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>

1 files changed, 22 insertions, 23 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2ed1a160a85b..03a89a2f464b 100644
--- 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));
 
         /*