[PATCH] fix get_user_pages bug

Checking pte_dirty instead of pte_write in __follow_page is problematic
for s390, and for copy_one_pte which leaves dirty when clearing write.

So revert __follow_page to check pte_write as before, and make
do_wp_page pass back a special extra VM_FAULT_WRITE bit to say it has
done its full job: once get_user_pages receives this value, it no longer
requires pte_write in __follow_page.

But most callers of handle_mm_fault, in the various architectures, have
switch statements which do not expect this new case. To avoid changing
them all in a hurry, make an inline wrapper function (using the old
name) that masks off the new bit, and use the extended interface with
double underscores.

Yes, we do have a call to do_wp_page from do_swap_page, but no need to
change that: in rare case it's needed, another do_wp_page will follow.

Signed-off-by: Hugh Dickins <[e-mail]>
[ Cleanups by Nick Piggin ]
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mm.h

···
  * Used to decide whether a process gets delivered SIGBUS or
  * just gets major/minor fault counters bumped up.
  */
-#define VM_FAULT_OOM	(-1)
-#define VM_FAULT_SIGBUS	0
-#define VM_FAULT_MINOR	1
-#define VM_FAULT_MAJOR	2
+#define VM_FAULT_OOM	0x00
+#define VM_FAULT_SIGBUS	0x01
+#define VM_FAULT_MINOR	0x02
+#define VM_FAULT_MAJOR	0x03
+
+/* 
+ * Special case for get_user_pages.
+ * Must be in a distinct bit from the above VM_FAULT_ flags.
+ */
+#define VM_FAULT_WRITE	0x10
 
 #define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK)
 
···
 extern pte_t *FASTCALL(pte_alloc_map(struct mm_struct *mm, pmd_t *pmd, unsigned long address));
 extern int install_page(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long addr, struct page *page, pgprot_t prot);
 extern int install_file_pte(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long addr, unsigned long pgoff, pgprot_t prot);
-extern int handle_mm_fault(struct mm_struct *mm,struct vm_area_struct *vma, unsigned long address, int write_access);
+extern int __handle_mm_fault(struct mm_struct *mm,struct vm_area_struct *vma, unsigned long address, int write_access);
+
+static inline int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long address, int write_access)
+{
+	return __handle_mm_fault(mm, vma, address, write_access) & (~VM_FAULT_WRITE);
+}
+
 extern int make_pages_present(unsigned long addr, unsigned long end);
 extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write);
 void install_arg_page(struct vm_area_struct *, struct page *, unsigned long);

mm/memory.c

···
 	pte = *ptep;
 	pte_unmap(ptep);
 	if (pte_present(pte)) {
-		if (write && !pte_dirty(pte))
+		if (write && !pte_write(pte))
 			goto out;
 		if (read && !pte_read(pte))
 			goto out;
 		pfn = pte_pfn(pte);
 		if (pfn_valid(pfn)) {
 			page = pfn_to_page(pfn);
-			if (accessed)
+			if (accessed) {
+				if (write && !pte_dirty(pte) &&!PageDirty(page))
+					set_page_dirty(page);
 				mark_page_accessed(page);
+			}
 			return page;
 		}
 	}
···
 		}
 		spin_lock(&mm->page_table_lock);
 		do {
+			int write_access = write;
 			struct page *page;
 
 			cond_resched_lock(&mm->page_table_lock);
-			while (!(page = follow_page(mm, start, write))) {
+			while (!(page = follow_page(mm, start, write_access))) {
 				/*
 				 * Shortcut for anonymous pages. We don't want
 				 * to force the creation of pages tables for
···
 					break;
 				}
 				spin_unlock(&mm->page_table_lock);
-				switch (handle_mm_fault(mm,vma,start,write)) {
+				switch (__handle_mm_fault(mm, vma, start,
+							write_access)) {
+				case VM_FAULT_WRITE:
+					/*
+					 * do_wp_page has broken COW when
+					 * necessary, even if maybe_mkwrite
+					 * decided not to set pte_write
+					 */
+					write_access = 0;
+					/* FALLTHRU */
 				case VM_FAULT_MINOR:
 					tsk->min_flt++;
 					break;
···
 	struct page *old_page, *new_page;
 	unsigned long pfn = pte_pfn(pte);
 	pte_t entry;
+	int ret;
 
 	if (unlikely(!pfn_valid(pfn))) {
 		/*
···
 			lazy_mmu_prot_update(entry);
 			pte_unmap(page_table);
 			spin_unlock(&mm->page_table_lock);
-			return VM_FAULT_MINOR;
+			return VM_FAULT_MINOR|VM_FAULT_WRITE;
 		}
 	}
 	pte_unmap(page_table);
···
 	/*
 	 * Re-check the pte - we dropped the lock
 	 */
+	ret = VM_FAULT_MINOR;
 	spin_lock(&mm->page_table_lock);
 	page_table = pte_offset_map(pmd, address);
 	if (likely(pte_same(*page_table, pte))) {
···
 
 		/* Free the old page.. */
 		new_page = old_page;
+		ret |= VM_FAULT_WRITE;
 	}
 	pte_unmap(page_table);
 	page_cache_release(new_page);
 	page_cache_release(old_page);
 	spin_unlock(&mm->page_table_lock);
-	return VM_FAULT_MINOR;
+	return ret;
 
 no_new_page:
 	page_cache_release(old_page);
···
 	if (write_access) {
 		if (!pte_write(entry))
 			return do_wp_page(mm, vma, address, pte, pmd, entry);
-
 		entry = pte_mkdirty(entry);
 	}
 	entry = pte_mkyoung(entry);
···
 /*
  * By the time we get here, we already hold the mm semaphore
  */
-int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct * vma,
+int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct * vma,
 		unsigned long address, int write_access)
 {
 	pgd_t *pgd;