linux-kernel - Re: [PATCH next] KVM: Fix debugfs_simple

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Date:   Tue, 7 Jan 2020 15:01:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Chen Wandun <chenwandun@...wei.com>, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Nicolai Stange <nstange@...e.de>
Subject: Re: [PATCH next] KVM: Fix debugfs_simple_attr.cocci warnings

On 24/12/19 02:41, Chen Wandun wrote:
> Use DEFINE_DEBUGFS_ATTRIBUTE rather than DEFINE_SIMPLE_ATTRIBUTE
> for debugfs files.
> 
> Semantic patch information:
> Rationale: DEFINE_SIMPLE_ATTRIBUTE + debugfs_create_file()
> imposes some significant overhead as compared to
> DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe().
> 
> Signed-off-by: Chen Wandun <chenwandun@...wei.com>

This patch was sent probably already two or three times, and every time
I've not been able to understand what is this significant overhead.

With DEFINE_DEBUGFS_ATTRIBUTE:

- the fops member is debugfs_open_proxy_file_operations, which calls
replace_fops so that the fops->read member is debugfs_attr_read on the
opened file

- debugfs_attr_read does

        ret = debugfs_file_get(dentry);
        if (unlikely(ret))
                return ret;
        ret = simple_attr_read(file, buf, len, ppos);
        debugfs_file_put(dentry);

With DEFINE_SIMPLE_ATTRIBUTE:

- the fops member is debugfs_full_proxy_open, and after
__full_proxy_fops_init fops->read is initialized to full_proxy_read

- full_proxy_read does

        r = debugfs_file_get(dentry);
        if (unlikely(r))
                return r;
        real_fops = debugfs_real_fops(filp);
        r = real_fops->name(args);
        debugfs_file_put(dentry);
        return r;

where real_fops->name is again just simple_attr_read.

So the overhead is really just one kzalloc every time the file is opened.

I could just apply the patch, but it wouldn't solve the main issue,
which is that there is a function with a scary name
("debugfs_create_file_unsafe") that can be used in very common
circumstances (with DEFINE_DEBUGFS_ATTRIBUTE.  Therefore, we could
instead fix the root cause and avoid using the scary API:

- remove from the .cocci patch the change from debugfs_create_file to
debugfs_create_file_unsafe.  Only switch DEFINE_SIMPLE_ATTRIBUTE to
DEFINE_DEBUGFS_ATTRIBUTE

- change debugfs_create_file to automatically detect the "easy" case
that does not need proxying of fops; something like this:

	const struct file_operations *proxy_fops;

	/*
	 * Any struct file_operations defined by means of
	 * DEFINE_DEBUGFS_ATTRIBUTE() is protected against file removals
	 * and thus does not need proxying of read and write fops.
	 */
	if (!fops ||
	    (fops->llseek == no_llseek &&
	     ((!fops->read && !fops->read_iter) ||
	      fops->read == debugfs_attr_read) &&
	     ((!fops->write && !fops->write_iter) ||
	      fops->write == debugfs_attr_write) &&
	     !fops->poll && !fops->unlocked_ioctl)
		return debugfs_create_file_unsafe(name, mode, parent,
						  data, fops);

	/* These are not supported by __full_proxy_fops_init.  */
	WARN_ON_ONCE(fops->read_iter || fops->write_iter);
	return __debugfs_create_file(name, mode, parent, data,
				    &debugfs_full_proxy_file_operations,
				     fops);

CCing Nicolai Stange who first introduced debugfs_create_file_unsafe.

Thanks,

Paolo