Re: [PATCH] mm/hotplug: Enumerate memory range offlining failure reasons

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 08/18/2020 11:35 AM, Michal Hocko wrote:
> On Tue 18-08-20 09:52:02, Anshuman Khandual wrote:
>> Currently a debug message is printed describing the reason for memory range
>> offline failure. This just enumerates existing reason codes which improves
>> overall readability and makes it cleaner. This does not add any functional
>> change.
> 
> Wasn't something like that posted already? To be honest I do not think

There was a similar one regarding bad page reason.

https://patchwork.kernel.org/patch/11464713/

> this is worth the additional LOC. We are talking about few strings used
> at a single place. I really do not see any simplification, constants are
> sometimes even longer than the strings they are describing.

I am still trying to understand why enumerating all potential offline
failure reasons in a single place (i.e via enum) is not a better idea
than strings scattered across the function. Besides being cleaner, it
classifies, organizes and provide a structure to the set of reasons.
It is not just about string replacement with constants.

> 
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This is based on 5.9-rc1
>>
>>  include/linux/memory.h | 28 ++++++++++++++++++++++++++++
>>  mm/memory_hotplug.c    | 18 +++++++++---------
>>  2 files changed, 37 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/memory.h b/include/linux/memory.h
>> index 439a89e758d8..4b52d706edc1 100644
>> --- a/include/linux/memory.h
>> +++ b/include/linux/memory.h
>> @@ -44,6 +44,34 @@ int set_memory_block_size_order(unsigned int order);
>>  #define	MEM_CANCEL_ONLINE	(1<<4)
>>  #define	MEM_CANCEL_OFFLINE	(1<<5)
>>  
>> +/*
>> + * Memory offline failure reasons
>> + */
>> +enum offline_failure_reason {
>> +	OFFLINE_FAILURE_MEMHOLES,
>> +	OFFLINE_FAILURE_MULTIZONE,
>> +	OFFLINE_FAILURE_ISOLATE,
>> +	OFFLINE_FAILURE_NOTIFIER,
>> +	OFFLINE_FAILURE_SIGNAL,
>> +	OFFLINE_FAILURE_UNMOVABLE,
>> +	OFFLINE_FAILURE_DISSOLVE,
>> +};
>> +
>> +static const char *const offline_failure_names[] = {
>> +	[OFFLINE_FAILURE_MEMHOLES]	= "memory holes",
>> +	[OFFLINE_FAILURE_MULTIZONE]	= "multizone range",
>> +	[OFFLINE_FAILURE_ISOLATE]	= "failure to isolate range",
>> +	[OFFLINE_FAILURE_NOTIFIER]	= "notifier failure",
>> +	[OFFLINE_FAILURE_SIGNAL]	= "signal backoff",
>> +	[OFFLINE_FAILURE_UNMOVABLE]	= "unmovable page",
>> +	[OFFLINE_FAILURE_DISSOLVE]	= "failure to dissolve huge pages",
>> +};
>> +
>> +static inline const char *offline_failure(enum offline_failure_reason reason)
>> +{
>> +	return offline_failure_names[reason];
>> +}
>> +
>>  struct memory_notify {
>>  	unsigned long start_pfn;
>>  	unsigned long nr_pages;
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index e9d5ab5d3ca0..b3fa36a09d7f 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1484,7 +1484,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  	unsigned long flags;
>>  	struct zone *zone;
>>  	struct memory_notify arg;
>> -	char *reason;
>> +	enum offline_failure_reason reason;
>>  
>>  	mem_hotplug_begin();
>>  
>> @@ -1500,7 +1500,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  			      count_system_ram_pages_cb);
>>  	if (nr_pages != end_pfn - start_pfn) {
>>  		ret = -EINVAL;
>> -		reason = "memory holes";
>> +		reason = OFFLINE_FAILURE_MEMHOLES;
>>  		goto failed_removal;
>>  	}
>>  
>> @@ -1509,7 +1509,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  	zone = test_pages_in_a_zone(start_pfn, end_pfn);
>>  	if (!zone) {
>>  		ret = -EINVAL;
>> -		reason = "multizone range";
>> +		reason = OFFLINE_FAILURE_MULTIZONE;
>>  		goto failed_removal;
>>  	}
>>  	node = zone_to_nid(zone);
>> @@ -1519,7 +1519,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  				       MIGRATE_MOVABLE,
>>  				       MEMORY_OFFLINE | REPORT_FAILURE);
>>  	if (ret < 0) {
>> -		reason = "failure to isolate range";
>> +		reason = OFFLINE_FAILURE_ISOLATE;
>>  		goto failed_removal;
>>  	}
>>  	nr_isolate_pageblock = ret;
>> @@ -1531,7 +1531,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  	ret = memory_notify(MEM_GOING_OFFLINE, &arg);
>>  	ret = notifier_to_errno(ret);
>>  	if (ret) {
>> -		reason = "notifier failure";
>> +		reason = OFFLINE_FAILURE_NOTIFIER;
>>  		goto failed_removal_isolated;
>>  	}
>>  
>> @@ -1540,7 +1540,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  		do {
>>  			if (signal_pending(current)) {
>>  				ret = -EINTR;
>> -				reason = "signal backoff";
>> +				reason = OFFLINE_FAILURE_SIGNAL;
>>  				goto failed_removal_isolated;
>>  			}
>>  
>> @@ -1558,7 +1558,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  		} while (!ret);
>>  
>>  		if (ret != -ENOENT) {
>> -			reason = "unmovable page";
>> +			reason = OFFLINE_FAILURE_UNMOVABLE;
>>  			goto failed_removal_isolated;
>>  		}
>>  
>> @@ -1569,7 +1569,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  		 */
>>  		ret = dissolve_free_huge_pages(start_pfn, end_pfn);
>>  		if (ret) {
>> -			reason = "failure to dissolve huge pages";
>> +			reason = OFFLINE_FAILURE_DISSOLVE;
>>  			goto failed_removal_isolated;
>>  		}
>>  		/* check again */
>> @@ -1627,7 +1627,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  	pr_debug("memory offlining [mem %#010llx-%#010llx] failed due to %s\n",
>>  		 (unsigned long long) start_pfn << PAGE_SHIFT,
>>  		 ((unsigned long long) end_pfn << PAGE_SHIFT) - 1,
>> -		 reason);
>> +		 offline_failure(reason));
>>  	/* pushback to free area */
>>  	mem_hotplug_done();
>>  	return ret;
>> -- 
>> 2.20.1
>>
>