Re: [PATCH] fs: Propagate FMODE_NOCMTIME flag to user-facing O_NOCMTIME

[Christoph Hellwig <hch@infradead.org>]

On Fri, Oct 03, 2025 at 12:32:13PM +0300, Pavel Emelyanov wrote:
> The FMODE_NOCMTIME flag tells that ctime and mtime stamps are not
> updated on IO. The flag was introduced long ago by 4d4be482a4 ([XFS]
> add a FMODE flag to make XFS invisible I/O less hacky. Back then it
> was suggested that this flag is propagated to a O_NOCMTIME one.

skipping c/mtime is dangerous.  The XFS handle code allows it to
support HSM where data is migrated out to tape, and requires
CAP_SYS_ADMIN.  Allowing it for any file owner would expand the scope
for too much as now everyone could skip timestamp updates.

> It can be used by workloads that want to write a file but don't care
> much about the preciese timestamp on it and can update it later with
> utimens() call.

The workload might not care, the rest of the system does.  ctime can't
bet set to arbitrary values, so it is important for backups and as
an audit trail.

> There's another reason for having this patch. When performing AIO write,
> the file_modified_flags() function checks whether or not to update inode
> times. In case update is needed and iocb carries the RWF_NOWAIT flag,
> the check return EINTR error that quickly propagates into cb completion
> without doing any IO. This restriction effectively prevents doing AIO
> writes with nowait flag, as file modifications really imply time update.

Well, we'll need to look into that, including maybe non-blockin
timestamp updates.