Re: [PATCH 1/4] mm: migrate: move migration validation into numa_migrate_prep()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 8/21/2023 10:20 AM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> Now there are 3 places will validate if a page can mirate or not, and
>> some validations are performed later, which will waste some CPU to call
>> numa_migrate_prep().
>>
>> Thus we can move all the migration validation into numa_migrate_prep(),
>> which is more maintainable as well as saving some CPU resources. Another
>> benefit is that it can serve as a preparation for supporting batch migration
>> in do_numa_page() in future.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/memory.c  | 19 +++++++++++++++++++
>>   mm/migrate.c | 19 -------------------
>>   2 files changed, 19 insertions(+), 19 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d003076b218d..bee9b1e86ef0 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4747,6 +4747,25 @@ int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
>>   		*flags |= TNF_FAULT_LOCAL;
>>   	}
>>   
>> +	/*
>> +	 * Don't migrate file pages that are mapped in multiple processes
>> +	 * with execute permissions as they are probably shared libraries.
>> +	 */
>> +	if (page_mapcount(page) != 1 && page_is_file_lru(page) &&
>> +	    (vma->vm_flags & VM_EXEC))
>> +		return NUMA_NO_NODE;
>> +
>> +	/*
>> +	 * Also do not migrate dirty pages as not all filesystems can move
>> +	 * dirty pages in MIGRATE_ASYNC mode which is a waste of cycles.
>> +	 */
>> +	if (page_is_file_lru(page) && PageDirty(page))
>> +		return NUMA_NO_NODE;
>> +
>> +	/* Do not migrate THP mapped by multiple processes */
>> +	if (PageTransHuge(page) && total_mapcount(page) > 1)
>> +		return NUMA_NO_NODE;
>> +
>>   	return mpol_misplaced(page, vma, addr);
> 
> In mpol_misplaced()->should_numa_migrate_memory(), accessing CPU and PID
> will be recorded.  So the code change above will introduce some behavior
> change.

Indeed.

> 
> How about move these checks into a separate function which is called
> between numa_migrate_prep() and migrate_misplaced_page() after unlocking
> PTL?

Sounds reasonable to me. Thanks for your input.

> 
> --
> Best Regards,
> Huang, Ying
> 
>>   }
>>   
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index e21d5a7e7447..9cc98fb1d6ec 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -2485,10 +2485,6 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
>>   
>>   	VM_BUG_ON_PAGE(order && !PageTransHuge(page), page);
>>   
>> -	/* Do not migrate THP mapped by multiple processes */
>> -	if (PageTransHuge(page) && total_mapcount(page) > 1)
>> -		return 0;
>> -
>>   	/* Avoid migrating to a node that is nearly full */
>>   	if (!migrate_balanced_pgdat(pgdat, nr_pages)) {
>>   		int z;
>> @@ -2533,21 +2529,6 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
>>   	LIST_HEAD(migratepages);
>>   	int nr_pages = thp_nr_pages(page);
>>   
>> -	/*
>> -	 * Don't migrate file pages that are mapped in multiple processes
>> -	 * with execute permissions as they are probably shared libraries.
>> -	 */
>> -	if (page_mapcount(page) != 1 && page_is_file_lru(page) &&
>> -	    (vma->vm_flags & VM_EXEC))
>> -		goto out;
>> -
>> -	/*
>> -	 * Also do not migrate dirty pages as not all filesystems can move
>> -	 * dirty pages in MIGRATE_ASYNC mode which is a waste of cycles.
>> -	 */
>> -	if (page_is_file_lru(page) && PageDirty(page))
>> -		goto out;
>> -
>>   	isolated = numamigrate_isolate_page(pgdat, page);
>>   	if (!isolated)
>>   		goto out;