linux-kernel - Re: [PATCH v1] drivers/base/node.c: get rid of get_nid_for

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Date:   Thu, 28 Nov 2019 11:25:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1] drivers/base/node.c: get rid of get_nid_for_pfn()



> Am 28.11.2019 um 11:20 schrieb Michal Hocko <mhocko@...nel.org>:
> 
> On Wed 27-11-19 18:41:26, David Hildenbrand wrote:
>> Since commit d84f2f5a7552 ("drivers/base/node.c: simplify
>> unregister_memory_block_under_nodes()") we only have a single user of
>> get_nid_for_pfn(). Let's just inline that code and get rid of
>> get_nid_for_pfn().
>> 
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
>> Cc: Michal Hocko <mhocko@...nel.org>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> I am not really sure this is an improvement. The code is ugly as hell
> and open coding it just makes register_mem_sect_under_node harder to
> read.

The issue I see is that this is a dangerous wrapper for pfn_to_nid() that is absolutely not obvious. The old second user on the memory removal path was completely buggy. IMHO nobody should be reusing that function. But it looks like a general „safe wrapper to get a nid“ - it‘s not.

How can we make that more obvious instead?

> 
> If anything get_nid_for_pfn deserves a comment why
> CONFIG_DEFERRED_STRUCT_PAGE_INIT calls for special case as
> early_pfn_to_nid is not bound to that config (it is defined when
> CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID || CONFIG_HAVE_MEMBLOCK_NODE_MAP
> 
>> ---
>> drivers/base/node.c | 23 +++++++----------------
>> 1 file changed, 7 insertions(+), 16 deletions(-)
>> 
>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>> index 98a31bafc8a2..735073fd2926 100644
>> --- a/drivers/base/node.c
>> +++ b/drivers/base/node.c
>> @@ -744,17 +744,6 @@ int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
>> }
>> 
>> #ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
>> -static int __ref get_nid_for_pfn(unsigned long pfn)
>> -{
>> -    if (!pfn_valid_within(pfn))
>> -        return -1;
>> -#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> -    if (system_state < SYSTEM_RUNNING)
>> -        return early_pfn_to_nid(pfn);
>> -#endif
>> -    return pfn_to_nid(pfn);
>> -}
>> -
>> /* register memory section under specified node if it spans that node */
>> static int register_mem_sect_under_node(struct memory_block *mem_blk,
>>                     void *arg)
>> @@ -766,8 +755,6 @@ static int register_mem_sect_under_node(struct memory_block *mem_blk,
>>    unsigned long pfn;
>> 
>>    for (pfn = start_pfn; pfn <= end_pfn; pfn++) {
>> -        int page_nid;
>> -
>>        /*
>>         * memory block could have several absent sections from start.
>>         * skip pfn range from absent section
>> @@ -784,11 +771,15 @@ static int register_mem_sect_under_node(struct memory_block *mem_blk,
>>         * block belong to the same node.
>>         */
>>        if (system_state == SYSTEM_BOOTING) {
>> -            page_nid = get_nid_for_pfn(pfn);
>> -            if (page_nid < 0)
>> +            if (!pfn_valid_within(pfn))
>>                continue;
>> -            if (page_nid != nid)
>> +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> +            if (early_pfn_to_nid(pfn) != nid)
>>                continue;
>> +#else
>> +            if (pfn_to_nid(pfn) != nid)
>> +                continue;
>> +#endif
>>        }
>> 
>>        /*
>> -- 
>> 2.21.0
> 
> -- 
> Michal Hocko
> SUSE Labs
>