[PATCH v2 3/6] mm: Speedup mremap on 1GB or larger regions

[Kalesh Singh]

Android needs to move large memory regions for garbage collection.
The GC requires moving physical pages of multi-gigabyte heap
using mremap. During this move, the application threads have to
be paused for correctness. It is critical to keep this pause as
short as possible to avoid jitters during user interaction.

Optimize mremap for >= 1GB-sized regions by moving at the PUD/PGD
level if the source and destination addresses are PUD-aligned.
For CONFIG_PGTABLE_LEVELS == 3, moving at the PUD level in effect moves
PGD entries, since the PUD entry is “folded back” onto the PGD entry.
Add HAVE_MOVE_PUD so that architectures where moving at the PUD level
isn't supported/tested can turn this off by not selecting the config.

Fix build test error from v1 of this series reported by
kernel test robot in [1].

[1] https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org/thread/CKPGL4FH4NG7TGH2CVYX2UX76L25BTA3/

Signed-off-by: Kalesh Singh <kaleshsingh at google.com>
Reported-by: kernel test robot <lkp at intel.com>
---
Changes in v2:
  - Update commit message with description of Android GC's use case.
  - Move set_pud_at() to a separate patch.
  - Use switch() instead of ifs in move_pgt_entry()
  - Fix build test error reported by kernel test robot on x86_64 in [1].
    Guard move_huge_pmd() with IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE),
    since this section doesn't get optimized out in the kernel test
    robot's build test when HAVE_MOVE_PUD is enabled.
  - Keep WARN_ON_ONCE(1) instead of BUILD_BUG() for the aforementioned
    reason.

 arch/Kconfig |   7 ++
 mm/mremap.c  | 220 ++++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 197 insertions(+), 30 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index af14a567b493..5eabaa00bf9b 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -602,6 +602,13 @@ config HAVE_IRQ_TIME_ACCOUNTING
 	  Archs need to ensure they use a high enough resolution clock to
 	  support irq time accounting and then call enable_sched_clock_irqtime().
 
+config HAVE_MOVE_PUD
+	bool
+	help
+	  Architectures that select this are able to move page tables at the
+	  PUD level. If there are only 3 page table levels, the move effectively
+	  happens at the PGD level.
+
 config HAVE_MOVE_PMD
 	bool
 	help
diff --git a/mm/mremap.c b/mm/mremap.c
index 138abbae4f75..c1d6ab667d70 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -249,14 +249,176 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
 
 	return true;
 }
+#else
+static inline bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
+		  unsigned long new_addr, pmd_t *old_pmd, pmd_t *new_pmd)
+{
+	return false;
+}
 #endif
 
+#ifdef CONFIG_HAVE_MOVE_PUD
+static pud_t *get_old_pud(struct mm_struct *mm, unsigned long addr)
+{
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+
+	pgd = pgd_offset(mm, addr);
+	if (pgd_none_or_clear_bad(pgd))
+		return NULL;
+
+	p4d = p4d_offset(pgd, addr);
+	if (p4d_none_or_clear_bad(p4d))
+		return NULL;
+
+	pud = pud_offset(p4d, addr);
+	if (pud_none_or_clear_bad(pud))
+		return NULL;
+
+	return pud;
+}
+
+static pud_t *alloc_new_pud(struct mm_struct *mm, struct vm_area_struct *vma,
+			    unsigned long addr)
+{
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+
+	pgd = pgd_offset(mm, addr);
+	p4d = p4d_alloc(mm, pgd, addr);
+	if (!p4d)
+		return NULL;
+	pud = pud_alloc(mm, p4d, addr);
+	if (!pud)
+		return NULL;
+
+	return pud;
+}
+
+static bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
+		  unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
+{
+	spinlock_t *old_ptl, *new_ptl;
+	struct mm_struct *mm = vma->vm_mm;
+	pud_t pud;
+
+	/*
+	 * The destination pud shouldn't be established, free_pgtables()
+	 * should have released it.
+	 */
+	if (WARN_ON_ONCE(!pud_none(*new_pud)))
+		return false;
+
+	/*
+	 * We don't have to worry about the ordering of src and dst
+	 * ptlocks because exclusive mmap_lock prevents deadlock.
+	 */
+	old_ptl = pud_lock(vma->vm_mm, old_pud);
+	new_ptl = pud_lockptr(mm, new_pud);
+	if (new_ptl != old_ptl)
+		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+	/* Clear the pud */
+	pud = *old_pud;
+	pud_clear(old_pud);
+
+	VM_BUG_ON(!pud_none(*new_pud));
+
+	/* Set the new pud */
+	set_pud_at(mm, new_addr, new_pud, pud);
+	flush_tlb_range(vma, old_addr, old_addr + PUD_SIZE);
+	if (new_ptl != old_ptl)
+		spin_unlock(new_ptl);
+	spin_unlock(old_ptl);
+
+	return true;
+}
+#else
+static inline bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
+		  unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
+{
+	return false;
+}
+#endif
+
+enum pgt_entry {
+	NORMAL_PMD,
+	HPAGE_PMD,
+	NORMAL_PUD,
+};
+
+/*
+ * Returns an extent of the corresponding size for the pgt_entry specified if valid.
+ * Else returns a smaller extent bounded by the end of the source and destination
+ * pgt_entry. Returns 0 if an invalid pgt_entry is specified.
+ */
+static unsigned long get_extent(enum pgt_entry entry, unsigned long old_addr,
+			unsigned long old_end, unsigned long new_addr)
+{
+	unsigned long next, extent, mask, size;
+
+	if (entry == NORMAL_PMD || entry == HPAGE_PMD) {
+		mask = PMD_MASK;
+		size = PMD_SIZE;
+	} else if (entry == NORMAL_PUD) {
+		mask = PUD_MASK;
+		size = PUD_SIZE;
+	} else
+		return 0;
+
+	next = (old_addr + size) & mask;
+	/* even if next overflowed, extent below will be ok */
+	extent = (next > old_end) ? old_end - old_addr : next - old_addr;
+	next = (new_addr + size) & mask;
+	if (extent > next - new_addr)
+		extent = next - new_addr;
+	return extent;
+}
+
+/*
+ * Attempts to speedup the move by moving entry at the level corresponding to
+ * pgt_entry. Returns true if the move was successful, else false.
+ */
+static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
+			unsigned long old_addr, unsigned long new_addr, void *old_entry,
+			void *new_entry, bool need_rmap_locks)
+{
+	bool moved = false;
+
+	/* See comment in move_ptes() */
+	if (need_rmap_locks)
+		take_rmap_locks(vma);
+
+	switch (entry) {
+	case NORMAL_PMD:
+		moved =  move_normal_pmd(vma, old_addr, new_addr, old_entry, new_entry);
+		break;
+	case NORMAL_PUD:
+		moved =  move_normal_pud(vma, old_addr, new_addr, old_entry, new_entry);
+		break;
+	case HPAGE_PMD:
+		moved =  IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+			move_huge_pmd(vma, old_addr, new_addr, old_entry, new_entry);
+		break;
+	default:
+		WARN_ON_ONCE(1);
+		break;
+	}
+
+	if (need_rmap_locks)
+		drop_rmap_locks(vma);
+
+	return moved;
+}
+
 unsigned long move_page_tables(struct vm_area_struct *vma,
 		unsigned long old_addr, struct vm_area_struct *new_vma,
 		unsigned long new_addr, unsigned long len,
 		bool need_rmap_locks)
 {
-	unsigned long extent, next, old_end;
+	unsigned long extent, old_end;
 	struct mmu_notifier_range range;
 	pmd_t *old_pmd, *new_pmd;
 
@@ -269,14 +431,27 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 
 	for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
 		cond_resched();
-		next = (old_addr + PMD_SIZE) & PMD_MASK;
-		/* even if next overflowed, extent below will be ok */
-		extent = next - old_addr;
-		if (extent > old_end - old_addr)
-			extent = old_end - old_addr;
-		next = (new_addr + PMD_SIZE) & PMD_MASK;
-		if (extent > next - new_addr)
-			extent = next - new_addr;
+#ifdef CONFIG_HAVE_MOVE_PUD
+		/*
+		 * If extent is PUD-sized try to speed up the move by moving at the
+		 * PUD level if possible.
+		 */
+		extent = get_extent(NORMAL_PUD, old_addr, old_end, new_addr);
+		if (extent == PUD_SIZE) {
+			pud_t *old_pud, *new_pud;
+
+			old_pud = get_old_pud(vma->vm_mm, old_addr);
+			if (!old_pud)
+				continue;
+			new_pud = alloc_new_pud(vma->vm_mm, vma, new_addr);
+			if (!new_pud)
+				break;
+			if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr,
+					   old_pud, new_pud, need_rmap_locks))
+				continue;
+		}
+#endif
+		extent = get_extent(NORMAL_PMD, old_addr, old_end, new_addr);
 		old_pmd = get_old_pmd(vma->vm_mm, old_addr);
 		if (!old_pmd)
 			continue;
@@ -284,18 +459,10 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 		if (!new_pmd)
 			break;
 		if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) || pmd_devmap(*old_pmd)) {
-			if (extent == HPAGE_PMD_SIZE) {
-				bool moved;
-				/* See comment in move_ptes() */
-				if (need_rmap_locks)
-					take_rmap_locks(vma);
-				moved = move_huge_pmd(vma, old_addr, new_addr,
-						      old_pmd, new_pmd);
-				if (need_rmap_locks)
-					drop_rmap_locks(vma);
-				if (moved)
-					continue;
-			}
+			if (extent == HPAGE_PMD_SIZE &&
+			    move_pgt_entry(HPAGE_PMD, vma, old_addr, new_addr, old_pmd,
+					   new_pmd, need_rmap_locks))
+				continue;
 			split_huge_pmd(vma, old_pmd, old_addr);
 			if (pmd_trans_unstable(old_pmd))
 				continue;
@@ -305,15 +472,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 			 * If the extent is PMD-sized, try to speed the move by
 			 * moving at the PMD level if possible.
 			 */
-			bool moved;
-
-			if (need_rmap_locks)
-				take_rmap_locks(vma);
-			moved = move_normal_pmd(vma, old_addr, new_addr,
-						old_pmd, new_pmd);
-			if (need_rmap_locks)
-				drop_rmap_locks(vma);
-			if (moved)
+			if (move_pgt_entry(NORMAL_PMD, vma, old_addr, new_addr, old_pmd,
+					   new_pmd, need_rmap_locks))
 				continue;
 #endif
 		}
-- 
2.28.0.806.g8561365e88-goog