linux-kernel - Re: [PATCH] fs: avoid calls to legitimize

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Message-ID: <20251110012705.GI2441659@ZenIV>
Date: Mon, 10 Nov 2025 01:27:05 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: avoid calls to legitimize_links() if possible

On Sun, Nov 09, 2025 at 07:54:09PM +0100, Mateusz Guzik wrote:

> @@ -882,8 +887,10 @@ static bool try_to_unlazy(struct nameidata *nd)
>  
>  	BUG_ON(!(nd->flags & LOOKUP_RCU));
>  
> -	if (unlikely(!legitimize_links(nd)))
> -		goto out1;
> +	if (unlikely(need_legitimize_links(nd))) {
> +		if (unlikely(!legitimize_links(nd)))
> +			goto out1;
> +	}
>  	if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
>  		goto out;
>  	if (unlikely(!legitimize_root(nd)))
> @@ -917,8 +924,10 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry)
>  	int res;
>  	BUG_ON(!(nd->flags & LOOKUP_RCU));
>  
> -	if (unlikely(!legitimize_links(nd)))
> -		goto out2;
> +	if (unlikely(need_legitimize_links(nd))) {
> +		if (unlikely(!legitimize_links(nd)))
> +			goto out2;
> +	}
>  	res = __legitimize_mnt(nd->path.mnt, nd->m_seq);
>  	if (unlikely(res)) {
>  		if (res > 0)

Seeing that odds of extra callers showing up are pretty much nil,
I think your need_legitimize_links() is only obfuscating things.
Let's just make it
	if (unlikely(nd->depth) && !legitimize_links(nd))
		goto ...
and be done with that.