File name is not persisted if opened with O_SYNC and O_TRUNC flags

File name is not persisted if opened with O_SYNC and O_TRUNC flags

[Vyacheslav Kovalevsky]

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

Hello, there seems to be an issue with O_SYNC flag when used together 
with O_TRUNC on various file systems.
Opening a file with O_SYNC (or using fsync(fd)) should persist directory 
entry.
However, if O_SYNC is used together with O_TRUNC the file will be 
missing if system crashes.
According to POSIX this is OK, but most file systems provide stronger 
guarantees (would be actually nice to have a more recent documentation 
on this behavior).
This happens on Btrfs, ext4, XFS, F2FS and likely other file systems.


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

Linux version 6.19-rc7, also tested on 6.17


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;

   status = creat("file", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
   printf("CREAT: %d\n", status);
   close(status);

   status = open("file", O_RDWR | O_TRUNC | O_SYNC);
   printf("OPEN: %d\n", status);
}
// after the crash `file` is missing
```

Steps:
1. Create and mount new 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 file is missing.

Re: File name is not persisted if opened with O_SYNC and O_TRUNC flags

[Christoph Hellwig]

On Thu, Feb 12, 2026 at 02:51:47PM +0300, Vyacheslav Kovalevsky wrote:
> Detailed description
> ====================
> 
> Hello, there seems to be an issue with O_SYNC flag when used together with
> O_TRUNC on various file systems.
> Opening a file with O_SYNC (or using fsync(fd)) should persist directory
> entry.

No, it should not.  I'm not sure who hallucinated, but O_SYNC has always
always applied to persistency semantics after writes and nothing else.

Re: File name is not persisted if opened with O_SYNC and O_TRUNC flags

[Vyacheslav Kovalevsky]

On 13/02/2026 10:33, Christoph Hellwig wrote:
> On Thu, Feb 12, 2026 at 02:51:47PM +0300, Vyacheslav Kovalevsky wrote:
>> Detailed description ==================== Hello, there seems to be an 
>> issue with O_SYNC flag when used together with O_TRUNC on various 
>> file systems. Opening a file with O_SYNC (or using fsync(fd)) should 
>> persist directory entry. 
> No, it should not. I'm not sure who hallucinated, but O_SYNC has 
> always always applied to persistency semantics after writes and 
> nothing else.
You are right, opening file with O_SYNC does not persist anything and 
ftruncate or O_TRUNC do not count as write I/O it seems. Also found an 
error related to these assumptions in our testing tool. Thanks.