thp: fix huge zero page logic for page with pfn == 0

Current implementation of huge zero page uses pfn value 0 to indicate
that the page hasn't allocated yet.  It assumes that buddy page
allocator can't return page with pfn == 0.

Let's rework the code to store 'struct page *' of huge zero page, not
its pfn.  This way we can avoid the weak assumption.

[akpm@linux-foundation.org: fix sparse warning]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Minchan Kim <minchan@kernel.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Wu Fengguang <fengguang.wu@intel.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 2ed1a16..03a89a2 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -163,35 +163,34 @@ static int start_khugepaged(void)
 }
 
 static atomic_t huge_zero_refcount;
-static unsigned long huge_zero_pfn __read_mostly;
+static struct page *huge_zero_page __read_mostly;
 
-static inline bool is_huge_zero_pfn(unsigned long pfn)
+static inline bool is_huge_zero_page(struct page *page)
 {
-       unsigned long zero_pfn = ACCESS_ONCE(huge_zero_pfn);
-       return zero_pfn && pfn == zero_pfn;
+       return ACCESS_ONCE(huge_zero_page) == page;
 }
 
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
-       return is_huge_zero_pfn(pmd_pfn(pmd));
+       return is_huge_zero_page(pmd_page(pmd));
 }
 
-static unsigned long get_huge_zero_page(void)
+static struct page *get_huge_zero_page(void)
 {
        struct page *zero_page;
 retry:
        if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
-               return ACCESS_ONCE(huge_zero_pfn);
+               return ACCESS_ONCE(huge_zero_page);
 
        zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
                        HPAGE_PMD_ORDER);
        if (!zero_page) {
                count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);
-               return 0;
+               return NULL;
        }
        count_vm_event(THP_ZERO_PAGE_ALLOC);
        preempt_disable();
-       if (cmpxchg(&huge_zero_pfn, 0, page_to_pfn(zero_page))) {
+       if (cmpxchg(&huge_zero_page, NULL, zero_page)) {
                preempt_enable();
                __free_page(zero_page);
                goto retry;
@@ -200,7 +199,7 @@ retry:
        /* We take additional reference here. It will be put back by shrinker */
        atomic_set(&huge_zero_refcount, 2);
        preempt_enable();
-       return ACCESS_ONCE(huge_zero_pfn);
+       return ACCESS_ONCE(huge_zero_page);
 }
 
 static void put_huge_zero_page(void)
@@ -220,9 +219,9 @@ static int shrink_huge_zero_page(struct shrinker *shrink,
                return atomic_read(&huge_zero_refcount) == 1 ? HPAGE_PMD_NR : 0;
 
        if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
-               unsigned long zero_pfn = xchg(&huge_zero_pfn, 0);
-               BUG_ON(zero_pfn == 0);
-               __free_page(__pfn_to_page(zero_pfn));
+               struct page *zero_page = xchg(&huge_zero_page, NULL);
+               BUG_ON(zero_page == NULL);
+               __free_page(zero_page);
        }
 
        return 0;
@@ -764,12 +763,12 @@ static inline struct page *alloc_hugepage(int defrag)
 
 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)
+               struct page *zero_page)
 {
        pmd_t entry;
        if (!pmd_none(*pmd))
                return false;
-       entry = pfn_pmd(zero_pfn, vma->vm_page_prot);
+       entry = mk_pmd(zero_page, vma->vm_page_prot);
        entry = pmd_wrprotect(entry);
        entry = pmd_mkhuge(entry);
        set_pmd_at(mm, haddr, pmd, entry);
@@ -794,20 +793,20 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                if (!(flags & FAULT_FLAG_WRITE) &&
                                transparent_hugepage_use_zero_page()) {
                        pgtable_t pgtable;
-                       unsigned long zero_pfn;
+                       struct page *zero_page;
                        bool set;
                        pgtable = pte_alloc_one(mm, haddr);
                        if (unlikely(!pgtable))
                                return VM_FAULT_OOM;
-                       zero_pfn = get_huge_zero_page();
-                       if (unlikely(!zero_pfn)) {
+                       zero_page = get_huge_zero_page();
+                       if (unlikely(!zero_page)) {
                                pte_free(mm, pgtable);
                                count_vm_event(THP_FAULT_FALLBACK);
                                goto out;
                        }
                        spin_lock(&mm->page_table_lock);
                        set = set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
-                                       zero_pfn);
+                                       zero_page);
                        spin_unlock(&mm->page_table_lock);
                        if (!set) {
                                pte_free(mm, pgtable);
@@ -886,16 +885,16 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
         * a page table.
         */
        if (is_huge_zero_pmd(pmd)) {
-               unsigned long zero_pfn;
+               struct page *zero_page;
                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();
+               zero_page = get_huge_zero_page();
                set = set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
-                               zero_pfn);
+                               zero_page);
                BUG_ON(!set); /* unexpected !pmd_none(dst_pmd) */
                ret = 0;
                goto out_unlock;
@@ -1812,7 +1811,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
        struct anon_vma *anon_vma;
        int ret = 1;
 
-       BUG_ON(is_huge_zero_pfn(page_to_pfn(page)));
+       BUG_ON(is_huge_zero_page(page));
        BUG_ON(!PageAnon(page));
 
        /*