mm/hmm: make hmm_range_fault return 0 or -1

hmm_vma_walk->last is supposed to be updated after every write to the
pfns, so that it can be returned by hmm_range_fault(). However, this is
not done consistently. Fortunately nothing checks the return code of
hmm_range_fault() for anything other than error.

More importantly last must be set before returning -EBUSY as it is used to
prevent reading an output pfn as an input flags when the loop restarts.

For clarity and simplicity make hmm_range_fault() return 0 or -ERRNO. Only
set last when returning -EBUSY.

Link: https://lore.kernel.org/r/2-v2-b4e84f444c7d+24f57-hmm_no_flags_jgg@mellanox.com
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

1 files changed, 9 insertions, 16 deletions

diff --git a/mm/hmm.c b/mm/hmm.c
index 280585833adf..f06bcac948a7 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -174,7 +174,6 @@ static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
 	}
 	if (required_fault)
 		return hmm_vma_fault(addr, end, required_fault, walk);
-	hmm_vma_walk->last = addr;
 	return hmm_pfns_fill(addr, end, range, HMM_PFN_NONE);
 }
 
@@ -207,7 +206,6 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 	pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++)
 		pfns[i] = hmm_device_entry_from_pfn(range, pfn) | cpu_flags;
-	hmm_vma_walk->last = end;
 	return 0;
 }
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
@@ -386,13 +384,10 @@ again:
 		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, pfns);
 		if (r) {
 			/* hmm_vma_handle_pte() did pte_unmap() */
-			hmm_vma_walk->last = addr;
 			return r;
 		}
 	}
 	pte_unmap(ptep - 1);
-
-	hmm_vma_walk->last = addr;
 	return 0;
 }
 
@@ -455,7 +450,6 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 		for (i = 0; i < npages; ++i, ++pfn)
 			pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
 				  cpu_flags;
-		hmm_vma_walk->last = end;
 		goto out_unlock;
 	}
 
@@ -500,7 +494,6 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 	for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
 		range->pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
 				 cpu_flags;
-	hmm_vma_walk->last = end;
 	spin_unlock(ptl);
 	return 0;
 }
@@ -537,7 +530,6 @@ static int hmm_vma_walk_test(unsigned long start, unsigned long end,
 		return -EFAULT;
 
 	hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
-	hmm_vma_walk->last = end;
 
 	/* Skip this vma and continue processing the next vma. */
 	return 1;
@@ -555,9 +547,7 @@ static const struct mm_walk_ops hmm_walk_ops = {
  * hmm_range_fault - try to fault some address in a virtual address range
  * @range:	argument structure
  *
- * Return: the number of valid pages in range->pfns[] (from range start
- * address), which may be zero.  On error one of the following status codes
- * can be returned:
+ * Returns 0 on success or one of the following error codes:
  *
  * -EINVAL:	Invalid arguments or mm or virtual address is in an invalid vma
  *		(e.g., device file vma).
@@ -572,7 +562,7 @@ static const struct mm_walk_ops hmm_walk_ops = {
  * This is similar to get_user_pages(), except that it can read the page tables
  * without mutating them (ie causing faults).
  */
-long hmm_range_fault(struct hmm_range *range)
+int hmm_range_fault(struct hmm_range *range)
 {
 	struct hmm_vma_walk hmm_vma_walk = {
 		.range = range,
@@ -590,10 +580,13 @@ long hmm_range_fault(struct hmm_range *range)
 			return -EBUSY;
 		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 				      &hmm_walk_ops, &hmm_vma_walk);
+		/*
+		 * When -EBUSY is returned the loop restarts with
+		 * hmm_vma_walk.last set to an address that has not been stored
+		 * in pfns. All entries < last in the pfn array are set to their
+		 * output, and all >= are still at their input values.
+		 */
 	} while (ret == -EBUSY);
-
-	if (ret)
-		return ret;
-	return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
+	return ret;
 }
 EXPORT_SYMBOL(hmm_range_fault);