Re: [PATCH v2] mm, compaction: Skip all non-migratable pages during scan

[Khalid Aziz <khalid.aziz@oracle.com>]

On 5/15/23 02:47, Huang, Ying wrote:
> Hi, Khalid,
> 
> Cced Mel.
> 
> Khalid Aziz <khalid.aziz@oracle.com> writes:
> 
>> Pages pinned in memory through extra refcounts can not be migrated.
>> Currently as isolate_migratepages_block() scans pages for
>> compaction, it skips any pinned anonymous pages. All non-migratable
>> pages should be skipped and not just the anonymous pinned pages.
>> This patch adds a check for extra refcounts on a page to determine
>> if the page can be migrated.  This was seen as a real issue on a
>> customer workload where a large number of pages were pinned by vfio
>> on the host and any attempts to allocate hugepages resulted in
>> significant amount of cpu time spent in either direct compaction or
>> in kcompatd scanning vfio pinned pages over and over again that can
> 
> s/kcompatd/kcompactd/

Thanks for the review. I will add the missing "c".

> 
>> not be migrated.
> 
> With the patch below, the cycles for kcompactd disappeared?

Yes, the test showed cpu time accumulated by kcompatcd during test run dropped significantly.

> 
>> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
>> Suggested-by: Steve Sistare <steven.sistare@oracle.com>
>> Cc: Khalid Aziz <khalid@kernel.org>
>> ---
>> v2:
>> 	- Update comments in the code (Suggested by Andrew)
>> 	- Use PagePrivate() instead of page_has_private() (Suggested
>> 	  by Matthew)
>> 	- Pass mapping to page_has_extrarefs() (Suggested by Matthew)
>> 	- Use page_ref_count() (Suggested by Matthew)
>> 	- Rename is_pinned_page() to reflect its function more
>> 	  accurately (Suggested by Matthew)
>>
>>   mm/compaction.c | 36 ++++++++++++++++++++++++++++++++----
>>   1 file changed, 32 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 5a9501e0ae01..837f20df2bbb 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -764,6 +764,34 @@ static bool too_many_isolated(pg_data_t *pgdat)
>>   	return too_many;
>>   }
>>   
>> +/*
>> + * Check if this base page should be skipped from isolation because
>> + * it has extra refcounts that will prevent it from being migrated.
>> + * This function is called for regular pages only, and not
>> + * for THP or hugetlbfs pages. This code is inspired by similar code
>> + * in migrate_vma_check_page(), can_split_folio() and
>> + * folio_migrate_mapping()
>> + */
>> +static inline bool page_has_extrarefs(struct page *page,
> 
> Better to be named as page_has_extra_refs()?

Sure, I can do that.

> 
>> +					struct address_space *mapping)
>> +{
>> +	unsigned long extra_refs;
>> +
>> +	/* anonymous page can have extra ref from swap cache */
>> +	if (mapping)
>> +		extra_refs = 1 + PagePrivate(page);
>> +	else
>> +		extra_refs = PageSwapCache(page) ? 1 : 0;
> 
> IIUC, mapping != NULL if PageSwapCache(page) is true.  Please check the
> implementation of page_mapping().

which should work out for refcount since it would go to if part of conditional which will add a 1 for swap cache 
reference, but this code could be written better. I will work on it.

> 
> And even if mapping == NULL, the extra_refs should be 1, because we have
> elevated the page refcount in isolate_migratepages_block() before
> checking whether the page is pinned.  IIUC, this is the original
> behavior.  Or, we can add "- 1" in the following checking.
> 

You are right. There is a get_page_unless_zero() earlier in the code. I will compensate for that.

>> +
>> +	/*
>> +	 * This is an admittedly racy check but good enough to determine
>> +	 * if a page is pinned and can not be migrated
>> +	 */
>> +	if ((page_ref_count(page) - extra_refs) > page_mapcount(page))
>> +		return true;
>> +	return false;
>> +}
>> +
>>   /**
>>    * isolate_migratepages_block() - isolate all migrate-able pages within
>>    *				  a single pageblock
>> @@ -992,12 +1020,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   			goto isolate_fail;
>>   
>>   		/*
>> -		 * Migration will fail if an anonymous page is pinned in memory,
>> -		 * so avoid taking lru_lock and isolating it unnecessarily in an
>> -		 * admittedly racy check.
>> +		 * Migration will fail if a page has extra refcounts
>> +		 * preventing it from migrating, so avoid taking
>> +		 * lru_lock and isolating it unnecessarily
>>   		 */
>>   		mapping = page_mapping(page);
>> -		if (!mapping && (page_count(page) - 1) > total_mapcount(page))
>> +		if (page_has_extrarefs(page, mapping))
>>   			goto isolate_fail_put;
>>   
>>   		/*
> 
> Best Regards,
> Huang, Ying

Thanks,
Khalid