mm/khugepaged: try to free transhuge swapcache when possible

Transhuge swapcaches won't be freed in __collapse_huge_page_copy().  It's
because release_pte_page() is not called for these pages and thus
free_page_and_swap_cache can't grab the page lock.  These pages won't be
freed from swap cache even if we are the only user until next time
reclaim.  It shouldn't hurt indeed, but we could try to free these pages
to save more memory for system.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Miaohe Lin <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Howells <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Zach O'Keefe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 95a5b7aa1ae9..6d11c51b2b62 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -455,6 +455,7 @@ static inline unsigned long total_swapcache_pages(void)
 	return global_node_page_state(NR_SWAPCACHE);
 }
 
+extern void free_swap_cache(struct page *page);
 extern void free_page_and_swap_cache(struct page *);
 extern void free_pages_and_swap_cache(struct page **, int);
 /* linux/mm/swapfile.c */
@@ -539,6 +540,10 @@ static inline void put_swap_device(struct swap_info_struct *si)
 /* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
 #define free_swap_and_cache(e) is_pfn_swap_entry(e)
 
+static inline void free_swap_cache(struct page *page)
+{
+}
+
 static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
 {
 	return 0;