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.
