mm: Always downgrade mmap_lock if requested

Now that stack growth must always hold the mmap_lock for write, we can
always downgrade the mmap_lock to read and safely unmap pages from the
page table, even if we're next to a stack.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index 3e5793e..141c618 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2551,19 +2551,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
        /* Point of no return */
        mm->locked_vm -= locked_vm;
        mm->map_count -= count;
-       /*
-        * Do not downgrade mmap_lock if we are next to VM_GROWSDOWN or
-        * VM_GROWSUP VMA. Such VMAs can change their size under
-        * down_read(mmap_lock) and collide with the VMA we are about to unmap.
-        */
-       if (downgrade) {
-               if (next && (next->vm_flags & VM_GROWSDOWN))
-                       downgrade = false;
-               else if (prev && (prev->vm_flags & VM_GROWSUP))
-                       downgrade = false;
-               else
-                       mmap_write_downgrade(mm);
-       }
+       if (downgrade)
+               mmap_write_downgrade(mm);
 
        /*
         * We can free page tables without write-locking mmap_lock because VMAs