[PATCH 2/3] hugetlb: fix prio_tree unit

Hugh Dickins
2006-10-25T02:35:41Z

hugetlb_vmtruncate_list was misconverted to prio_tree: its prio_tree is
in units of PAGE_SIZE (PAGE_CACHE_SIZE) like any other, not HPAGE_SIZE
(whereas its radix_tree is kept in units of HPAGE_SIZE, otherwise slots
would be absurdly sparse).

At first I thought the error benign, just calling __unmap_hugepage_range
on more vmas than necessary; but on 32-bit machines, when the prio_tree
is searched correctly, it happens to ensure the v_offset calculation won't
overflow. As it stood, when truncating at or beyond 4GB, it was liable
to discard pages COWed from lower offsets; or even to clear pmd entries
of preceding vmas, triggering exit_mmap's BUG_ON(nr_ptes).

Signed-off-by: Hugh Dickins <hugh@veritas.com>
___

fs/hugetlbfs/inode.c | 24 +++++++++++-------------
1 file changed, 11 insertions(+), 13 deletions(-)

--- 2.6.19-rc3/fs/hugetlbfs/inode.c 2006-10-24 04:34:28.000000000 +0100
+++ linux/fs/hugetlbfs/inode.c 2006-10-24 17:43:08.000000000 +0100
@@ -271,26 +271,24 @@ static void hugetlbfs_drop_inode(struct
hugetlbfs_forget_inode(inode);
}

-/*
- * h_pgoff is in HPAGE_SIZE units.
- * vma->vm_pgoff is in PAGE_SIZE units.
- */
static inline void
-hugetlb_vmtruncate_list(struct prio_tree_root *root, unsigned long h_pgoff)
+hugetlb_vmtruncate_list(struct prio_tree_root *root, pgoff_t pgoff)
{
struct vm_area_struct *vma;
struct prio_tree_iter iter;

- vma_prio_tree_foreach(vma, &iter, root, h_pgoff, ULONG_MAX) {
- unsigned long h_vm_pgoff;
+ vma_prio_tree_foreach(vma, &iter, root, pgoff, ULONG_MAX) {
unsigned long v_offset;

- h_vm_pgoff = vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT);
- v_offset = (h_pgoff - h_vm_pgoff) << HPAGE_SHIFT;
/*
- * Is this VMA fully outside the truncation point?
+ * Can the expression below overflow on 32-bit arches?
+ * No, because the prio_tree returns us only those vmas
+ * which overlap the truncated area starting at pgoff,
+ * and no vma on a 32-bit arch can span beyond the 4GB.
*/
- if (h_vm_pgoff >= h_pgoff)
+ if (vma->vm_pgoff < pgoff)
+ v_offset = (pgoff - vma->vm_pgoff) << PAGE_SHIFT;
+ else
v_offset = 0;

__unmap_hugepage_range(vma,
@@ -303,14 +301,14 @@ hugetlb_vmtruncate_list(struct prio_tree
*/
static int hugetlb_vmtruncate(struct inode *inode, loff_t offset)
{
- unsigned long pgoff;
+ pgoff_t pgoff;
struct address_space *mapping = inode->i_mapping;

if (offset > inode->i_size)
return -EINVAL;

BUG_ON(offset & ~HPAGE_MASK);
- pgoff = offset >> HPAGE_SHIFT;
+ pgoff = offset >> PAGE_SHIFT;

inode->i_size = offset;
spin_lock(&mapping->i_mmap_lock);