[PATCH v6 6/7] mm: Introduce deferred freeing for kernel page tables

[Lu Baolu <baolu.lu@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

This introduces a conditional asynchronous mechanism, enabled by
CONFIG_ASYNC_KERNEL_PGTABLE_FREE. When enabled, this mechanism defers the
freeing of pages that are used as page tables for kernel address mappings.
These pages are now queued to a work struct instead of being freed
immediately.

This deferred freeing allows for batch-freeing of page tables, providing
a safe context for performing a single expensive operation (TLB flush)
for a batch of kernel page tables instead of performing that expensive
operation for each page table.

On x86, CONFIG_ASYNC_KERNEL_PGTABLE_FREE is selected if CONFIG_IOMMU_SVA
is enabled, because both Intel and AMD IOMMU architectures could
potentially cache kernel page table entries in their paging structure
cache, regardless of the permission.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
---
 arch/x86/Kconfig     |  1 +
 mm/Kconfig           |  3 +++
 include/linux/mm.h   | 16 +++++++++++++---
 mm/pgtable-generic.c | 37 +++++++++++++++++++++++++++++++++++++
 4 files changed, 54 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index fa3b616af03a..ded29ee848fd 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -279,6 +279,7 @@ config X86
 	select HAVE_PCI
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select ASYNC_KERNEL_PGTABLE_FREE 	if IOMMU_SVA
 	select MMU_GATHER_RCU_TABLE_FREE
 	select MMU_GATHER_MERGE_VMAS
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
diff --git a/mm/Kconfig b/mm/Kconfig
index 0e26f4fc8717..a83df9934acd 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -908,6 +908,9 @@ config PAGE_MAPCOUNT
 config PGTABLE_HAS_HUGE_LEAVES
 	def_bool TRANSPARENT_HUGEPAGE || HUGETLB_PAGE
 
+config ASYNC_KERNEL_PGTABLE_FREE
+	def_bool n
+
 # TODO: Allow to be enabled without THP
 config ARCH_SUPPORTS_HUGE_PFNMAP
 	def_bool n
diff --git a/include/linux/mm.h b/include/linux/mm.h
index bb235a9f991e..fe5515725c46 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3031,6 +3031,14 @@ static inline void __pagetable_free(struct ptdesc *pt)
 	__free_pages(page, compound_order(page));
 }
 
+#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE
+void pagetable_free_kernel(struct ptdesc *pt);
+#else
+static inline void pagetable_free_kernel(struct ptdesc *pt)
+{
+	__pagetable_free(pt);
+}
+#endif
 /**
  * pagetable_free - Free pagetables
  * @pt:	The page table descriptor
@@ -3040,10 +3048,12 @@ static inline void __pagetable_free(struct ptdesc *pt)
  */
 static inline void pagetable_free(struct ptdesc *pt)
 {
-	if (ptdesc_test_kernel(pt))
+	if (ptdesc_test_kernel(pt)) {
 		ptdesc_clear_kernel(pt);
-
-	__pagetable_free(pt);
+		pagetable_free_kernel(pt);
+	} else {
+		__pagetable_free(pt);
+	}
 }
 
 #if defined(CONFIG_SPLIT_PTE_PTLOCKS)
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 567e2d084071..1c7caa8ef164 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -406,3 +406,40 @@ pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
 	pte_unmap_unlock(pte, ptl);
 	goto again;
 }
+
+#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE
+static void kernel_pgtable_work_func(struct work_struct *work);
+
+static struct {
+	struct list_head list;
+	/* protect above ptdesc lists */
+	spinlock_t lock;
+	struct work_struct work;
+} kernel_pgtable_work = {
+	.list = LIST_HEAD_INIT(kernel_pgtable_work.list),
+	.lock = __SPIN_LOCK_UNLOCKED(kernel_pgtable_work.lock),
+	.work = __WORK_INITIALIZER(kernel_pgtable_work.work, kernel_pgtable_work_func),
+};
+
+static void kernel_pgtable_work_func(struct work_struct *work)
+{
+	struct ptdesc *pt, *next;
+	LIST_HEAD(page_list);
+
+	spin_lock(&kernel_pgtable_work.lock);
+	list_splice_tail_init(&kernel_pgtable_work.list, &page_list);
+	spin_unlock(&kernel_pgtable_work.lock);
+
+	list_for_each_entry_safe(pt, next, &page_list, pt_list)
+		__pagetable_free(pt);
+}
+
+void pagetable_free_kernel(struct ptdesc *pt)
+{
+	spin_lock(&kernel_pgtable_work.lock);
+	list_add(&pt->pt_list, &kernel_pgtable_work.list);
+	spin_unlock(&kernel_pgtable_work.lock);
+
+	schedule_work(&kernel_pgtable_work.work);
+}
+#endif
-- 
2.43.0