On 5/25/2023 3:58 PM, Matthew Wilcox wrote:
> On Thu, May 25, 2023 at 01:15:07PM -0600, Khalid Aziz wrote:
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 5a9501e0ae01..b548e05f0349 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -764,6 +764,42 @@ 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 code is inspired by similar code in migrate_vma_check_page(),
>> + * can_split_folio() and folio_migrate_mapping()
>> + */
>> +static inline bool page_has_extra_refs(struct page *page,
>> + struct address_space *mapping)
>> +{
>> + unsigned long extra_refs;
>> + struct folio *folio;
>> +
>> + /*
>> + * Skip this check for pages in ZONE_MOVABLE or MIGRATE_CMA
>> + * pages that can not be long term pinned
>> + */
>> + if (is_zone_movable_page(page) || is_migrate_cma_page(page))
>> + return false;
>> +
>> + folio = page_folio(page);
>> +
>> + /*
>> + * caller holds a ref already from get_page_unless_zero()
>> + * which is accounted for in folio_expected_refs()
>> + */
>> + extra_refs = folio_expected_refs(mapping, folio);
>> +
>> + /*
>> + * This is an admittedly racy check but good enough to determine
>> + * if a page is pinned and can not be migrated
>> + */
>> + if ((folio_ref_count(folio) - extra_refs) > folio_mapcount(folio))
>> + return true;
>> + return false;
>> +}
>> +
>> /**
>> * isolate_migratepages_block() - isolate all migrate-able pages within
>> * a single pageblock
>> @@ -992,12 +1028,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>> goto isolate_fail;
>
> Just out of shot, we have ...
>
> if (unlikely(!get_page_unless_zero(page)))
>
> This is the perfect opportunity to use folio_get_nontail_page() instead.
> You get back the folio without having to cast the pointer yourself
> or call page_folio(). Now you can use a folio throughout your new
> function, saving a call to compound_head().
>
> For a followup patch, everything in this loop below this point can use
> the folio ... that's quite a lot of change.
>
>> /*
>> - * 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
>> + * from long term pinning preventing it from migrating,
>> + * so avoid taking lru_lock and isolating it unnecessarily.
>> */
>
> Isn't "long term pinning" the wrong description of the problem? Long term
> pins suggest to me FOLL_LONGTERM. I think this is simple short term
> pins that we care about here.
vfio pins are held for a long time - Steve
