[PATCH v4 21/66] mm/mmap: Change do_brk_flags() to expand existing VMA and add do_brk_munmap()

[Liam Howlett <liam.howlett@oracle.com>]

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

Avoid allocating a new VMA when it a vma modification can occur.  When a
brk() can expand or contract a VMA, then the single store operation will
only modify one index of the maple tree instead of causing a node to
split or coalesce.  This avoids unnecessary allocations/frees of maple
tree nodes and VMAs.

Use the advanced API for the maple tree to avoid unnecessary walks of
the tree.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
 mm/mmap.c | 258 +++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 207 insertions(+), 51 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index c5f92666d145..e4c8ce377f2c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -188,17 +188,22 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
 	return next;
 }
 
-static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
-		struct list_head *uf);
+static int do_brk_munmap(struct ma_state *mas, struct vm_area_struct *vma,
+			 unsigned long newbrk, unsigned long oldbrk,
+			 struct list_head *uf);
+static int do_brk_flags(struct ma_state *mas, struct vm_area_struct *brkvma,
+			unsigned long addr, unsigned long request,
+			unsigned long flags);
 SYSCALL_DEFINE1(brk, unsigned long, brk)
 {
 	unsigned long newbrk, oldbrk, origbrk;
 	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *next;
+	struct vm_area_struct *brkvma, *next = NULL;
 	unsigned long min_brk;
 	bool populate;
 	bool downgraded = false;
 	LIST_HEAD(uf);
+	MA_STATE(mas, &mm->mm_mt, 0, 0);
 
 	if (mmap_write_lock_killable(mm))
 		return -EINTR;
@@ -238,37 +243,56 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
 		goto success;
 	}
 
-	/*
-	 * Always allow shrinking brk.
-	 * __do_munmap() may downgrade mmap_lock to read.
-	 */
-	if (brk <= mm->brk) {
+	mas_set(&mas, newbrk);
+	brkvma = mas_walk(&mas);
+	if (brkvma) { // munmap necessary, there is something at newbrk.
+		/*
+		 * Always allow shrinking brk.
+		 * do_brk_munmap() may downgrade mmap_lock to read.
+		 */
 		int ret;
 
+		if (brkvma->vm_start >= oldbrk)
+			goto out; // mapping intersects with an existing non-brk vma.
 		/*
-		 * mm->brk must to be protected by write mmap_lock so update it
-		 * before downgrading mmap_lock. When __do_munmap() fails,
-		 * mm->brk will be restored from origbrk.
+		 * mm->brk must be protected by write mmap_lock.
+		 * do_brk_munmap() may downgrade the lock,  so update it
+		 * before calling do_brk_munmap().
 		 */
 		mm->brk = brk;
-		ret = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
-		if (ret < 0) {
-			mm->brk = origbrk;
-			goto out;
-		} else if (ret == 1) {
+		mas.last = oldbrk - 1;
+		ret = do_brk_munmap(&mas, brkvma, newbrk, oldbrk, &uf);
+		if (ret == 1)  {
 			downgraded = true;
-		}
-		goto success;
-	}
+			goto success;
+		} else if (!ret)
+			goto success;
 
+		mm->brk = origbrk;
+		goto out;
+	}
+	/* Only check if the next VMA is within the stack_guard_gap of the
+	 * expansion area */
+	next = mas_next(&mas, newbrk + PAGE_SIZE + stack_guard_gap);
 	/* Check against existing mmap mappings. */
-	next = find_vma(mm, oldbrk);
 	if (next && newbrk + PAGE_SIZE > vm_start_gap(next))
 		goto out;
 
+	brkvma = mas_prev(&mas, mm->start_brk);
+	if (brkvma) {
+		if (brkvma->vm_start >= oldbrk)
+			goto out; // Trying to map over another vma.
+
+		if (brkvma->vm_end <= min_brk) {
+			brkvma = NULL;
+			mas_reset(&mas);
+		}
+	}
+
 	/* Ok, looks good - let it rip. */
-	if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
+	if (do_brk_flags(&mas, brkvma, oldbrk, newbrk - oldbrk, 0) < 0)
 		goto out;
+
 	mm->brk = brk;
 
 success:
@@ -1989,6 +2013,7 @@ EXPORT_SYMBOL(get_unmapped_area);
 struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
 {
 	struct vm_area_struct *vma;
+	MA_STATE(mas, &mm->mm_mt, addr, addr);
 
 	mmap_assert_locked(mm);
 	/* Check the cache first. */
@@ -1996,7 +2021,7 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
 	if (likely(vma))
 		return vma;
 
-	vma = mt_find(&mm->mm_mt, &addr, ULONG_MAX);
+	vma = mas_find(&mas, -1);
 	if (vma)
 		vmacache_update(addr, vma);
 	return vma;
@@ -2713,16 +2738,105 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
 }
 
 /*
- *  this is really a simplified "do_mmap".  it only handles
- *  anonymous maps.  eventually we may be able to do some
- *  brk-specific accounting here.
+ * brk_munmap() - Unmap a parital vma.
+ * @mas: The maple tree state.
+ * @vma: The vma to be modified
+ * @newbrk: the start of the address to unmap
+ * @oldbrk: The end of the address to unmap
+ * @uf: The userfaultfd list_head
+ *
+ * Returns: 1 on success.
+ * unmaps a partial VMA mapping.  Does not handle alignment, downgrades lock if
+ * possible.
+ */
+static int do_brk_munmap(struct ma_state *mas, struct vm_area_struct *vma,
+			 unsigned long newbrk, unsigned long oldbrk,
+			 struct list_head *uf)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	struct vm_area_struct unmap;
+	unsigned long unmap_pages;
+	int ret = 1;
+
+	arch_unmap(mm, newbrk, oldbrk);
+
+	if (likely(vma->vm_start >= newbrk)) { // remove entire mapping(s)
+		mas_set(mas, newbrk);
+		if (vma->vm_start != newbrk)
+			mas_reset(mas); // cause a re-walk for the first overlap.
+		ret = __do_munmap(mm, newbrk, oldbrk - newbrk, uf, true);
+		goto munmap_full_vma;
+	}
+
+	vma_init(&unmap, mm);
+	unmap.vm_start = newbrk;
+	unmap.vm_end = oldbrk;
+	ret = userfaultfd_unmap_prep(&unmap, newbrk, oldbrk, uf);
+	if (ret)
+		return ret;
+	ret = 1;
+
+	// Change the oldbrk of vma to the newbrk of the munmap area
+	vma_adjust_trans_huge(vma, vma->vm_start, newbrk, 0);
+	if (vma->anon_vma) {
+		anon_vma_lock_write(vma->anon_vma);
+		anon_vma_interval_tree_pre_update_vma(vma);
+	}
+
+	vma->vm_end = newbrk;
+	if (vma_mas_remove(&unmap, mas))
+		goto mas_store_fail;
+
+	vmacache_invalidate(vma->vm_mm);
+	if (vma->anon_vma) {
+		anon_vma_interval_tree_post_update_vma(vma);
+		anon_vma_unlock_write(vma->anon_vma);
+	}
+
+	unmap_pages = vma_pages(&unmap);
+	if (unmap.vm_flags & VM_LOCKED) {
+		mm->locked_vm -= unmap_pages;
+		munlock_vma_pages_range(&unmap, newbrk, oldbrk);
+	}
+
+	mmap_write_downgrade(mm);
+	unmap_region(mm, &unmap, vma, newbrk, oldbrk);
+	/* Statistics */
+	vm_stat_account(mm, unmap.vm_flags, -unmap_pages);
+	if (unmap.vm_flags & VM_ACCOUNT)
+		vm_unacct_memory(unmap_pages);
+
+munmap_full_vma:
+	validate_mm_mt(mm);
+	return ret;
+
+mas_store_fail:
+	vma->vm_end = oldbrk;
+	if (vma->anon_vma) {
+		anon_vma_interval_tree_post_update_vma(vma);
+		anon_vma_unlock_write(vma->anon_vma);
+	}
+	return -ENOMEM;
+}
+
+/*
+ * do_brk_flags() - Increase the brk vma if the flags match.
+ * @mas: The maple tree state.
+ * @addr: The start address
+ * @len: The length of the increase
+ * @vma: The vma,
+ * @flags: The VMA Flags
+ *
+ * Extend the brk VMA from addr to addr + len.  If the VMA is NULL or the flags
+ * do not match then create a new anonymous VMA.  Eventually we may be able to
+ * do some brk-specific accounting here.
  */
-static int do_brk_flags(unsigned long addr, unsigned long len,
-			unsigned long flags, struct list_head *uf)
+static int do_brk_flags(struct ma_state *mas, struct vm_area_struct *vma,
+			unsigned long addr, unsigned long len,
+			unsigned long flags)
 {
 	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma, *prev;
-	pgoff_t pgoff = addr >> PAGE_SHIFT;
+	struct vm_area_struct *prev = NULL;
 	int error;
 	unsigned long mapped_addr;
 	validate_mm_mt(mm);
@@ -2740,11 +2854,7 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
 	if (error)
 		return error;
 
-	/* Clear old maps, set up prev and uf */
-	if (munmap_vma_range(mm, addr, len, &prev, uf))
-		return -ENOMEM;
-
-	/* Check against address space limits *after* clearing old maps... */
+	/* Check against address space limits by the changed size */
 	if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT))
 		return -ENOMEM;
 
@@ -2754,28 +2864,57 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
 	if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
 		return -ENOMEM;
 
-	/* Can we just expand an old private anonymous mapping? */
-	vma = vma_merge(mm, prev, addr, addr + len, flags,
-			NULL, NULL, pgoff, NULL, NULL_VM_UFFD_CTX);
-	if (vma)
-		goto out;
+	mas->last = addr + len - 1;
+	if (vma) {
+		/* Expand the existing vma if possible; almost never a singular
+		 * list, so this will almost always fail. */
 
-	/*
-	 * create a vma struct for an anonymous mapping
-	 */
-	vma = vm_area_alloc(mm);
-	if (!vma) {
-		vm_unacct_memory(len >> PAGE_SHIFT);
-		return -ENOMEM;
+		if ((!vma->anon_vma ||
+		     list_is_singular(&vma->anon_vma_chain)) &&
+		     ((vma->vm_flags & ~VM_SOFTDIRTY) == flags)){
+			mas->index = vma->vm_start;
+
+			vma_adjust_trans_huge(vma, addr, addr + len, 0);
+			if (vma->anon_vma) {
+				anon_vma_lock_write(vma->anon_vma);
+				anon_vma_interval_tree_pre_update_vma(vma);
+			}
+			vma->vm_end = addr + len;
+			vma->vm_flags |= VM_SOFTDIRTY;
+			if (mas_store_gfp(mas, vma, GFP_KERNEL))
+				goto mas_mod_fail;
+
+			if (vma->anon_vma) {
+				anon_vma_interval_tree_post_update_vma(vma);
+				anon_vma_unlock_write(vma->anon_vma);
+			}
+			khugepaged_enter_vma_merge(vma, flags);
+			goto out;
+		}
+		prev = vma;
 	}
+	mas->index = addr;
+	mas_walk(mas);
+
+	/* create a vma struct for an anonymous mapping */
+	vma = vm_area_alloc(mm);
+	if (!vma)
+		goto vma_alloc_fail;
 
 	vma_set_anonymous(vma);
 	vma->vm_start = addr;
 	vma->vm_end = addr + len;
-	vma->vm_pgoff = pgoff;
+	vma->vm_pgoff = addr >> PAGE_SHIFT;
 	vma->vm_flags = flags;
 	vma->vm_page_prot = vm_get_page_prot(flags);
-	vma_link(mm, vma, prev);
+	if (vma_mas_store(vma, mas))
+		goto mas_store_fail;
+
+	if (!prev)
+		prev = mas_prev(mas, 0);
+
+	__vma_link_list(mm, vma, prev);
+	mm->map_count++;
 out:
 	perf_event_mmap(vma);
 	mm->total_vm += len >> PAGE_SHIFT;
@@ -2785,15 +2924,31 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
 	vma->vm_flags |= VM_SOFTDIRTY;
 	validate_mm_mt(mm);
 	return 0;
+
+mas_store_fail:
+	vm_area_free(vma);
+vma_alloc_fail:
+	vm_unacct_memory(len >> PAGE_SHIFT);
+	return -ENOMEM;
+
+mas_mod_fail:
+	vma->vm_end = addr;
+	if (vma->anon_vma) {
+		anon_vma_interval_tree_post_update_vma(vma);
+		anon_vma_unlock_write(vma->anon_vma);
+	}
+	return -ENOMEM;
+
 }
 
 int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
 {
 	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma = NULL;
 	unsigned long len;
 	int ret;
 	bool populate;
-	LIST_HEAD(uf);
+	MA_STATE(mas, &mm->mm_mt, addr, addr);
 
 	len = PAGE_ALIGN(request);
 	if (len < request)
@@ -2804,10 +2959,11 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
 	if (mmap_write_lock_killable(mm))
 		return -EINTR;
 
-	ret = do_brk_flags(addr, len, flags, &uf);
+	// This vma left intentionally blank.
+	mas_walk(&mas);
+	ret = do_brk_flags(&mas, vma, addr, len, flags);
 	populate = ((mm->def_flags & VM_LOCKED) != 0);
 	mmap_write_unlock(mm);
-	userfaultfd_unmap_complete(mm, &uf);
 	if (populate && !ret)
 		mm_populate(addr, len);
 	return ret;
-- 
2.30.2