thp: avoid race on multiple parallel page faults to the same page

pmd value is stable only with mm->page_table_lock taken. After taking
the lock we need to check that nobody modified the pmd before changing it.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Reviewed-by: Bob Liu <lliubbo@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 72835ce..827d9c8 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -769,17 +769,20 @@ static inline struct page *alloc_hugepage(int defrag)
 }
 #endif
 
-static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
+static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
                struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
                unsigned long zero_pfn)
 {
        pmd_t entry;
+       if (!pmd_none(*pmd))
+               return false;
        entry = pfn_pmd(zero_pfn, vma->vm_page_prot);
        entry = pmd_wrprotect(entry);
        entry = pmd_mkhuge(entry);
        set_pmd_at(mm, haddr, pmd, entry);
        pgtable_trans_huge_deposit(mm, pgtable);
        mm->nr_ptes++;
+       return true;
 }
 
 int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
@@ -799,6 +802,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                                transparent_hugepage_use_zero_page()) {
                        pgtable_t pgtable;
                        unsigned long zero_pfn;
+                       bool set;
                        pgtable = pte_alloc_one(mm, haddr);
                        if (unlikely(!pgtable))
                                return VM_FAULT_OOM;
@@ -809,9 +813,13 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                                goto out;
                        }
                        spin_lock(&mm->page_table_lock);
-                       set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
+                       set = set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
                                        zero_pfn);
                        spin_unlock(&mm->page_table_lock);
+                       if (!set) {
+                               pte_free(mm, pgtable);
+                               put_huge_zero_page();
+                       }
                        return 0;
                }
                page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
@@ -885,14 +893,16 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
         */
        if (is_huge_zero_pmd(pmd)) {
                unsigned long zero_pfn;
+               bool set;
                /*
                 * get_huge_zero_page() will never allocate a new page here,
                 * since we already have a zero page to copy. It just takes a
                 * reference.
                 */
                zero_pfn = get_huge_zero_page();
-               set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
+               set = set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
                                zero_pfn);
+               BUG_ON(!set); /* unexpected !pmd_none(dst_pmd) */
                ret = 0;
                goto out_unlock;
        }
@@ -949,7 +959,7 @@ unlock:
 
 static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
                struct vm_area_struct *vma, unsigned long address,
-               pmd_t *pmd, unsigned long haddr)
+               pmd_t *pmd, pmd_t orig_pmd, unsigned long haddr)
 {
        pgtable_t pgtable;
        pmd_t _pmd;
@@ -978,6 +988,9 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
        mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
 
        spin_lock(&mm->page_table_lock);
+       if (unlikely(!pmd_same(*pmd, orig_pmd)))
+               goto out_free_page;
+
        pmdp_clear_flush(vma, haddr, pmd);
        /* leave pmd empty until pte is filled */
 
@@ -1010,6 +1023,12 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
        ret |= VM_FAULT_WRITE;
 out:
        return ret;
+out_free_page:
+       spin_unlock(&mm->page_table_lock);
+       mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
+       mem_cgroup_uncharge_page(page);
+       put_page(page);
+       goto out;
 }
 
 static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
@@ -1156,7 +1175,7 @@ alloc:
                count_vm_event(THP_FAULT_FALLBACK);
                if (is_huge_zero_pmd(orig_pmd)) {
                        ret = do_huge_pmd_wp_zero_page_fallback(mm, vma,
-                                       address, pmd, haddr);
+                                       address, pmd, orig_pmd, haddr);
                } else {
                        ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
                                        pmd, orig_pmd, page, haddr);