mm/hmm: make full use of walk_page_range()

hmm_range_fault() calls find_vma() and walk_page_range() in a loop.  This
is unnecessary duplication since walk_page_range() calls find_vma() in a
loop already.

Simplify hmm_range_fault() by defining a walk_test() callback function to
filter unhandled vmas.

This also fixes a bug where hmm_range_fault() was not checking start >=
vma->vm_start before checking vma->vm_flags so hmm_range_fault() could
return an error based on the wrong vma for the requested range.

It also fixes a bug when the vma has no read access and the caller did not
request a fault, there shouldn't be any error return code.

Link: https://lore.kernel.org/r/20191104222141.5173-2-rcampbell@nvidia.comSigned-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

mm/hmm.c

@@ -65,18 +65,15 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
 	return -EFAULT;
 }
 
-static int hmm_pfns_bad(unsigned long addr,
-			unsigned long end,
-			struct mm_walk *walk)
+static int hmm_pfns_fill(unsigned long addr, unsigned long end,
+		struct hmm_range *range, enum hmm_pfn_value_e value)
 {
-	struct hmm_vma_walk *hmm_vma_walk = walk->private;
-	struct hmm_range *range = hmm_vma_walk->range;
 	uint64_t *pfns = range->pfns;
 	unsigned long i;
 
 	i = (addr - range->start) >> PAGE_SHIFT;
 	for (; addr < end; addr += PAGE_SIZE, i++)
-		pfns[i] = range->values[HMM_PFN_ERROR];
+		pfns[i] = range->values[value];
 
 	return 0;
 }
@@ -403,7 +400,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 		}
 		return 0;
 	} else if (!pmd_present(pmd))
-		return hmm_pfns_bad(start, end, walk);
+		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 
 	if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
 		/*
@@ -431,7 +428,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 	 * recover.
 	 */
 	if (pmd_bad(pmd))
-		return hmm_pfns_bad(start, end, walk);
+		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 
 	ptep = pte_offset_map(pmdp, addr);
 	i = (addr - range->start) >> PAGE_SHIFT;
@@ -589,13 +586,47 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 #define hmm_vma_walk_hugetlb_entry NULL
 #endif /* CONFIG_HUGETLB_PAGE */
 
-static void hmm_pfns_clear(struct hmm_range *range,
-			   uint64_t *pfns,
-			   unsigned long addr,
-			   unsigned long end)
+static int hmm_vma_walk_test(unsigned long start, unsigned long end,
+			     struct mm_walk *walk)
 {
-	for (; addr < end; addr += PAGE_SIZE, pfns++)
-		*pfns = range->values[HMM_PFN_NONE];
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
+	struct vm_area_struct *vma = walk->vma;
+
+	/*
+	 * Skip vma ranges that don't have struct page backing them or
+	 * map I/O devices directly.
+	 */
+	if (vma->vm_flags & (VM_IO | VM_PFNMAP | VM_MIXEDMAP))
+		return -EFAULT;
+
+	/*
+	 * If the vma does not allow read access, then assume that it does not
+	 * allow write access either. HMM does not support architectures
+	 * that allow write without read.
+	 */
+	if (!(vma->vm_flags & VM_READ)) {
+		bool fault, write_fault;
+
+		/*
+		 * Check to see if a fault is requested for any page in the
+		 * range.
+		 */
+		hmm_range_need_fault(hmm_vma_walk, range->pfns +
+					((start - range->start) >> PAGE_SHIFT),
+					(end - start) >> PAGE_SHIFT,
+					0, &fault, &write_fault);
+		if (fault || write_fault)
+			return -EFAULT;
+
+		hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
+		hmm_vma_walk->last = end;
+
+		/* Skip this vma and continue processing the next vma. */
+		return 1;
+	}
+
+	return 0;
 }
 
 static const struct mm_walk_ops hmm_walk_ops = {
@@ -603,6 +634,7 @@ static const struct mm_walk_ops hmm_walk_ops = {
 	.pmd_entry	= hmm_vma_walk_pmd,
 	.pte_hole	= hmm_vma_walk_hole,
 	.hugetlb_entry	= hmm_vma_walk_hugetlb_entry,
+	.test_walk	= hmm_vma_walk_test,
 };
 
 /**
@@ -635,11 +667,12 @@ static const struct mm_walk_ops hmm_walk_ops = {
  */
 long hmm_range_fault(struct hmm_range *range, unsigned int flags)
 {
-	const unsigned long device_vma = VM_IO | VM_PFNMAP | VM_MIXEDMAP;
-	unsigned long start = range->start, end;
-	struct hmm_vma_walk hmm_vma_walk;
+	struct hmm_vma_walk hmm_vma_walk = {
+		.range = range,
+		.last = range->start,
+		.flags = flags,
+	};
 	struct mm_struct *mm = range->notifier->mm;
-	struct vm_area_struct *vma;
 	int ret;
 
 	lockdep_assert_held(&mm->mmap_sem);
@@ -649,53 +682,12 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
 		if (mmu_interval_check_retry(range->notifier,
 					     range->notifier_seq))
 			return -EBUSY;
+		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
+				      &hmm_walk_ops, &hmm_vma_walk);
+	} while (ret == -EBUSY);
 
-		vma = find_vma(mm, start);
-		if (vma == NULL || (vma->vm_flags & device_vma))
-			return -EFAULT;
-
-		if (!(vma->vm_flags & VM_READ)) {
-			/*
-			 * If vma do not allow read access, then assume that it
-			 * does not allow write access, either. HMM does not
-			 * support architecture that allow write without read.
-			 */
-			hmm_pfns_clear(range, range->pfns,
-				range->start, range->end);
-			return -EPERM;
-		}
-
-		hmm_vma_walk.pgmap = NULL;
-		hmm_vma_walk.last = start;
-		hmm_vma_walk.flags = flags;
-		hmm_vma_walk.range = range;
-		end = min(range->end, vma->vm_end);
-
-		walk_page_range(vma->vm_mm, start, end, &hmm_walk_ops,
-				&hmm_vma_walk);
-
-		do {
-			ret = walk_page_range(vma->vm_mm, start, end,
-					&hmm_walk_ops, &hmm_vma_walk);
-			start = hmm_vma_walk.last;
-
-			/* Keep trying while the range is valid. */
-		} while (ret == -EBUSY &&
-			 !mmu_interval_check_retry(range->notifier,
-						   range->notifier_seq));
-
-		if (ret) {
-			unsigned long i;
-
-			i = (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
-			hmm_pfns_clear(range, &range->pfns[i],
-				hmm_vma_walk.last, range->end);
-			return ret;
-		}
-		start = end;
-
-	} while (start < range->end);
-
+	if (ret)
+		return ret;
 	return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
 }
 EXPORT_SYMBOL(hmm_range_fault);