linux-kernel - Re: [RFC PATCH] vfs: dodge strlen() in vfs_readlink() when ->i

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Message-ID: <20241118115359.mzzx3avongvfqaha@quack3>
Date: Mon, 18 Nov 2024 12:53:59 +0100
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] vfs: dodge strlen() in vfs_readlink() when ->i_link
 is populated

On Mon 18-11-24 09:53:57, Mateusz Guzik wrote:
> This gives me about 1.5% speed up when issuing readlink on /initrd.img
> on ext4.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> ---
> 
> I had this running with the following debug:
> 
> if (strlen(link) != inode->i_size)
>        printk(KERN_CRIT "mismatch [%s] %l %l\n", link,
>            strlen(link), inode->i_size);
> 
> nothing popped up

Then you didn't run with UDF I guess ;). There inode->i_size is the length
of on-disk symlink encoding which is definitely different from the length
of the string we return to VFS (it uses weird standards-defined cross OS
compatible encoding of a path and I'm not even mentioning its own special
encoding of character sets somewhat resembling UCS-2).

> I would leave something of that sort in if it was not defeating the
> point of the change.
> 
> However, I'm a little worried some crap fs *does not* fill this in
> despite populating i_link.
> 
> Perhaps it would make sense to keep the above with the patch hanging out
> in next and remove later?
> 
> Anyhow, worst case, should it turn out i_size does not work there are at
> least two 4-byte holes which can be used to store the length (and
> chances are some existing field can be converted into a union instead).

I'm not sure I completely follow your proposal here...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR