mm/hmm: add missing unmaps of the ptep during hmm_vma_handle_pte()

Many of the direct returns of error skipped doing the pte_unmap(). All non
zero exit paths must unmap the pte.

The pte_unmap() is split unnaturally like this because some of the error
exit paths trigger a sleep and must release the lock before sleeping.

Fixes: 992de9a8 ("mm/hmm: allow to mirror vma of a file on a DAX backed filesystem")
Fixes: 53f5c3f4 ("mm/hmm: factor out pte and pmd handling to simplify hmm_vma_walk_pmd()")
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

mm/hmm.c

@@ -325,6 +325,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		}
 
 		/* Report error for everything else */
+		pte_unmap(ptep);
 		*pfn = range->values[HMM_PFN_ERROR];
 		return -EFAULT;
 	} else {
@@ -339,10 +340,13 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	if (pte_devmap(pte)) {
 		hmm_vma_walk->pgmap = get_dev_pagemap(pte_pfn(pte),
 					      hmm_vma_walk->pgmap);
-		if (unlikely(!hmm_vma_walk->pgmap))
+		if (unlikely(!hmm_vma_walk->pgmap)) {
+			pte_unmap(ptep);
 			return -EBUSY;
+		}
 	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
 		if (!is_zero_pfn(pte_pfn(pte))) {
+			pte_unmap(ptep);
 			*pfn = range->values[HMM_PFN_SPECIAL];
 			return -EFAULT;
 		}
@@ -437,7 +441,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 
 		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, &pfns[i]);
 		if (r) {
-			/* hmm_vma_handle_pte() did unmap pte directory */
+			/* hmm_vma_handle_pte() did pte_unmap() */
 			hmm_vma_walk->last = addr;
 			return r;
 		}