Re: [PATCH] fsnotify: move fsnotify_open() hook into do_dentry_open()

[Christian Brauner <brauner@kernel.org>]

On Sun, Jun 11, 2023 at 03:24:29PM +0300, Amir Goldstein wrote:
> fsnotify_open() hook is called only from high level system calls
> context and not called for the very many helpers to open files.
> 
> This may makes sense for many of the special file open cases, but it is
> inconsistent with fsnotify_close() hook that is called for every last
> fput() of on a file object with FMODE_OPENED.
> 
> As a result, it is possible to observe ACCESS, MODIFY and CLOSE events
> without ever observing an OPEN event.
> 
> Fix this inconsistency by replacing all the fsnotify_open() hooks with
> a single hook inside do_dentry_open().
> 
> If there are special cases that would like to opt-out of the possible
> overhead of fsnotify() call in fsnotify_open(), they would probably also
> want to avoid the overhead of fsnotify() call in the rest of the fsnotify
> hooks, so they should be opening that file with the __FMODE_NONOTIFY flag.
> 
> However, in the majority of those cases, the s_fsnotify_connectors
> optimization in fsnotify_parent() would be sufficient to avoid the
> overhead of fsnotify() call anyway.
> 
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
> 
> Jan,
> 
> I found out about this problem when I tested the work to remove
> FMODE_NONOTIFY from overlayfs internal files - even after I enabled
> notifications on the underlying fs, the LTS tests [2] did not observe
> the OPEN events.
> 
> Because this change is independent of the ovl work and has implications
> on other subsystems as well (e.g. cachefiles), I think it is better
> if the change came through your tree.
> 
> This change has a potential to regress some micro-benchmarks, so if
> you could queue it up for soaking in linux-next that would be great.
> 
> Thanks,
> Amir.
> 
> 
> [1] https://lore.kernel.org/linux-fsdevel/20230609073239.957184-1-amir73il@gmail.com/
> [2] https://github.com/amir73il/ltp/commits/ovl_encode_fid
> 
>  fs/exec.c            | 5 -----
>  fs/fhandle.c         | 1 -
>  fs/open.c            | 6 +++++-
>  io_uring/openclose.c | 1 -
>  4 files changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index a466e797c8e2..238473de1ec5 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -152,8 +152,6 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>  			 path_noexec(&file->f_path)))
>  		goto exit;
>  
> -	fsnotify_open(file);
> -
>  	error = -ENOEXEC;
>  
>  	read_lock(&binfmt_lock);
> @@ -934,9 +932,6 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  	if (err)
>  		goto exit;
>  
> -	if (name->name[0] != '\0')
> -		fsnotify_open(file);
> -
>  out:
>  	return file;
>  
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index fd0d6a3b3699..6ea8d35a9382 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -242,7 +242,6 @@ static long do_handle_open(int mountdirfd, struct file_handle __user *ufh,
>  		retval =  PTR_ERR(file);
>  	} else {
>  		retval = fd;
> -		fsnotify_open(file);
>  		fd_install(fd, file);
>  	}
>  	path_put(&path);
> diff --git a/fs/open.c b/fs/open.c
> index 4478adcc4f3a..81444ebf6091 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -969,6 +969,11 @@ static int do_dentry_open(struct file *f,
>  		}
>  	}
>  
> +	/*
> +	 * Once we return a file with FMODE_OPENED, __fput() will call
> +	 * fsnotify_close(), so we need fsnotify_open() here for symetry.

s/symetry/symmetry/