mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()

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.

Move some limit & flag verifications out of the do_brk_flags() function to
use only relevant checks in the code path of bkr() and vm_brk_flags().

Set the vma to check if it can expand in vm_brk_flags() if extra criteria
are met.

Drop userfaultfd from do_brk_flags() path and only use it in
vm_brk_flags() path since that is the only place a munmap will happen.

Add a wraper for munmap for the brk case called do_brk_munmap().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Liam R. Howlett <[e-mail]>
Tested-by: Yu Zhao <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: SeongJae Park <[e-mail]>
Cc: Sven Schnelle <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/mmap.c

···
 	return next;
 }
 
-static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
-		struct list_head *uf);
+/*
+ * check_brk_limits() - Use platform specific check of range & verify mlock
+ * limits.
+ * @addr: The address to check
+ * @len: The size of increase.
+ *
+ * Return: 0 on success.
+ */
+static int check_brk_limits(unsigned long addr, unsigned long len)
+{
+	unsigned long mapped_addr;
+
+	mapped_addr = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
+	if (IS_ERR_VALUE(mapped_addr))
+		return mapped_addr;
+
+	return mlock_future_check(current->mm, current->mm->def_flags, len);
+}
+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;
···
 
 	/*
 	 * Always allow shrinking brk.
-	 * __do_munmap() may downgrade mmap_lock to read.
+	 * do_brk_munmap() may downgrade mmap_lock to read.
 	 */
 	if (brk <= mm->brk) {
 		int ret;
 
+		/* Search one past newbrk */
+		mas_set(&mas, newbrk);
+		brkvma = mas_find(&mas, oldbrk);
+		BUG_ON(brkvma == NULL);
+		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;
 	}
 
-	/* Check against existing mmap mappings. */
-	next = find_vma(mm, oldbrk);
+	if (check_brk_limits(oldbrk, newbrk - oldbrk))
+		goto out;
+
+	/*
+	 * Only check if the next VMA is within the stack_guard_gap of the
+	 * expansion area
+	 */
+	mas_set(&mas, oldbrk);
+	next = mas_find(&mas, newbrk - 1 + PAGE_SIZE + stack_guard_gap);
 	if (next && newbrk + PAGE_SIZE > vm_start_gap(next))
 		goto out;
 
+	brkvma = mas_prev(&mas, mm->start_brk);
 	/* 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:
···
 }
 
 /*
- *  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_flags(unsigned long addr, unsigned long len,
-			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)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	int ret;
+
+	arch_unmap(mm, newbrk, oldbrk);
+	ret = __do_munmap(mm, newbrk, oldbrk - newbrk, uf, true);
+	validate_mm_mt(mm);
+	return ret;
+}
+
+/*
+ * 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(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;
-	int error;
-	unsigned long mapped_addr;
+	struct vm_area_struct *prev = NULL;
+
 	validate_mm_mt(mm);
-
-	/* Until we need other flags, refuse anything except VM_EXEC. */
-	if ((flags & (~VM_EXEC)) != 0)
-		return -EINVAL;
+	/*
+	 * Check against address space limits by the changed size
+	 * Note: This happens *after* clearing old mappings in some code paths.
+	 */
 	flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
-
-	mapped_addr = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
-	if (IS_ERR_VALUE(mapped_addr))
-		return mapped_addr;
-
-	error = mlock_future_check(mm, mm->def_flags, 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... */
 	if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT))
 		return -ENOMEM;
 
···
 	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, NULL);
-	if (vma)
-		goto out;
-
 	/*
-	 * create a vma struct for an anonymous mapping
+	 * Expand the existing vma if possible; Note that singular lists do not
+	 * occur after forking, so the expand will only happen on new VMAs.
 	 */
-	vma = vm_area_alloc(mm);
-	if (!vma) {
-		vm_unacct_memory(len >> PAGE_SHIFT);
-		return -ENOMEM;
+	if (vma &&
+	    (!vma->anon_vma || list_is_singular(&vma->anon_vma_chain)) &&
+	    ((vma->vm_flags & ~VM_SOFTDIRTY) == flags)) {
+		mas->index = vma->vm_start;
+		mas->last = addr + len - 1;
+		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_expand_failed;
+
+		if (vma->anon_vma) {
+			anon_vma_interval_tree_post_update_vma(vma);
+			anon_vma_unlock_write(vma->anon_vma);
+		}
+		khugepaged_enter_vma(vma, flags);
+		goto out;
 	}
+	prev = vma;
+
+	/* 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);
-	if (vma_link(mm, vma, prev))
-		goto no_vma_link;
+	mas_set_range(mas, vma->vm_start, addr + len - 1);
+	if (mas_store_gfp(mas, vma, GFP_KERNEL))
+		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;
···
 	validate_mm_mt(mm);
 	return 0;
 
-no_vma_link:
+mas_store_fail:
 	vm_area_free(vma);
+vma_alloc_fail:
+	vm_unacct_memory(len >> PAGE_SHIFT);
+	return -ENOMEM;
+
+mas_expand_failed:
+	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)
···
 	if (mmap_write_lock_killable(mm))
 		return -EINTR;
 
-	ret = do_brk_flags(addr, len, flags, &uf);
+	/* Until we need other flags, refuse anything except VM_EXEC. */
+	if ((flags & (~VM_EXEC)) != 0)
+		return -EINVAL;
+
+	ret = check_brk_limits(addr, len);
+	if (ret)
+		goto limits_failed;
+
+	if (find_vma_intersection(mm, addr, addr + len))
+		ret = do_munmap(mm, addr, len, &uf);
+
+	if (ret)
+		goto munmap_failed;
+
+	vma = mas_prev(&mas, 0);
+	if (!vma || vma->vm_end != addr || vma_policy(vma) ||
+	    !can_vma_merge_after(vma, flags, NULL, NULL,
+				 addr >> PAGE_SHIFT, NULL_VM_UFFD_CTX, NULL))
+		vma = NULL;
+
+	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;
+
+munmap_failed:
+limits_failed:
+	mmap_write_unlock(mm);
 	return ret;
 }
 EXPORT_SYMBOL(vm_brk_flags);