linux-kernel - [PATCH RFC v3 31/35] khugepaged: arm64: Don't collapse MTE enabled VMAs

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Message-Id: <20240125164256.4147-32-alexandru.elisei@arm.com>
Date: Thu, 25 Jan 2024 16:42:52 +0000
From: Alexandru Elisei <alexandru.elisei@....com>
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Subject: [PATCH RFC v3 31/35] khugepaged: arm64: Don't collapse MTE enabled VMAs

copy_user_highpage() will do memory allocation if there are saved tags for
the destination page, and the page is missing tag storage.

After commit a349d72fd9ef ("mm/pgtable: add rcu_read_lock() and
rcu_read_unlock()s"), collapse_huge_page() calls
__collapse_huge_page_copy() -> .. -> copy_user_highpage() with the RCU lock
held, which means that copy_user_highpage() can only allocate memory using
GFP_ATOMIC or equivalent.

Get around this by refusing to collapse pages into a transparent huge page
if the VMA is MTE-enabled.

Signed-off-by: Alexandru Elisei <alexandru.elisei@....com>
---

Changes since rfc v2:

* New patch. I think an agreement on whether copy*_user_highpage() should be
always allowed to sleep, or should not be allowed, would be useful.

 arch/arm64/include/asm/pgtable.h    | 3 +++
 arch/arm64/kernel/mte_tag_storage.c | 5 +++++
 include/linux/khugepaged.h          | 5 +++++
 mm/khugepaged.c                     | 4 ++++
 4 files changed, 17 insertions(+)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 87ae59436162..d0473538c926 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1120,6 +1120,9 @@ static inline bool arch_alloc_cma(gfp_t gfp_mask)
 	return true;
 }
 
+bool arch_hugepage_vma_revalidate(struct vm_area_struct *vma, unsigned long address);
+#define arch_hugepage_vma_revalidate arch_hugepage_vma_revalidate
+
 #endif /* CONFIG_ARM64_MTE_TAG_STORAGE */
 #endif /* CONFIG_ARM64_MTE */
 
diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
index ac7b9c9c585c..a99959b70573 100644
--- a/arch/arm64/kernel/mte_tag_storage.c
+++ b/arch/arm64/kernel/mte_tag_storage.c
@@ -636,3 +636,8 @@ void arch_alloc_page(struct page *page, int order, gfp_t gfp)
 	if (tag_storage_enabled() && alloc_requires_tag_storage(gfp))
 		reserve_tag_storage(page, order, gfp);
 }
+
+bool arch_hugepage_vma_revalidate(struct vm_area_struct *vma, unsigned long address)
+{
+	return !(vma->vm_flags & VM_MTE);
+}
diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index f68865e19b0b..461e4322dff2 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h
@@ -38,6 +38,11 @@ static inline void khugepaged_exit(struct mm_struct *mm)
 	if (test_bit(MMF_VM_HUGEPAGE, &mm->flags))
 		__khugepaged_exit(mm);
 }
+
+#ifndef arch_hugepage_vma_revalidate
+#define arch_hugepage_vma_revalidate(vma, address) 1
+#endif
+
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 static inline void khugepaged_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 2b219acb528e..cb9a9ddb4d86 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -935,6 +935,10 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	 */
 	if (expect_anon && (!(*vmap)->anon_vma || !vma_is_anonymous(*vmap)))
 		return SCAN_PAGE_ANON;
+
+	if (!arch_hugepage_vma_revalidate(vma, address))
+		return SCAN_VMA_CHECK;
+
 	return SCAN_SUCCEED;
 }
 
-- 
2.43.0