linux-kernel - Re: [PATCH SLAB 1/2 v3] duplicate the cache name in SLUB's saved

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Message-ID: <1341884144.2562.18.camel@ThinkPad-T420>
Date:	Tue, 10 Jul 2012 09:35:44 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linux-mm <linux-mm@...ck.org>,
	PowerPC email list <linuxppc-dev@...ts.ozlabs.org>,
	Wanlong Gao <gaowanlong@...fujitsu.com>,
	Glauber Costa <glommer@...allels.com>
Subject: Re: [PATCH SLAB 1/2 v3] duplicate the cache name in SLUB's
 saved_alias list, SLAB, and SLOB

On Mon, 2012-07-09 at 09:01 -0500, Christoph Lameter wrote:
> > I was pointed by Glauber to the slab common code patches. I need some
> > more time to read the patches. Now I think the slab/slot changes in this
> > v3 are not needed, and can be ignored.
> 
> That may take some kernel cycles. You have a current issue here that needs
> to be fixed.

I'm a little confused ... and what need I do for the next step? 

> 
> > >  	down_write(&slub_lock);
> > > -	s = find_mergeable(size, align, flags, name, ctor);
> > > +	s = find_mergeable(size, align, flags, n, ctor);
> > >  	if (s) {
> > >  		s->refcount++;
> > >  		/*
> >
> > 		......
> > 		up_write(&slub_lock);
> > 		return s;
> > 	}
> >
> > Here, the function returns without name string n be kfreed.
> 
> That is intentional since the string n is still referenced by the entry
> that sysfs_slab_alias has created.

I'm not sure whether the "referenced by ..." you mentioned is what I
understood. From my understanding:

if slab_state == SYS_FS, after 
	return sysfs_create_link(&slab_kset->kobj, &s->kobj, name); 
is called, the name string passed in sysfs_slab_alias is no longer
referenced (sysfs_new_dirent duplicates the string for sysfs to use).

else, the name sting is referenced by 
	al->name = name;
temporarily. After slab_sysfs_init is finished, the name is not
referenced any more.

So in my patch (slub part), the string is duplicated here, and kfreed in
slab_sysfs_init.

> > But we couldn't kfree n here, because in sysfs_slab_alias(), if
> > (slab_state < SYS_FS), the name need to be kept valid until
> > slab_sysfs_init() is finished adding the entry into sysfs.
> 
> Right that is why it is not freed and that is what fixes the issue you
> see.
> 


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