Re: [PATCH v3] mm, compaction: Skip all non-migratable pages during scan

[Huang, Ying]

Khalid Aziz <khalid.aziz@xxxxxxxxxx> writes:

> On 5/17/23 19:21, Huang, Ying wrote:
>> Khalid Aziz <khalid.aziz@xxxxxxxxxx> 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 kcompactd scanning vfio pinned pages over and over again that can
>>> not be migrated.
>>>
>>> Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>>> Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
>>> ---
>>> v3:
>>> 	- Account for extra ref added by get_page_unless_zero() earlier
>>> 	  in isolate_migratepages_block() (Suggested by Huang, Ying)
>>> 	- Clean up computation of extra refs to be consistent
>>> 	  (Suggested by Huang, Ying)
>>>
>>> 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..f04c00981172 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 appears duplicated with the comments in caller.  It's OK to
>> keep
>> one only?
>
> I don't see any harm in explaining what the function is doing and then
> why this function is being called. This allows one to understand code
> in either place without having to refer to the function and function
> call both to understand what the code is doing.

Duplicated comments will waste reader's time.

>> 
>>> + * 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()
>> It's not good to duplicate code.  Why not just use
>> folio_expected_refs()?
>
> compaction.c has not been converted over to folios, so I want to not
> do piecemeal conversion. When compaction code is converted over, it
> would make sense to see if this could also use the folio function.
> folio_expected_refs() currently is a static function in mm/migrate.c.

page_folio() is handy.

Best Regards,
Huang, Ying

>> 
>>> + */
>>> +static inline bool page_has_extra_refs(struct page *page)
>>> +{
>>> +	/* caller holds a ref already from get_page_unless_zero() */
>>> +	unsigned long extra_refs = 1;
>>> +
>>> +	/* anonymous page can have extra ref from swap cache */
>>> +	if (PageAnon(page))
>>> +		extra_refs += PageSwapCache(page) ? 1 : 0;
>>> +	else
>>> +		extra_refs += 1 + PagePrivate(page);
>>> +
>>> +	/*
>>> +	 * 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_extra_refs(page))
>>>   			goto isolate_fail_put;
>>>     		/*