linux-kernel - [PATCH 2/2] mm: rmap: Move the cache flushing to the correct place for hugetlb PMD sharing

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Date:   Sun, 24 Apr 2022 22:50:14 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     akpm@...ux-foundation.org, mike.kravetz@...cle.com
Cc:     almasrymina@...gle.com, songmuchun@...edance.com,
        baolin.wang@...ux.alibaba.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] mm: rmap: Move the cache flushing to the correct place for hugetlb PMD sharing

The cache level flush will always be first when changing an existing
virtual–>physical mapping to a new value, since this allows us to
properly handle systems whose caches are strict and require a
virtual–>physical translation to exist for a virtual address. So we
should move the cache flushing before huge_pmd_unshare().

As Muchun pointed out[1], now the architectures whose supporting hugetlb
PMD sharing have no cache flush issues in practice. But I think we
should still follow the cache/TLB flushing rules when changing a valid
virtual address mapping in case of potential issues in future.

[1] https://lore.kernel.org/all/YmT%2F%2FhuUbFX+KHcy@FVFYT0MHHV2J.usts.net/
Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
---
 mm/rmap.c | 40 ++++++++++++++++++++++------------------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 61e63db..81872bb 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1535,15 +1535,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 * do this outside rmap routines.
 			 */
 			VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
+			/*
+			 * huge_pmd_unshare unmapped an entire PMD page.
+			 * There is no way of knowing exactly which PMDs may
+			 * be cached for this mm, so we must flush them all.
+			 * start/end were already adjusted above to cover this
+			 * range.
+			 */
+			flush_cache_range(vma, range.start, range.end);
+
 			if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
-				/*
-				 * huge_pmd_unshare unmapped an entire PMD
-				 * page.  There is no way of knowing exactly
-				 * which PMDs may be cached for this mm, so
-				 * we must flush them all.  start/end were
-				 * already adjusted above to cover this range.
-				 */
-				flush_cache_range(vma, range.start, range.end);
 				flush_tlb_range(vma, range.start, range.end);
 				mmu_notifier_invalidate_range(mm, range.start,
 							      range.end);
@@ -1560,13 +1561,14 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				page_vma_mapped_walk_done(&pvmw);
 				break;
 			}
+		} else {
+			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
 		}
 
 		/*
 		 * Nuke the page table entry. When having to clear
 		 * PageAnonExclusive(), we always have to flush.
 		 */
-		flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
 		if (should_defer_flush(mm, flags) && !anon_exclusive) {
 			/*
 			 * We clear the PTE but do not flush so potentially
@@ -1890,15 +1892,16 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			 * do this outside rmap routines.
 			 */
 			VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
+			/*
+			 * huge_pmd_unshare unmapped an entire PMD page.
+			 * There is no way of knowing exactly which PMDs may
+			 * be cached for this mm, so we must flush them all.
+			 * start/end were already adjusted above to cover this
+			 * range.
+			 */
+			flush_cache_range(vma, range.start, range.end);
+
 			if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
-				/*
-				 * huge_pmd_unshare unmapped an entire PMD
-				 * page.  There is no way of knowing exactly
-				 * which PMDs may be cached for this mm, so
-				 * we must flush them all.  start/end were
-				 * already adjusted above to cover this range.
-				 */
-				flush_cache_range(vma, range.start, range.end);
 				flush_tlb_range(vma, range.start, range.end);
 				mmu_notifier_invalidate_range(mm, range.start,
 							      range.end);
@@ -1915,10 +1918,11 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				page_vma_mapped_walk_done(&pvmw);
 				break;
 			}
+		} else {
+			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
 		}
 
 		/* Nuke the page table entry. */
-		flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
 		pteval = ptep_clear_flush(vma, address, pvmw.pte);
 
 		/* Set the dirty flag on the folio now the pte is gone. */
-- 
1.8.3.1