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

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Message-ID: <CAGudoHHezVS1Z00N1EvC-QC5Z_R7pAbJw+B0Z1rijEN_OdFO1g@mail.gmail.com>
Date: Mon, 18 Nov 2024 13:20:09 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, 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, Nov 18, 2024 at 12:53 PM Jan Kara <jack@...e.cz> wrote:
>
> 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).
>

Indeed I did not, thanks. :>

> > 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...
>

I am saying if the size has to be explicitly stored specifically for
symlinks, 2 options are:
- fill up one of the holes
- find a field which is never looked at for symlink inodes and convert
into a union

I'm going to look into it.

-- 
Mateusz Guzik <mjguzik gmail.com>