linux-kernel - Re: [RFC PATCH v2 1/3] meminfo_extra: introduce meminfo extra

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Date:   Tue, 24 Mar 2020 11:11:10 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
Cc:     leon@...nel.org, vbabka@...e.cz, adobriyan@...il.com,
        akpm@...ux-foundation.org, labbott@...hat.com,
        sumit.semwal@...aro.org, minchan@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, kasong@...hat.com,
        bhe@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jaewon31.kim@...il.com, linux-api@...r.kernel.org,
        kexec@...ts.infradead.org
Subject: Re: [RFC PATCH v2 1/3] meminfo_extra: introduce meminfo extra

On Tue, Mar 24, 2020 at 06:11:17PM +0900, Jaewon Kim wrote:
> On 2020년 03월 23일 18:53, Greg KH wrote:
> >> +int register_meminfo_extra(atomic_long_t *val, int shift, const char *name)
> >> +{
> >> +	struct meminfo_extra *meminfo, *memtemp;
> >> +	int len;
> >> +	int error = 0;
> >> +
> >> +	meminfo = kzalloc(sizeof(*meminfo), GFP_KERNEL);
> >> +	if (!meminfo) {
> >> +		error = -ENOMEM;
> >> +		goto out;
> >> +	}
> >> +
> >> +	meminfo->val = val;
> >> +	meminfo->shift_for_page = shift;
> >> +	strncpy(meminfo->name, name, NAME_SIZE);
> >> +	len = strlen(meminfo->name);
> >> +	meminfo->name[len] = ':';
> >> +	strncpy(meminfo->name_pad, meminfo->name, NAME_BUF_SIZE);
> >> +	while (++len < NAME_BUF_SIZE - 1)
> >> +		meminfo->name_pad[len] = ' ';
> >> +
> >> +	spin_lock(&meminfo_lock);
> >> +	list_for_each_entry_rcu(memtemp, &meminfo_head, list) {
> >> +		if (memtemp->val == val) {
> >> +			error = -EINVAL;
> >> +			break;
> >> +		}
> >> +	}
> >> +	if (!error)
> >> +		list_add_tail_rcu(&meminfo->list, &meminfo_head);
> >> +	spin_unlock(&meminfo_lock);
> > If you have a lock, why are you needing rcu?
> I think _rcu should be removed out of list_for_each_entry_rcu.
> But I'm confused about what you meant.
> I used rcu_read_lock on __meminfo_extra,
> and I think spin_lock is also needed for addition and deletion to handle multiple modifiers.

If that's the case, then that's fine, it just didn't seem like that was
needed.  Or I might have been reading your rcu logic incorrectly...

> >> +	if (error)
> >> +		kfree(meminfo);
> >> +out:
> >> +
> >> +	return error;
> >> +}
> >> +EXPORT_SYMBOL(register_meminfo_extra);
> > EXPORT_SYMBOL_GPL()?  I have to ask :)
> I can use EXPORT_SYMBOL_GPL.
> >
> > thanks,
> >
> > greg k-h
> >
> >
> 
> Hello
> Thank you for your comment.
> 
> By the way there was not resolved discussion on v1 patch as I mentioned on cover page.
> I'd like to hear your opinion on this /proc/meminfo_extra node.

I think it is the propagation of an old and obsolete interface that you
will have to support for the next 20+ years and yet not actually be
useful :)

> Do you think this is meaningful or cannot co-exist with other future
> sysfs based API.

What sysfs-based API?

I still don't know _why_ you want this.  The ION stuff is not needed as
that code is about to be deleted, so who else wants this?  What is the
use-case for it that is so desperately needed that parsing
yet-another-proc file is going to solve the problem?

thanks,

greg k-h