Subject: [PATCH v3 09/13] mm/khugepaged: retract_page_tables() without mmap or vma lock

From: Hugh Dickins <[email-masked]>
Date: Tue, 11 Jul 2023 21:41:04 -0700 (PDT)

Simplify shmem and file THP collapse's retract_page_tables(), and relax
its locking: to improve its success rate and to lessen impact on others.

Instead of its MADV_COLLAPSE case doing set_huge_pmd() at target_addr of
target_mm, leave that part of the work to madvise_collapse() calling
collapse_pte_mapped_thp() afterwards: just adjust collapse_file()'s
result code to arrange for that.  That spares retract_page_tables() four
arguments; and since it will be successful in retracting all of the page
tables expected of it, no need to track and return a result code itself.

It needs i_mmap_lock_read(mapping) for traversing the vma interval tree,
but it does not need i_mmap_lock_write() for that: page_vma_mapped_walk()
allows for pte_offset_map_lock() etc to fail, and uses pmd_lock() for
THPs.  retract_page_tables() just needs to use those same spinlocks to
exclude it briefly, while transitioning pmd from page table to none: so
restore its use of pmd_lock() inside of which pte lock is nested.

Users of pte_offset_map_lock() etc all now allow for them to fail:
so retract_page_tables() now has no use for mmap_write_trylock() or
vma_try_start_write().  In common with rmap and page_vma_mapped_walk(),
it does not even need the mmap_read_lock().

But those users do expect the page table to remain a good page table,
until they unlock and rcu_read_unlock(): so the page table cannot be
freed immediately, but rather by the recently added pte_free_defer().

Use the (usually a no-op) pmdp_get_lockless_sync() to send an interrupt
when PAE, and pmdp_collapse_flush() did not already do so: to make sure
that the start,pmdp_get_lockless(),end sequence in __pte_offset_map()
cannot pick up a pmd entry with mismatched pmd_low and pmd_high.

retract_page_tables() can be enhanced to replace_page_tables(), which
inserts the final huge pmd without mmap lock: going through an invalid
state instead of pmd_none() followed by fault.  But that enhancement
does raise some more questions: leave it until a later release.

Signed-off-by: Hugh Dickins <[email-masked]>
---
 mm/khugepaged.c | 184 ++++++++++++++++++++------------------------------
 1 file changed, 75 insertions(+), 109 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 78c8d5d8b628..3bb05147961b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1615,9 +1615,8 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 		break;
 	case SCAN_PMD_NONE:
 		/*
-		 * In MADV_COLLAPSE path, possible race with khugepaged where
-		 * all pte entries have been removed and pmd cleared.  If so,
-		 * skip all the pte checks and just update the pmd mapping.
+		 * All pte entries have been removed and pmd cleared.
+		 * Skip all the pte checks and just update the pmd mapping.
 		 */
 		goto maybe_install_pmd;
 	default:
@@ -1748,123 +1747,88 @@ static void khugepaged_collapse_pte_mapped_thps(struct khugepaged_mm_slot *mm_sl
 	mmap_write_unlock(mm);
 }
 
-static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
-			       struct mm_struct *target_mm,
-			       unsigned long target_addr, struct page *hpage,
-			       struct collapse_control *cc)
+static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 {
 	struct vm_area_struct *vma;
-	int target_result = SCAN_FAIL;
 
-	i_mmap_lock_write(mapping);
+	i_mmap_lock_read(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
-		int result = SCAN_FAIL;
-		struct mm_struct *mm = NULL;
-		unsigned long addr = 0;
-		pmd_t *pmd;
-		bool is_target = false;
+		struct mmu_notifier_range range;
+		struct mm_struct *mm;
+		unsigned long addr;
+		pmd_t *pmd, pgt_pmd;
+		spinlock_t *pml;
+		spinlock_t *ptl;
+		bool skipped_uffd = false;
 
 		/*
 		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
-		 * got written to. These VMAs are likely not worth investing
-		 * mmap_write_lock(mm) as PMD-mapping is likely to be split
-		 * later.
-		 *
-		 * Note that vma->anon_vma check is racy: it can be set up after
-		 * the check but before we took mmap_lock 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. It would also probably require locking
-		 * the anon_vma.
+		 * got written to. These VMAs are likely not worth removing
+		 * page tables from, as PMD-mapping is likely to be split later.
 		 */
-		if (READ_ONCE(vma->anon_vma)) {
-			result = SCAN_PAGE_ANON;
-			goto next;
-		}
+		if (READ_ONCE(vma->anon_vma))
+			continue;
+
 		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
 		if (addr & ~HPAGE_PMD_MASK ||
-		    vma->vm_end < addr + HPAGE_PMD_SIZE) {
-			result = SCAN_VMA_CHECK;
-			goto next;
-		}
-		mm = vma->vm_mm;
-		is_target = mm == target_mm && addr == target_addr;
-		result = find_pmd_or_thp_or_none(mm, addr, &pmd);
-		if (result != SCAN_SUCCEED)
-			goto next;
-		/*
-		 * We need exclusive mmap_lock to retract page table.
-		 *
-		 * We use trylock due to lock inversion: we need to acquire
-		 * mmap_lock while holding page lock. Fault path does it in
-		 * reverse order. Trylock is a way to avoid deadlock.
-		 *
-		 * Also, it's not MADV_COLLAPSE's job to collapse other
-		 * mappings - let khugepaged take care of them later.
-		 */
-		result = SCAN_PTE_MAPPED_HUGEPAGE;
-		if ((cc->is_khugepaged || is_target) &&
-		    mmap_write_trylock(mm)) {
-			/* trylock for the same lock inversion as above */
-			if (!vma_try_start_write(vma))
-				goto unlock_next;
-
-			/*
-			 * Re-check whether we have an ->anon_vma, because
-			 * collapse_and_free_pmd() requires that either no
-			 * ->anon_vma exists or the anon_vma is locked.
-			 * We already checked ->anon_vma above, but that check
-			 * is racy because ->anon_vma can be populated under the
-			 * mmap lock in read mode.
-			 */
-			if (vma->anon_vma) {
-				result = SCAN_PAGE_ANON;
-				goto unlock_next;
-			}
-			/*
-			 * When a vma is registered with uffd-wp, we can't
-			 * recycle the pmd pgtable because there can be pte
-			 * markers installed.  Skip it only, so the rest mm/vma
-			 * can still have the same file mapped hugely, however
-			 * it'll always mapped in small page size for uffd-wp
-			 * registered ranges.
-			 */
-			if (hpage_collapse_test_exit(mm)) {
-				result = SCAN_ANY_PROCESS;
-				goto unlock_next;
-			}
-			if (userfaultfd_wp(vma)) {
-				result = SCAN_PTE_UFFD_WP;
-				goto unlock_next;
-			}
-			collapse_and_free_pmd(mm, vma, addr, pmd);
-			if (!cc->is_khugepaged && is_target)
-				result = set_huge_pmd(vma, addr, pmd, hpage);
-			else
-				result = SCAN_SUCCEED;
-
-unlock_next:
-			mmap_write_unlock(mm);
-			goto next;
-		}
-		/*
-		 * Calling context will handle target mm/addr. Otherwise, let
-		 * khugepaged try again later.
-		 */
-		if (!is_target) {
-			khugepaged_add_pte_mapped_thp(mm, addr);
+		    vma->vm_end < addr + HPAGE_PMD_SIZE)
 			continue;
+
+		mm = vma->vm_mm;
+		if (find_pmd_or_thp_or_none(mm, addr, &pmd) != SCAN_SUCCEED)
+			continue;
+
+		if (hpage_collapse_test_exit(mm))
+			continue;
+		/*
+		 * When a vma is registered with uffd-wp, we cannot recycle
+		 * the page table because there may be pte markers installed.
+		 * Other vmas can still have the same file mapped hugely, but
+		 * skip this one: it will always be mapped in small page size
+		 * for uffd-wp registered ranges.
+		 */
+		if (userfaultfd_wp(vma))
+			continue;
+
+		/* PTEs were notified when unmapped; but now for the PMD? */
+		mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
+					addr, addr + HPAGE_PMD_SIZE);
+		mmu_notifier_invalidate_range_start(&range);
+
+		pml = pmd_lock(mm, pmd);
+		ptl = pte_lockptr(mm, pmd);
+		if (ptl != pml)
+			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+
+		/*
+		 * Huge page lock is still held, so normally the page table
+		 * must remain empty; and we have already skipped anon_vma
+		 * and userfaultfd_wp() vmas.  But since the mmap_lock is not
+		 * held, it is still possible for a racing userfaultfd_ioctl()
+		 * to have inserted ptes or markers.  Now that we hold ptlock,
+		 * repeating the anon_vma check protects from one category,
+		 * and repeating the userfaultfd_wp() check from another.
+		 */
+		if (unlikely(vma->anon_vma || userfaultfd_wp(vma))) {
+			skipped_uffd = true;
+		} else {
+			pgt_pmd = pmdp_collapse_flush(vma, addr, pmd);
+			pmdp_get_lockless_sync();
+		}
+
+		if (ptl != pml)
+			spin_unlock(ptl);
+		spin_unlock(pml);
+
+		mmu_notifier_invalidate_range_end(&range);
+
+		if (!skipped_uffd) {
+			mm_dec_nr_ptes(mm);
+			page_table_check_pte_clear_range(mm, addr, pgt_pmd);
+			pte_free_defer(mm, pmd_pgtable(pgt_pmd));
 		}
-next:
-		if (is_target)
-			target_result = result;
 	}
-	i_mmap_unlock_write(mapping);
-	return target_result;
+	i_mmap_unlock_read(mapping);
 }
 
 /**
@@ -2259,9 +2223,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 
 	/*
 	 * Remove pte page tables, so we can re-fault the page as huge.
+	 * If MADV_COLLAPSE, adjust result to call collapse_pte_mapped_thp().
 	 */
-	result = retract_page_tables(mapping, start, mm, addr, hpage,
-				     cc);
+	retract_page_tables(mapping, start);
+	if (cc && !cc->is_khugepaged)
+		result = SCAN_PTE_MAPPED_HUGEPAGE;
 	unlock_page(hpage);
 
 	/*
-- 
2.35.3