[PATCH] mm: remove unnecessary page_table_lock on stack expansion

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Ever since commit 8d7071af8907 ("mm: always expand the stack with the mmap
write lock held") we have been expanding the stack with the mmap write lock
held.

This is true in all code paths:

get_arg_page()
  -> expand_downwards()
setup_arg_pages()
  -> expand_stack_locked()
    -> expand_downwards() / expand_upwards()
lock_mm_and_find_vma()
  -> expand_stack_locked()
    -> expand_downwards() / expand_upwards()
create_elf_tables()
  -> find_extend_vma_locked()
    -> expand_stack_locked()
expand_stack()
  -> vma_expand_down()
    -> expand_downwards()
expand_stack()
  -> vma_expand_up()
    -> expand_upwards()

Each of which acquire the mmap write lock before doing so. Despite this, we
maintain code that acquires a page table lock in the expand_upwards() and
expand_downwards() code, stating that we hold a shared mmap lock and thus
this is necessary.

It is not, we do not have to worry about concurrent VMA expansions so we
can simply drop this, and update comments accordingly.

We do not even need be concerned with racing page faults, as
vma_start_write() is invoked in both cases.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/mmap.c | 38 ++++++--------------------------------
 1 file changed, 6 insertions(+), 32 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f904b3bba962..386429f7db5a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1039,6 +1039,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 	if (!(vma->vm_flags & VM_GROWSUP))
 		return -EFAULT;
 
+	mmap_assert_write_locked(mm);
+
 	/* Guard against exceeding limits of the address space. */
 	address &= PAGE_MASK;
 	if (address >= (TASK_SIZE & PAGE_MASK))
@@ -1074,11 +1076,7 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 
 	/* Lock the VMA before expanding to prevent concurrent page faults */
 	vma_start_write(vma);
-	/*
-	 * vma->vm_start/vm_end cannot change under us because the caller
-	 * is required to hold the mmap_lock in read mode.  We need the
-	 * anon_vma lock to serialize against concurrent expand_stacks.
-	 */
+	/* We update the anon VMA tree. */
 	anon_vma_lock_write(vma->anon_vma);
 
 	/* Somebody else might have raced and expanded it already */
@@ -1092,16 +1090,6 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 		if (vma->vm_pgoff + (size >> PAGE_SHIFT) >= vma->vm_pgoff) {
 			error = acct_stack_growth(vma, size, grow);
 			if (!error) {
-				/*
-				 * We only hold a shared mmap_lock lock here, so
-				 * we need to protect against concurrent vma
-				 * expansions.  anon_vma_lock_write() doesn't
-				 * help here, as we don't guarantee that all
-				 * growable vmas in a mm share the same root
-				 * anon vma.  So, we reuse mm->page_table_lock
-				 * to guard against concurrent vma expansions.
-				 */
-				spin_lock(&mm->page_table_lock);
 				if (vma->vm_flags & VM_LOCKED)
 					mm->locked_vm += grow;
 				vm_stat_account(mm, vma->vm_flags, grow);
@@ -1110,7 +1098,6 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 				/* Overwrite old entry in mtree. */
 				vma_iter_store(&vmi, vma);
 				anon_vma_interval_tree_post_update_vma(vma);
-				spin_unlock(&mm->page_table_lock);
 
 				perf_event_mmap(vma);
 			}
@@ -1137,6 +1124,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 	if (!(vma->vm_flags & VM_GROWSDOWN))
 		return -EFAULT;
 
+	mmap_assert_write_locked(mm);
+
 	address &= PAGE_MASK;
 	if (address < mmap_min_addr || address < FIRST_USER_ADDRESS)
 		return -EPERM;
@@ -1166,11 +1155,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 
 	/* Lock the VMA before expanding to prevent concurrent page faults */
 	vma_start_write(vma);
-	/*
-	 * vma->vm_start/vm_end cannot change under us because the caller
-	 * is required to hold the mmap_lock in read mode.  We need the
-	 * anon_vma lock to serialize against concurrent expand_stacks.
-	 */
+	/* We update the anon VMA tree. */
 	anon_vma_lock_write(vma->anon_vma);
 
 	/* Somebody else might have raced and expanded it already */
@@ -1184,16 +1169,6 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 		if (grow <= vma->vm_pgoff) {
 			error = acct_stack_growth(vma, size, grow);
 			if (!error) {
-				/*
-				 * We only hold a shared mmap_lock lock here, so
-				 * we need to protect against concurrent vma
-				 * expansions.  anon_vma_lock_write() doesn't
-				 * help here, as we don't guarantee that all
-				 * growable vmas in a mm share the same root
-				 * anon vma.  So, we reuse mm->page_table_lock
-				 * to guard against concurrent vma expansions.
-				 */
-				spin_lock(&mm->page_table_lock);
 				if (vma->vm_flags & VM_LOCKED)
 					mm->locked_vm += grow;
 				vm_stat_account(mm, vma->vm_flags, grow);
@@ -1203,7 +1178,6 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 				/* Overwrite old entry in mtree. */
 				vma_iter_store(&vmi, vma);
 				anon_vma_interval_tree_post_update_vma(vma);
-				spin_unlock(&mm->page_table_lock);
 
 				perf_event_mmap(vma);
 			}
-- 
2.47.0