linux-kernel - [PATCH 1/2] mm: hugetlb: Considering PMD sharing when flushing cache/TLBs

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Message-Id: <ad5ad7739a0e2d1d2db9f17f0d672313ae63bad6.1650810915.git.baolin.wang@linux.alibaba.com>
Date:   Sun, 24 Apr 2022 22:50:13 +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 1/2] mm: hugetlb: Considering PMD sharing when flushing cache/TLBs

When moving hugetlb page tables, the cache flushing is called in
move_page_tables() without considering the shared PMDs, which may
be cause cache issues on some architectures.

Thus we should move the hugetlb cache flushing into
move_hugetlb_page_tables() with considering the shared PMDs ranges,
calculated by adjust_range_if_pmd_sharing_possible(). Meanwhile also
expanding the TLBs flushing range in case of shared PMDs.

Note this is discovered via code inspection, and did not meet a real
problem in practice so far.

Fixes: 550a7d60bd5e ("mm, hugepages: add mremap() support for hugepage backed vma")
Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
---
 mm/hugetlb.c | 17 +++++++++++++++--
 mm/mremap.c  |  2 +-
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1945dfb..d3a6094 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4937,10 +4937,17 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
 	unsigned long old_addr_copy;
 	pte_t *src_pte, *dst_pte;
 	struct mmu_notifier_range range;
+	bool shared_pmd = false;
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, old_addr,
 				old_end);
 	adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
+	/*
+	 * In case of shared PMDs, we should cover the maximum possible
+	 * range.
+	 */
+	flush_cache_range(vma, range.start, range.end);
+
 	mmu_notifier_invalidate_range_start(&range);
 	/* Prevent race with file truncation */
 	i_mmap_lock_write(mapping);
@@ -4957,8 +4964,10 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
 		 */
 		old_addr_copy = old_addr;
 
-		if (huge_pmd_unshare(mm, vma, &old_addr_copy, src_pte))
+		if (huge_pmd_unshare(mm, vma, &old_addr_copy, src_pte)) {
+			shared_pmd = true;
 			continue;
+		}
 
 		dst_pte = huge_pte_alloc(mm, new_vma, new_addr, sz);
 		if (!dst_pte)
@@ -4966,7 +4975,11 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
 
 		move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte);
 	}
-	flush_tlb_range(vma, old_end - len, old_end);
+
+	if (shared_pmd)
+		flush_tlb_range(vma, range.start, range.end);
+	else
+		flush_tlb_range(vma, old_end - len, old_end);
 	mmu_notifier_invalidate_range_end(&range);
 	i_mmap_unlock_write(mapping);
 
diff --git a/mm/mremap.c b/mm/mremap.c
index 98f50e6..0970025 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -490,12 +490,12 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 		return 0;
 
 	old_end = old_addr + len;
-	flush_cache_range(vma, old_addr, old_end);
 
 	if (is_vm_hugetlb_page(vma))
 		return move_hugetlb_page_tables(vma, new_vma, old_addr,
 						new_addr, len);
 
+	flush_cache_range(vma, old_addr, old_end);
 	mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
 				old_addr, old_end);
 	mmu_notifier_invalidate_range_start(&range);
-- 
1.8.3.1