Re: [PATCH v2 3/3] mm: rmap: Use flush_cache_range() to flush cache for hugetlb pages

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> · Wed, 4 May 2022 10:49:23 +0800

On 5/4/2022 4:17 AM, Mike Kravetz wrote:
On 4/27/22 03:52, Baolin Wang wrote:
Now we will use flush_cache_page() to flush cache for anonymous hugetlb
pages when unmapping or migrating a hugetlb page mapping, but the
flush_cache_page() only handles a PAGE_SIZE range on some architectures
(like arm32, arc and so on), which will cause potential cache issues.
Thus change to use flush_cache_range() to cover the whole size of a
hugetlb page.

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
  mm/rmap.c | 90 ++++++++++++++++++++++++++++++++++-----------------------------
  1 file changed, 48 insertions(+), 42 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 4f0d115..6fdd198 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1528,13 +1528,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
  		anon_exclusive = folio_test_anon(folio) &&
  				 PageAnonExclusive(subpage);
  
-		if (folio_test_hugetlb(folio) && !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(!(flags & TTU_RMAP_LOCKED));
+		if (folio_test_hugetlb(folio)) {
  			/*
  			 * huge_pmd_unshare may unmap an entire PMD page.
  			 * There is no way of knowing exactly which PMDs may
@@ -1544,22 +1538,31 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
  			 */
  			flush_cache_range(vma, range.start, range.end);
  
-			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);
-
+			if (!folio_test_anon(folio)) {
  				/*
-				 * 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.
+				 * To call huge_pmd_unshare, i_mmap_rwsem must be
+				 * held in write mode.  Caller needs to explicitly
+				 * do this outside rmap routines.
  				 */
-				page_vma_mapped_walk_done(&pvmw);
-				break;
+				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;
+				}
  			}
  		} else {
  			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
@@ -1885,13 +1888,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
  		anon_exclusive = folio_test_anon(folio) &&
  				 PageAnonExclusive(subpage);
  
-		if (folio_test_hugetlb(folio) && !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(!(flags & TTU_RMAP_LOCKED));
+		if (folio_test_hugetlb(folio)) {
  			/*
  			 * huge_pmd_unshare may unmap an entire PMD page.
  			 * There is no way of knowing exactly which PMDs may
@@ -1901,22 +1898,31 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
  			 */
  			flush_cache_range(vma, range.start, range.end);
  
-			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);
-
+			if (!folio_test_anon(folio)) {
  				/*
-				 * 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.
+				 * To call huge_pmd_unshare, i_mmap_rwsem must be
+				 * held in write mode.  Caller needs to explicitly
+				 * do this outside rmap routines.
  				 */
-				page_vma_mapped_walk_done(&pvmw);
-				break;
+				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;
+				}
  			}
  		} else {
  			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));

Thanks,
The code looks fine.  It is unfortunate that we need so many levels of
indenting and exceed 80 columns.  But, that is OK.

I'll do a cleanup to make it more readable with factoring the hugetlb 
case into a new function after the fix series[1].

[1] 
https://lore.kernel.org/linux-arm-kernel/20220503120343.6264e126@thinkpad/T/


Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

I see you have a followup series to address the call to ptep_clear_flush()
for hugetlb pages not unmapped via huge_pmd_share and will take a look at
that soon.

Thanks.







