linux-kernel - Re: [PATCH] powerpc/nvram: Replace kmalloc with kzalloc in the error message

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Date:   Wed, 3 Jun 2020 16:18:41 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Markus Elfring <Markus.Elfring@....de>,
        Liao Pingfang <liao.pingfang@....com.cn>,
        linuxppc-dev@...ts.ozlabs.org, Joe Perches <joe@...ches.com>,
        Allison Randal <allison@...utok.net>,
        Anton Vorontsov <anton@...msg.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Colin Cross <ccross@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Wang Liang <wang.liang82@....com.cn>,
        Xue Zhihong <xue.zhihong@....com.cn>,
        Yi Wang <wang.yi59@....com.cn>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] powerpc/nvram: Replace kmalloc with kzalloc in the error
 message

On Wed, Jun 03, 2020 at 09:37:18PM +1000, Michael Ellerman wrote:
> Dan Carpenter <dan.carpenter@...cle.com> writes:
> > On Tue, Jun 02, 2020 at 09:23:57PM +1000, Michael Ellerman wrote:
> >> Markus Elfring <Markus.Elfring@....de> writes:
> >> >>>> Please just remove the message instead, it's a tiny allocation that's
> >> >>>> unlikely to ever fail, and the caller will print an error anyway.
> >> >>>
> >> >>> How do you think about to take another look at a previous update suggestion
> >> >>> like the following?
> >> >>>
> >> >>> powerpc/nvram: Delete three error messages for a failed memory allocation
> >> >>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/00845261-8528-d011-d3b8-e9355a231d3a@users.sourceforge.net/
> >> >>> https://lore.kernel.org/linuxppc-dev/00845261-8528-d011-d3b8-e9355a231d3a@users.sourceforge.net/
> >> >>> https://lore.kernel.org/patchwork/patch/752720/
> >> >>> https://lkml.org/lkml/2017/1/19/537
> >> >>
> >> >> That deleted the messages from nvram_scan_partitions(), but neither of
> >> >> the callers of nvram_scan_paritions() check its return value or print
> >> >> anything if it fails. So removing those messages would make those
> >> >> failures silent which is not what we want.
> >> >
> >> > * How do you think about information like the following?
> >> >   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/coding-style.rst?id=f359287765c04711ff54fbd11645271d8e5ff763#n883
> >> > “…
> >> > These generic allocation functions all emit a stack dump on failure when used
> >> > without __GFP_NOWARN so there is no use in emitting an additional failure
> >> > message when NULL is returned.
> >> > …”
> >> 
> >> Are you sure that's actually true?
> >> 
> >> A quick look around in slub.c leads me to:
> >> 
> >> slab_out_of_memory(struct kmem_cache *s, gfp_t gfpflags, int nid)
> >> {
> >> #ifdef CONFIG_SLUB_DEBUG
> >
> > You first have to enable EXPERT mode before you can disable SLUB_DEBUG.
> 
> I see ~175 defconfigs with CONFIG_EXPERT=y, so that's not really a high
> bar unfortunately.
> 
> And there's 38 defconfigs with SLUB_DEBUG=n.
> 
> So for kernels built with those defconfigs that documentation is plain
> wrong and misleading.
> 
> And then there's SLOB which doesn't dump stack anywhere AFAICS.
> 
> In fact slab_out_of_memory() doesn't emit a stack dump either, it just
> prints a bunch of slab related info!
> 
> > So that hopefully means you *really* want to save memory.  It doesn't
> > make sense to add a bunch of memory wasting printks when the users want
> > to go to extra lengths to conserve memory.
> 
> I agree that in many cases those printks are just a waste of space in
> the source and the binary and should be removed.
> 
> But I dislike being told "these generic allocation functions all emit a
> stack dump" only to find out that actually they don't, they print some
> other debug info, and depending on config settings they actually don't
> print _anything_.

Wait...  It *does* print a stack trace.  We must but looking at the
wrong function.  Huh...  The stack trace comes from warn_alloc().  What
happen is this:

mm/slub.c
  2673  
  2674          freelist = new_slab_objects(s, gfpflags, node, &c);
  2675  
  2676          if (unlikely(!freelist)) {
  2677                  slab_out_of_memory(s, gfpflags, node);
  2678                  return NULL;
  2679          }
  2680  

The new_slab_objects() will call allocate_slab() which calls
__alloc_pages_slowpath() which calls warn_alloc() on failure.

There are some error paths from alloc_pages() which look like they
could return without the stack dump, but those are impossible paths from
kmalloc or error injection.

regards,
dan carpenter