[PATCH] mm: unlink_file_vma, remove_vma

Divide remove_vm_struct into two parts: first anon_vma_unlink plus
unlink_file_vma, to unlink the vma from the list and tree by which rmap or
vmtruncate might find it; then remove_vma to close, fput and free.

The intention here is to do the anon_vma_unlink and unlink_file_vma earlier,
in free_pgtables before freeing any page tables: so we can be sure that any
page tables traversed by rmap and vmtruncate are stable (and other, ordinary
cases are stabilized by holding mmap_sem).

This will be crucial to traversing pgd,pud,pmd without page_table_lock.  But
testing the split-out patch showed that lifting the page_table_lock is
symbiotically necessary to make this change - the lock ordering is wrong to
move those unlinks into free_pgtables while it's under ptlock.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 376a466743bc3144c7f592717d25b574ff16990b..0c64484d8ae0ca61da9a07c7b46d6787c8294d6b 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -834,6 +834,7 @@ extern int split_vma(struct mm_struct *,
 extern int insert_vm_struct(struct mm_struct *, struct vm_area_struct *);
 extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
        struct rb_node **, struct rb_node *);
+extern void unlink_file_vma(struct vm_area_struct *);
 extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
        unsigned long addr, unsigned long len, pgoff_t pgoff);
 extern void exit_mmap(struct mm_struct *);

diff --git a/mm/mmap.c b/mm/mmap.c
index eeefe19a0facb2c3add503b821b21097c0572fa8..a3984fad3fc2cb61257e3a1f585195a8a5f9afb7 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -181,26 +181,44 @@ static void __remove_shared_vm_struct(struct vm_area_struct *vma,
 }
 
 /*
- * Remove one vm structure and free it.
+ * Unlink a file-based vm structure from its prio_tree, to hide
+ * vma from rmap and vmtruncate before freeing its page tables.
  */
-static void remove_vm_struct(struct vm_area_struct *vma)
+void unlink_file_vma(struct vm_area_struct *vma)
 {
        struct file *file = vma->vm_file;
 
-       might_sleep();
        if (file) {
                struct address_space *mapping = file->f_mapping;
                spin_lock(&mapping->i_mmap_lock);
                __remove_shared_vm_struct(vma, file, mapping);
                spin_unlock(&mapping->i_mmap_lock);
        }
+}
+
+/*
+ * Close a vm structure and free it, returning the next.
+ */
+static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
+{
+       struct vm_area_struct *next = vma->vm_next;
+
+       /*
+        * Hide vma from rmap and vmtruncate before freeing page tables:
+        * to be moved into free_pgtables once page_table_lock is lifted
+        * from it, but until then lock ordering forbids that move.
+        */
+       anon_vma_unlink(vma);
+       unlink_file_vma(vma);
+
+       might_sleep();
        if (vma->vm_ops && vma->vm_ops->close)
                vma->vm_ops->close(vma);
-       if (file)
-               fput(file);
-       anon_vma_unlink(vma);
+       if (vma->vm_file)
+               fput(vma->vm_file);
        mpol_free(vma_policy(vma));
        kmem_cache_free(vm_area_cachep, vma);
+       return next;
 }
 
 asmlinkage unsigned long sys_brk(unsigned long brk)
@@ -1612,15 +1630,13 @@ find_extend_vma(struct mm_struct * mm, unsigned long addr)
 static void remove_vma_list(struct mm_struct *mm, struct vm_area_struct *vma)
 {
        do {
-               struct vm_area_struct *next = vma->vm_next;
                long nrpages = vma_pages(vma);
 
                mm->total_vm -= nrpages;
                if (vma->vm_flags & VM_LOCKED)
                        mm->locked_vm -= nrpages;
                vm_stat_account(mm, vma->vm_flags, vma->vm_file, -nrpages);
-               remove_vm_struct(vma);
-               vma = next;
+               vma = remove_vma(vma);
        } while (vma);
        validate_mm(mm);
 }
@@ -1944,11 +1960,8 @@ void exit_mmap(struct mm_struct *mm)
         * Walk the list again, actually closing and freeing it
         * without holding any MM locks.
         */
-       while (vma) {
-               struct vm_area_struct *next = vma->vm_next;
-               remove_vm_struct(vma);
-               vma = next;
-       }
+       while (vma)
+               vma = remove_vma(vma);
 
        BUG_ON(mm->nr_ptes > (FIRST_USER_ADDRESS+PMD_SIZE-1)>>PMD_SHIFT);
 }