linux-kernel - Re: [PATCH] evm: stop avoidably reading i_writecount in evm_file

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Message-ID: <CAGudoHGBTRJqKAE6Db3PyVne6rrJR4vsF2MNH2qKMy-44XReZw@mail.gmail.com>
Date: Mon, 23 Sep 2024 07:26:37 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: zohar@...ux.ibm.com, roberto.sassu@...wei.com, paul@...l-moore.com, 
	jmorris@...ei.org, serge@...lyn.com, linux-kernel@...r.kernel.org, 
	linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] evm: stop avoidably reading i_writecount in evm_file_release

On Fri, Aug 16, 2024 at 1:53 PM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
>
> On Tue, 2024-08-06 at 15:36 +0200, Mateusz Guzik wrote:
> > The EVM_NEW_FILE flag is unset if the file already existed at the time
> > of open and this can be checked without looking at i_writecount.
>
> Agreed. EVM_NEW_FILE is not going to be set during the open(), only
> before, in evm_post_path_mknod().
>
> Looks good to me.
>
> Reviewed-by: Roberto Sassu <roberto.sassu@...wei.com>
>
> Thanks

thanks for the review

are there plans to pick this up for this merge window?

>
> Roberto
>
> > Not accessing it reduces traffic on the cacheline during parallel open
> > of the same file and drop the evm_file_release routine from second place
> > to bottom of the profile.
> >
> > Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> > ---
> >
> > The context is that I'm writing a patch which removes one lockref
> > get/put cycle on parallel open. An operational WIP reduces ping-pong in
> > that area and made do_dentry_open skyrocket along with evm_file_release,
> > due to i_writecount access. With the patch they go down again and
> > apparmor takes the rightful first place.
> >
> > The patch accounts for about 5% speed up at 20 cores running open3 from
> > will-it-scale on top of the above wip. (the apparmor + lockref thing
> > really don't scale, that's next)
> >
> > I would provide better measurements, but the wip is not ready (as the
> > description suggests) and I need evm out of the way for the actual
> > patch.
> >
> >  security/integrity/evm/evm_main.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index 62fe66dd53ce..309630f319e2 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -1084,7 +1084,8 @@ static void evm_file_release(struct file *file)
> >       if (!S_ISREG(inode->i_mode) || !(mode & FMODE_WRITE))
> >               return;
> >
> > -     if (iint && atomic_read(&inode->i_writecount) == 1)
> > +     if (iint && iint->flags & EVM_NEW_FILE &&
> > +         atomic_read(&inode->i_writecount) == 1)
> >               iint->flags &= ~EVM_NEW_FILE;
> >  }
> >
>


-- 
Mateusz Guzik <mjguzik gmail.com>