[PATCH v4 14/21] mm/mmap: Avoid zeroing vma tree in mmap_region()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>

Instead of zeroing the vma tree and then overwriting the area, let the
area be overwritten and then clean up the gathered vmas using
vms_complete_munmap_vmas().

If a driver is mapping over an existing vma, then clear the ptes before
the call_mmap() invocation.  This is done using the vms_clear_ptes()
helper.

Temporarily keep track of the number of pages that will be removed and
reduce the charged amount.

This also drops the validate_mm() call in the vma_expand() function.
It is necessary to drop the validate as it would fail since the mm
map_count would be incorrect during a vma expansion, prior to the
cleanup from vms_complete_munmap_vmas().

Clean up the error handing of the vms_gather_munmap_vmas() by calling
the verification within the function.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
 mm/internal.h |  1 +
 mm/mmap.c     | 80 +++++++++++++++++++++++++++------------------------
 2 files changed, 44 insertions(+), 37 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 11e90c6e5a3e..dd4eede1be0f 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1503,6 +1503,7 @@ struct vma_munmap_struct {
 	unsigned long stack_vm;
 	unsigned long data_vm;
 	bool unlock;			/* Unlock after the munmap */
+	bool clear_ptes;		/* If there are outstanding PTE to be cleared */
 };
 
 void __meminit __init_single_page(struct page *page, unsigned long pfn,
diff --git a/mm/mmap.c b/mm/mmap.c
index 870c2d04ad6b..58cf42e22bfe 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -401,17 +401,21 @@ anon_vma_interval_tree_post_update_vma(struct vm_area_struct *vma)
 }
 
 static unsigned long count_vma_pages_range(struct mm_struct *mm,
-		unsigned long addr, unsigned long end)
+		unsigned long addr, unsigned long end,
+		unsigned long *nr_accounted)
 {
 	VMA_ITERATOR(vmi, mm, addr);
 	struct vm_area_struct *vma;
 	unsigned long nr_pages = 0;
 
+	*nr_accounted = 0;
 	for_each_vma_range(vmi, vma, end) {
 		unsigned long vm_start = max(addr, vma->vm_start);
 		unsigned long vm_end = min(end, vma->vm_end);
 
 		nr_pages += PHYS_PFN(vm_end - vm_start);
+		if (vma->vm_flags & VM_ACCOUNT)
+			*nr_accounted += PHYS_PFN(vm_end - vm_start);
 	}
 
 	return nr_pages;
@@ -524,6 +528,7 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms,
 	vms->exec_vm = vms->stack_vm = vms->data_vm = 0;
 	vms->unmap_start = FIRST_USER_ADDRESS;
 	vms->unmap_end = USER_PGTABLES_CEILING;
+	vms->clear_ptes = false;	/* No PTEs to clear yet */
 }
 
 /*
@@ -732,7 +737,6 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	vma_iter_store(vmi, vma);
 
 	vma_complete(&vp, vmi, vma->vm_mm);
-	validate_mm(vma->vm_mm);
 	return 0;
 
 nomem:
@@ -2606,11 +2610,14 @@ static inline void abort_munmap_vmas(struct ma_state *mas_detach)
 }
 
 
-static void vms_complete_pte_clear(struct vma_munmap_struct *vms,
+static inline void vms_clear_ptes(struct vma_munmap_struct *vms,
 		struct ma_state *mas_detach, bool mm_wr_locked)
 {
 	struct mmu_gather tlb;
 
+	if (!vms->clear_ptes) /* Nothing to do */
+		return;
+
 	/*
 	 * We can free page tables without write-locking mmap_lock because VMAs
 	 * were isolated before we downgraded mmap_lock.
@@ -2624,6 +2631,7 @@ static void vms_complete_pte_clear(struct vma_munmap_struct *vms,
 	/* start and end may be different if there is no prev or next vma. */
 	free_pgtables(&tlb, mas_detach, vms->vma, vms->unmap_start, vms->unmap_end, mm_wr_locked);
 	tlb_finish_mmu(&tlb);
+	vms->clear_ptes = false;
 }
 
 /*
@@ -2647,7 +2655,7 @@ static void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
 	if (vms->unlock)
 		mmap_write_downgrade(mm);
 
-	vms_complete_pte_clear(vms, mas_detach, !vms->unlock);
+	vms_clear_ptes(vms, mas_detach, !vms->unlock);
 	/* Update high watermark before we lower total_vm */
 	update_hiwater_vm(mm);
 	/* Stat accounting */
@@ -2799,6 +2807,9 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
 	while (vma_iter_addr(vms->vmi) > vms->start)
 		vma_iter_prev_range(vms->vmi);
 
+	/* There are now PTEs that need to be cleared */
+	vms->clear_ptes = true;
+
 	return 0;
 
 userfaultfd_error:
@@ -2807,6 +2818,7 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
 	abort_munmap_vmas(mas_detach);
 start_split_failed:
 map_count_exceeded:
+	validate_mm(vms->mm);
 	return error;
 }
 
@@ -2851,8 +2863,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 
 clear_tree_failed:
 	abort_munmap_vmas(&mas_detach);
-gather_failed:
 	validate_mm(mm);
+gather_failed:
 	return error;
 }
 
@@ -2940,24 +2952,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	unsigned long merge_start = addr, merge_end = end;
 	bool writable_file_mapping = false;
 	pgoff_t vm_pgoff;
-	int error;
+	int error = -ENOMEM;
 	VMA_ITERATOR(vmi, mm, addr);
+	unsigned long nr_pages, nr_accounted;
 
-	/* Check against address space limit. */
-	if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT)) {
-		unsigned long nr_pages;
-
-		/*
-		 * MAP_FIXED may remove pages of mappings that intersects with
-		 * requested mapping. Account for the pages it would unmap.
-		 */
-		nr_pages = count_vma_pages_range(mm, addr, end);
-
-		if (!may_expand_vm(mm, vm_flags,
-					(len >> PAGE_SHIFT) - nr_pages))
-			return -ENOMEM;
-	}
+	nr_pages = count_vma_pages_range(mm, addr, end, &nr_accounted);
 
+	/*
+	 * Check against address space limit.
+	 * MAP_FIXED may remove pages of mappings that intersects with requested
+	 * mapping. Account for the pages it would unmap.
+	 */
+	if (!may_expand_vm(mm, vm_flags, (len >> PAGE_SHIFT) - nr_pages))
+		return -ENOMEM;
 
 	if (unlikely(!can_modify_mm(mm, addr, end)))
 		return -EPERM;
@@ -2974,18 +2981,13 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
 		/* Prepare to unmap any existing mapping in the area */
 		if (vms_gather_munmap_vmas(&vms, &mas_detach))
-			goto gather_failed;
-
-		/* Remove any existing mappings from the vma tree */
-		if (vma_iter_clear_gfp(&vmi, addr, end, GFP_KERNEL))
-			goto clear_tree_failed;
+			return -ENOMEM;
 
-		/* Unmap any existing mapping in the area */
-		vms_complete_munmap_vmas(&vms, &mas_detach);
 		next = vms.next;
 		prev = vms.prev;
 		vma = NULL;
 	} else {
+		/* Minimal setup of vms */
 		next = vma_next(&vmi);
 		prev = vma_prev(&vmi);
 		if (prev)
@@ -2997,8 +2999,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	 */
 	if (accountable_mapping(file, vm_flags)) {
 		charged = len >> PAGE_SHIFT;
+		charged -= nr_accounted;
 		if (security_vm_enough_memory_mm(mm, charged))
-			return -ENOMEM;
+			goto abort_munmap;
+		vms.nr_accounted = 0;
 		vm_flags |= VM_ACCOUNT;
 	}
 
@@ -3047,10 +3051,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	 * not unmapped, but the maps are removed from the list.
 	 */
 	vma = vm_area_alloc(mm);
-	if (!vma) {
-		error = -ENOMEM;
+	if (!vma)
 		goto unacct_error;
-	}
 
 	vma_iter_config(&vmi, addr, end);
 	vma_set_range(vma, addr, end, pgoff);
@@ -3059,6 +3061,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 	if (file) {
 		vma->vm_file = get_file(file);
+		/* call_mmap() may map PTE, so ensure there are no existing PTEs */
+		vms_clear_ptes(&vms, &mas_detach, true);
 		error = call_mmap(file, vma);
 		if (error)
 			goto unmap_and_free_vma;
@@ -3149,6 +3153,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 expanded:
 	perf_event_mmap(vma);
 
+	/* Unmap any existing mapping in the area */
+	if (vms.nr_pages)
+		vms_complete_munmap_vmas(&vms, &mas_detach);
+
 	vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
 	if (vm_flags & VM_LOCKED) {
 		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
@@ -3196,14 +3204,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 unacct_error:
 	if (charged)
 		vm_unacct_memory(charged);
-	validate_mm(mm);
-	return error;
 
-clear_tree_failed:
-	abort_munmap_vmas(&mas_detach);
-gather_failed:
+abort_munmap:
+	if (vms.nr_pages)
+		abort_munmap_vmas(&mas_detach);
 	validate_mm(mm);
-	return -ENOMEM;
+	return error;
 }
 
 static int __vm_munmap(unsigned long start, size_t len, bool unlock)
-- 
2.43.0