[PATCH] khugepaged: Add comments for retract_page_tables()

[Kirill A. Shutemov]

Oleg Nesterov pointed that logic behind checks in retract_page_tables()
are not obvious.

Add comments to clarify the reasoning for the checks and why they are
safe.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/khugepaged.c | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5c0a5f0826b2..00cec6a127aa 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1421,7 +1421,22 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 
 	i_mmap_lock_write(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
-		/* probably overkill */
+		/*
+		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
+		 * got written to. These VMAs are likely not worth investing
+		 * down_write(mmap_sem) as PMD-mapping is likely to be split
+		 * later.
+		 *
+		 * Not that vma->anon_vma check is racy: it can be set up after
+		 * the check but before we took mmap_sem by the fault path.
+		 * But page lock would prevent establishing any new ptes of the
+		 * page, so we are safe.
+		 *
+		 * An alternative would be drop the check, but check that page
+		 * table is clear before calling pmdp_collapse_flush() under
+		 * ptl. It has higher chance to recover THP for the VMA, but
+		 * has higher cost too.
+		 */
 		if (vma->anon_vma)
 			continue;
 		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
@@ -1434,9 +1449,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			continue;
 		/*
 		 * We need exclusive mmap_sem to retract page table.
-		 * If trylock fails we would end up with pte-mapped THP after
-		 * re-fault. Not ideal, but it's more important to not disturb
-		 * the system too much.
+		 *
+		 * We use trylock due to lock inversion: we need to acquire
+		 * mmap_sem while holding page lock. Fault path does it in
+		 * reverse order. Trylock is a way to avoid deadlock.
 		 */
 		if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
 			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
@@ -1446,8 +1462,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			up_write(&vma->vm_mm->mmap_sem);
 			mm_dec_nr_ptes(vma->vm_mm);
 			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
-		} else
+		} else {
+			/* Try again later */
 			khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
+		}
 	}
 	i_mmap_unlock_write(mapping);
 }
-- 
 Kirill A. Shutemov