linux-kernel - RE: [PATCH 1/2] fs: reorganize path

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Message-ID: <858f6fb6afcd450d85d1ff900f82d396@AcuMS.aculab.com>
Date: Wed, 24 Apr 2024 09:17:57 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Stas Sergeev' <stsp2@...dex.ru>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: Eric Biederman <ebiederm@...ssion.com>, Alexander Viro
	<viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara
	<jack@...e.cz>, Andy Lutomirski <luto@...nel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH 1/2] fs: reorganize path_openat()

From: Stas Sergeev
> Sent: 22 April 2024 09:45

I seem to have 5 copies of this patch.....

> This patch moves the call to alloc_empty_file() below the call to
> path_init(). That changes is needed for the next patch, which adds
> a cred override for alloc_empty_file(). The needed cred info is only
> available after the call to path_init().
> 
> No functional changes are intended by that patch.
..
> ---
>  fs/namei.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index c5b2a25be7d0..2fde2c320ae9 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3782,22 +3782,30 @@ static struct file *path_openat(struct nameidata *nd,
>  	struct file *file;
>  	int error;
> 
> -	file = alloc_empty_file(op->open_flag, current_cred());
> -	if (IS_ERR(file))
> -		return file;
> -
> -	if (unlikely(file->f_flags & __O_TMPFILE)) {
> +	if (unlikely(op->open_flag & __O_TMPFILE)) {
> +		file = alloc_empty_file(op->open_flag, current_cred());
> +		if (IS_ERR(file))
> +			return file;
>  		error = do_tmpfile(nd, flags, op, file);
> -	} else if (unlikely(file->f_flags & O_PATH)) {
> +	} else if (unlikely(op->open_flag & O_PATH)) {
> +		file = alloc_empty_file(op->open_flag, current_cred());
> +		if (IS_ERR(file))
> +			return file;
>  		error = do_o_path(nd, flags, file);

You probably ought to merge the two 'unlikely' tests.
Otherwise there'll be two conditionals in the 'hot path'.
(There probably always were.)
So something like:
	if (unlikely(op->open_flag & (__O_TMPFILE | O_PATH))) {
		file = alloc_empty_file(op->open_flag, current_cred());
		if (IS_ERR(file))
			return file;
		if (op->open_flag & __O_TMFILE)
			error = do_tmpfile(nd, flags, op, file);
		else
			error = do_o_path(nd, flags, file);
	} else {
Copying op->open_flag to a local may also generate better code.

	David
		
>  	} else {
>  		const char *s = path_init(nd, flags);
> -		while (!(error = link_path_walk(s, nd)) &&
> -		       (s = open_last_lookups(nd, file, op)) != NULL)
> -			;
> +		file = alloc_empty_file(op->open_flag, current_cred());
> +		error = PTR_ERR_OR_ZERO(file);
> +		if (!error) {
> +			while (!(error = link_path_walk(s, nd)) &&
> +			       (s = open_last_lookups(nd, file, op)) != NULL)
> +				;
> +		}
>  		if (!error)
>  			error = do_open(nd, file, op);
>  		terminate_walk(nd);
> +		if (IS_ERR(file))
> +			return file;
>  	}
>  	if (likely(!error)) {
>  		if (likely(file->f_mode & FMODE_OPENED))
> --
> 2.44.0
> 

-
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