Re: [PATCH STABLE 4.9 1/1] mm, gup: add missing refcount overflow checks on x86 and s390

On 12/3/19 1:22 PM, Ben Hutchings wrote:

...

•     || !page_cache_get_speculative(head)))
  return 0;
  

  if (unlikely(pte_val(pte) != pte_val(*ptep))) { put_page(head);

[...]

...

--- a/arch/x86/mm/gup.c +++ b/arch/x86/mm/gup.c @@ -202,10 +202,12 @@ static int __gup_device_huge_pmd(pmd_t pmd, unsigned long addr, undo_dev_pagemap(nr, nr_start, pages); return 0; }

• if (unlikely(!try_get_page(page))) {
  

• 	put_dev_pagemap(pgmap);
  

• 	return 0;
  

• }
  

  SetPageReferenced(page); pages[*nr] = page;

• get_page(page);
  

• put_dev_pagemap(pgmap);
  

This leaks a pgmap reference on success!

Good catch, deleted one line too many!

...

(*nr)++;
pfn++;

} while (addr += PAGE_SIZE, addr != end); @@ -230,6 +232,8 @@ static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr, refs = 0; head = pmd_page(pmd);

• if (WARN_ON_ONCE(page_ref_count(head) <= 0))

Why <= 0, given we use < 0 elsewhere?

The code uses get_head_page_multiple() which boils down to atomic_add and not add_unless_zero(), so it assumes a pre-existing pin that must not go away or it's a bug (one that I've been hunting recently in this area). The check makes it explicit.

...

• return 0;
  

  page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT); do { VM_BUG_ON_PAGE(compound_head(page) != head, page);

@@ -289,6 +293,8 @@ static noinline int gup_huge_pud(pud_t pud, unsigned long addr, refs = 0; head = pud_page(pud);

• if (WARN_ON_ONCE(page_ref_count(head) <= 0))

Same question here.

Same as above.

Ben.

...

• return 0;
  

  page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT); do { VM_BUG_ON_PAGE(compound_head(page) != head, page);