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

Dan Carpenter
2020-06-03T13:19:59Z

On Wed, Jun 03, 2020 at 09:37:18PM +1000, Michael Ellerman wrote:
> Dan Carpenter <dan.carpenter@oracle.com> writes:
> > On Tue, Jun 02, 2020 at 09:23:57PM +1000, Michael Ellerman wrote:
> >> Markus Elfring <Markus.Elfring@web.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