Re: [PATCH v1] drivers/base/node.c: get rid of get_nid_for_pfn()

[David Hildenbrand <david@redhat.com>]

On 28.11.19 13:01, Michal Hocko wrote:
> On Thu 28-11-19 12:52:16, David Hildenbrand wrote:
>> On 28.11.19 12:50, Michal Hocko wrote:
>>> On Thu 28-11-19 12:23:08, David Hildenbrand wrote:
>>> [...]
>>>> >From fc13fd540a1702592e389e821f6266098e41e2bd Mon Sep 17 00:00:00 2001
>>>> From: David Hildenbrand <david@redhat.com>
>>>> Date: Wed, 27 Nov 2019 16:18:42 +0100
>>>> Subject: [PATCH] drivers/base/node.c: optimize get_nid_for_pfn()
>>>>
>>>> Since commit d84f2f5a7552 ("drivers/base/node.c: simplify
>>>> unregister_memory_block_under_nodes()") we only have a single user of
>>>> get_nid_for_pfn(). The remaining user calls this function when booting -
>>>> where all added memory is online.
>>>>
>>>> Make it clearer that this function should only be used during boot (
>>>> e.g., calling it on offline memory would be bad) by renaming the
>>>> function to something meaningful, optimize out the ifdef and the additional
>>>> system_state check, and add a comment why CONFIG_DEFERRED_STRUCT_PAGE_INIT
>>>> handling is in place at all.
>>>>
>>>> Also, optimize the call site. There is no need to check against
>>>> page_nid < 0 - it will never match the nid (nid >= 0).
>>>>
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>>>> Cc: Michal Hocko <mhocko@kernel.org>
>>>> Cc: Oscar Salvador <osalvador@suse.de>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>
>>> Yes this looks much better! I am not sure this will pass all weird
>>> config combinations because IS_ENABLED will not hide early_pfn_to_nid
>>> from the early compiler stages so it might complain. But if this passes
>>> 0day compile scrutiny then this is much much better. If not then we just
>>> have to use ifdef which is a minor thing.
>>
>> The compiler should optimize out
>>
>> if (0)
>> 	code
>>
>> and therefore never link to early_pfn_to_nid.
> 
> You are right, but there is a catch. The optimization phase is much
> later than the syntactic check so if the code doesn't make sense
> for the syntactic point of view then it will complain. This is a notable
> difference to #ifdef which just removes the whole block in the
> preprocessor phase.
> 

We should always have a declaration of early_pfn_to_nid(). The
interesting part AFAIKS is include/linux/mmzone.h:

#if !defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID) && \
	!defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP)
static inline unsigned long early_pfn_to_nid(unsigned long pfn)
{
	BUILD_BUG_ON(IS_ENABLED(CONFIG_NUMA));
	return 0;
}
#endif

so we would have

if (IS_ENABLED(...))
	BUILD_BUG_ON(IS_ENABLED(CONFIG_NUMA));

Let's see how that will turn out :) Will do some test builds
(CONFIG_NUMA, !CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID and
!CONFIG_HAVE_MEMBLOCK_NODE_MAP) ... if possible

-- 
Thanks,

David / dhildenb