dax: wrprotect pmd_t in dax_mapping_entry_mkclean

Currently dax_mapping_entry_mkclean() fails to clean and write protect
the pmd_t of a DAX PMD entry during an *sync operation.  This can result
in data loss in the following sequence:

1) mmap write to DAX PMD, dirtying PMD radix tree entry and making the
   pmd_t dirty and writeable
2) fsync, flushing out PMD data and cleaning the radix tree entry. We
   currently fail to mark the pmd_t as clean and write protected.
3) more mmap writes to the PMD.  These don't cause any page faults since
   the pmd_t is dirty and writeable.  The radix tree entry remains clean.
4) fsync, which fails to flush the dirty PMD data because the radix tree
   entry was clean.
5) crash - dirty data that should have been fsync'd as part of 4) could
   still have been in the processor cache, and is lost.

Fix this by marking the pmd_t clean and write protected in
dax_mapping_entry_mkclean(), which is called as part of the fsync
operation 2).  This will cause the writes in step 3) above to generate
page faults where we'll re-dirty the PMD radix tree entry, resulting in
flushes in the fsync that happens in step 4).

Fixes: 4b4bb46d00 ("dax: clear dirty entry tags on cache flush")
Link: http://lkml.kernel.org/r/1482272586-21177-3-git-send-email-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dax.c

@@ -691,8 +691,8 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
 				      pgoff_t index, unsigned long pfn)
 {
 	struct vm_area_struct *vma;
-	pte_t *ptep;
-	pte_t pte;
+	pte_t pte, *ptep = NULL;
+	pmd_t *pmdp = NULL;
 	spinlock_t *ptl;
 	bool changed;
 
@@ -707,12 +707,32 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
 
 		address = pgoff_address(index, vma);
 		changed = false;
-		if (follow_pte(vma->vm_mm, address, &ptep, &ptl))
+		if (follow_pte_pmd(vma->vm_mm, address, &ptep, &pmdp, &ptl))
 			continue;
+
+		if (pmdp) {
+#ifdef CONFIG_FS_DAX_PMD
+			pmd_t pmd;
+
+			if (pfn != pmd_pfn(*pmdp))
+				goto unlock_pmd;
+			if (!pmd_dirty(*pmdp) && !pmd_write(*pmdp))
+				goto unlock_pmd;
+
+			flush_cache_page(vma, address, pfn);
+			pmd = pmdp_huge_clear_flush(vma, address, pmdp);
+			pmd = pmd_wrprotect(pmd);
+			pmd = pmd_mkclean(pmd);
+			set_pmd_at(vma->vm_mm, address, pmdp, pmd);
+			changed = true;
+unlock_pmd:
+			spin_unlock(ptl);
+#endif
+		} else {
 			if (pfn != pte_pfn(*ptep))
-			goto unlock;
+				goto unlock_pte;
 			if (!pte_dirty(*ptep) && !pte_write(*ptep))
-			goto unlock;
+				goto unlock_pte;
 
 			flush_cache_page(vma, address, pfn);
 			pte = ptep_clear_flush(vma, address, ptep);
@@ -720,8 +740,9 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
 			pte = pte_mkclean(pte);
 			set_pte_at(vma->vm_mm, address, ptep, pte);
 			changed = true;
-unlock:
+unlock_pte:
 			pte_unmap_unlock(ptep, ptl);
+		}
 
 		if (changed)
 			mmu_notifier_invalidate_page(vma->vm_mm, address);

include/linux/mm.h

@@ -1210,8 +1210,6 @@ int copy_page_range(struct mm_struct *dst, struct mm_struct *src,
 			struct vm_area_struct *vma);
 void unmap_mapping_range(struct address_space *mapping,
 		loff_t const holebegin, loff_t const holelen, int even_cows);
-int follow_pte(struct mm_struct *mm, unsigned long address, pte_t **ptepp,
-	       spinlock_t **ptlp);
 int follow_pte_pmd(struct mm_struct *mm, unsigned long address,
 			     pte_t **ptepp, pmd_t **pmdpp, spinlock_t **ptlp);
 int follow_pfn(struct vm_area_struct *vma, unsigned long address,

mm/memory.c

@@ -3819,8 +3819,8 @@ out:
 	return -EINVAL;
 }
 
-int follow_pte(struct mm_struct *mm, unsigned long address, pte_t **ptepp,
-	       spinlock_t **ptlp)
+static inline int follow_pte(struct mm_struct *mm, unsigned long address,
+			     pte_t **ptepp, spinlock_t **ptlp)
 {
 	int res;