mm: rmap: simplify the hugetlb handling when unmapping or migration

According to previous discussion [1], there are so many levels of
indenting to handle the hugetlb case when unmapping or migration.  We can
combine folio_test_anon() and huge_pmd_unshare() to save one level of
indenting, by adding a local variable and moving the VM_BUG_ON() a little
forward.

No intended functional changes in this patch.

[1] https://lore.kernel.org/all/0b986dc4-5843-3e2d-c2df-5a2e9f13e6ab@oracle.com/

Link: https://lkml.kernel.org/r/28414b1b96f095e838c1e548074f8e0fc70d78cf.1655724713.git.baolin.wang@linux.alibaba.comSigned-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/rmap.c

@@ -1537,6 +1537,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 PageAnonExclusive(subpage);
 
 		if (folio_test_hugetlb(folio)) {
+			bool anon = folio_test_anon(folio);
+
 			/*
 			 * The try_to_unmap() is only passed a hugetlb page
 			 * in the case where the hugetlb page is poisoned.
@@ -1551,31 +1553,28 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 */
 			flush_cache_range(vma, range.start, range.end);
 
-			if (!folio_test_anon(folio)) {
+			/*
+			 * To call huge_pmd_unshare, i_mmap_rwsem must be
+			 * held in write mode.  Caller needs to explicitly
+			 * do this outside rmap routines.
+			 */
+			VM_BUG_ON(!anon && !(flags & TTU_RMAP_LOCKED));
+			if (!anon && huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
+				flush_tlb_range(vma, range.start, range.end);
+				mmu_notifier_invalidate_range(mm, range.start,
+							      range.end);
+
 				/*
-				 * To call huge_pmd_unshare, i_mmap_rwsem must be
-				 * held in write mode.  Caller needs to explicitly
-				 * do this outside rmap routines.
+				 * The ref count of the PMD page was dropped
+				 * which is part of the way map counting
+				 * is done for shared PMDs.  Return 'true'
+				 * here.  When there is no other sharing,
+				 * huge_pmd_unshare returns false and we will
+				 * unmap the actual page and drop map count
+				 * to zero.
 				 */
-				VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
-
-				if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
-					flush_tlb_range(vma, range.start, range.end);
-					mmu_notifier_invalidate_range(mm, range.start,
-								      range.end);
-
-					/*
-					 * The ref count of the PMD page was dropped
-					 * which is part of the way map counting
-					 * is done for shared PMDs.  Return 'true'
-					 * here.  When there is no other sharing,
-					 * huge_pmd_unshare returns false and we will
-					 * unmap the actual page and drop map count
-					 * to zero.
-					 */
-					page_vma_mapped_walk_done(&pvmw);
-					break;
-				}
+				page_vma_mapped_walk_done(&pvmw);
+				break;
 			}
 			pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
 		} else {
@@ -1906,6 +1905,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				 PageAnonExclusive(subpage);
 
 		if (folio_test_hugetlb(folio)) {
+			bool anon = folio_test_anon(folio);
+
 			/*
 			 * huge_pmd_unshare may unmap an entire PMD page.
 			 * There is no way of knowing exactly which PMDs may
@@ -1915,31 +1916,28 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			 */
 			flush_cache_range(vma, range.start, range.end);
 
-			if (!folio_test_anon(folio)) {
+			/*
+			 * To call huge_pmd_unshare, i_mmap_rwsem must be
+			 * held in write mode.  Caller needs to explicitly
+			 * do this outside rmap routines.
+			 */
+			VM_BUG_ON(!anon && !(flags & TTU_RMAP_LOCKED));
+			if (!anon && huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
+				flush_tlb_range(vma, range.start, range.end);
+				mmu_notifier_invalidate_range(mm, range.start,
+							      range.end);
+
 				/*
-				 * To call huge_pmd_unshare, i_mmap_rwsem must be
-				 * held in write mode.  Caller needs to explicitly
-				 * do this outside rmap routines.
+				 * The ref count of the PMD page was dropped
+				 * which is part of the way map counting
+				 * is done for shared PMDs.  Return 'true'
+				 * here.  When there is no other sharing,
+				 * huge_pmd_unshare returns false and we will
+				 * unmap the actual page and drop map count
+				 * to zero.
 				 */
-				VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
-
-				if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
-					flush_tlb_range(vma, range.start, range.end);
-					mmu_notifier_invalidate_range(mm, range.start,
-								      range.end);
-
-					/*
-					 * The ref count of the PMD page was dropped
-					 * which is part of the way map counting
-					 * is done for shared PMDs.  Return 'true'
-					 * here.  When there is no other sharing,
-					 * huge_pmd_unshare returns false and we will
-					 * unmap the actual page and drop map count
-					 * to zero.
-					 */
-					page_vma_mapped_walk_done(&pvmw);
-					break;
-				}
+				page_vma_mapped_walk_done(&pvmw);
+				break;
 			}
 
 			/* Nuke the hugetlb page table entry */