linux-kernel - Re: [PATCH] mm/cma_debug: Check for null tmp in cma_debugfs_add

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Date:   Thu, 21 Feb 2019 09:45:25 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Yue Hu <zbestahu@...il.com>, akpm@...ux-foundation.org,
        rientjes@...gle.com, joe@...ches.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, huyue2@...ong.com
Subject: Re: [PATCH] mm/cma_debug: Check for null tmp in cma_debugfs_add_one()

On Thu 21-02-19 09:36:24, Greg KH wrote:
> On Thu, Feb 21, 2019 at 09:23:09AM +0100, Michal Hocko wrote:
> > On Thu 21-02-19 12:01:30, Yue Hu wrote:
> > > From: Yue Hu <huyue2@...ong.com>
> > > 
> > > If debugfs_create_dir() failed, the following debugfs_create_file()
> > > will be meanless since it depends on non-NULL tmp dentry and it will
> > > only waste CPU resource.
> > 
> > The file will be created in the debugfs root. But, more importantly.
> > Greg (CCed now) is working on removing the failure paths because he
> > believes they do not really matter for debugfs and they make code more
> > ugly. More importantly a check for NULL is not correct because you
> > get ERR_PTR after recent changes IIRC.
> > 
> > > 
> > > Signed-off-by: Yue Hu <huyue2@...ong.com>
> > > ---
> > >  mm/cma_debug.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/mm/cma_debug.c b/mm/cma_debug.c
> > > index 2c2c869..3e9d984 100644
> > > --- a/mm/cma_debug.c
> > > +++ b/mm/cma_debug.c
> > > @@ -169,6 +169,8 @@ static void cma_debugfs_add_one(struct cma *cma, struct dentry *root_dentry)
> > >  	scnprintf(name, sizeof(name), "cma-%s", cma->name);
> > >  
> > >  	tmp = debugfs_create_dir(name, root_dentry);
> > > +	if (!tmp)
> > > +		return;
> 
> Ick, yes, this patch isn't ok, I've been doing lots of work to rip these
> checks out :)

Btw. I believe that it would help to clarify this stance in the
kerneldoc otherwise these checks will be returning back because the
general kernel development attitude is to check for errors. As I've said
previously debugfs being different is ugly but decision is yours.

-- 
Michal Hocko
SUSE Labs