int root_fd;

   status = mkdir("foo", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
   printf("MKDIR: %d\n", status);

   sync();

   status = rmdir("foo");
   printf("RMDIR: %d\n", status);

   status = mkdir("dir1", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
   printf("MKDIR: %d\n", status);

   status = mkdir("dir2", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
   printf("MKDIR: %d\n", status);

   status = symlink("bar", "foo");
   printf("SYMLINK: %d\n", status);

   status = link("foo", "dir2/link");
   printf("LINK: %d\n", status);

   status = open("dir2", O_RDONLY);
   printf("OPEN: %d\n", status);
   dir_fd = status;

   status = fsync(dir_fd);
   printf("FSYNC: %d\n", status);

   status = open(".", O_RDONLY);
   printf("OPEN: %d\n", status);
   root_fd = status;

   status = fsync(root_fd);
   printf("FSYNC: %d\n", status);
}
// after crash `dir1` is missing
```

Steps:

1. Create and mount new btrfs file system in default configuration.
2. Change directory to root of the file system and run the compiled test.
3. Cause hard system crash (e.g. QEMU `system_reset` command).
4. Remount file system after crash.
5. Observe that directory `dir1` is missing.

Re: Directory is not persisted after removing another directory and creating a hard linked symlink with the same name if system crashes

[Filipe Manana]

On Tue, Mar 3, 2026 at 1:32 PM Vyacheslav Kovalevsky
<slava.kovalevskiy.2014@gmail.com> wrote:
>
> Detailed description
> ====================
>
> Hello, there seems to be an issue with btrfs crash behavior:
>
> 1. Create new directory `foo`, sync, then remove the directory.
> 2. Create new directories `dir1` and `dir2`.
> 3. Create new symlink named `foo` (NOTE: must have the same name as the
> removed directory).

It doesn't need to be a symlink, a regular file is enough.

Fix here:   https://lore.kernel.org/linux-btrfs/9a367f025abf4ea19c96aead75d206e129b2c56e.1772558089.git.fdmanana@suse.com/

Btw, I think you may have sent a message with one of the longest subjects ever.

Thanks.

> 4. Create new (hard) link to the symlink inside `dir2` (NOTE: this is
> also important).
> 5. Sync directory `dir2`.
> 6. Sync root directory.
>
> Directory `dir1` will be missing even though it was synced in the last step.
>
>
> System info
> ===========
>
> Linux version 7.0-rc2, also tested on 6.19.2
>
>
> How to reproduce
> ================
>
> ```
> #include <errno.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <string.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <unistd.h>
>
> int main() {
>    int status;
>    int dir_fd;
>    int root_fd;
>
>    status = mkdir("foo", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
>    printf("MKDIR: %d\n", status);
>
>    sync();
>
>    status = rmdir("foo");
>    printf("RMDIR: %d\n", status);
>
>    status = mkdir("dir1", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
>    printf("MKDIR: %d\n", status);
>
>    status = mkdir("dir2", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
>    printf("MKDIR: %d\n", status);
>
>    status = symlink("bar", "foo");
>    printf("SYMLINK: %d\n", status);
>
>    status = link("foo", "dir2/link");
>    printf("LINK: %d\n", status);
>
>    status = open("dir2", O_RDONLY);
>    printf("OPEN: %d\n", status);
>    dir_fd = status;
>
>    status = fsync(dir_fd);
>    printf("FSYNC: %d\n", status);
>
>    status = open(".", O_RDONLY);
>    printf("OPEN: %d\n", status);
>    root_fd = status;
>
>    status = fsync(root_fd);
>    printf("FSYNC: %d\n", status);
> }
> // after crash `dir1` is missing
> ```
>
> Steps:
>
> 1. Create and mount new btrfs file system in default configuration.
> 2. Change directory to root of the file system and run the compiled test.
> 3. Cause hard system crash (e.g. QEMU `system_reset` command).
> 4. Remount file system after crash.
> 5. Observe that directory `dir1` is missing.
>
>