mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

When the user no longer requires the pages, they would use
madvise(MADV_FREE) to mark the pages as lazy free.  Subsequently, they
typically would not re-write to that memory again.

During memory reclaim, if we detect that the large folio and its PMD are
both still marked as clean and there are no unexpected references (such as
GUP), so we can just discard the memory lazily, improving the efficiency
of memory reclamation in this case.

On an Intel i5 CPU, reclaiming 1GiB of lazyfree THPs using
mem_cgroup_force_empty() results in the following runtimes in seconds
(shorter is better):

--------------------------------------------
|     Old       |      New       |  Change  |
--------------------------------------------
|   0.683426    |    0.049197    |  -92.80% |
--------------------------------------------

[[email protected]: minor changes per David]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Lance Yang <[email protected]>
Suggested-by: Zi Yan <[email protected]>
Suggested-by: David Hildenbrand <[email protected]>
Cc: Bang Li <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Fangrui Song <[email protected]>
Cc: Jeff Xie <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Cc: Zach O'Keefe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 16 insertions, 10 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 42ffdcdfcd27..69cbd7ac2a5c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1678,16 +1678,22 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			goto walk_abort;
 		}
 
-		if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
-			/*
-			 * We temporarily have to drop the PTL and start once
-			 * again from that now-PTE-mapped page table.
-			 */
-			split_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
-					      false, folio);
-			flags &= ~TTU_SPLIT_HUGE_PMD;
-			page_vma_mapped_walk_restart(&pvmw);
-			continue;
+		if (!pvmw.pte) {
+			if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
+						  folio))
+				goto walk_done;
+
+			if (flags & TTU_SPLIT_HUGE_PMD) {
+				/*
+				 * We temporarily have to drop the PTL and
+				 * restart so we can process the PTE-mapped THP.
+				 */
+				split_huge_pmd_locked(vma, pvmw.address,
+						      pvmw.pmd, false, folio);
+				flags &= ~TTU_SPLIT_HUGE_PMD;
+				page_vma_mapped_walk_restart(&pvmw);
+				continue;
+			}
 		}
 
 		/* Unexpected PMD-mapped THP? */