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;
>>> /*
