mm: introduce deferred freeing for kernel page tables

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: Lu Baolu <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Mike Rapoport (Microsoft) <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Betkov <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jean-Philippe Brucker <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Robin Murohy <[email protected]>
Cc: Thomas Gleinxer <[email protected]>
Cc: "Uladzislau Rezki (Sony)" <[email protected]>
Cc: Vasant Hegde <[email protected]>
Cc: Vinicius Costa Gomes <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Yi Lai <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

include/linux/mm.h

@@ -3053,6 +3053,14 @@ 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
@@ -3062,10 +3070,12 @@ 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)

mm/Kconfig

@@ -906,6 +906,9 @@ config HAVE_GIGANTIC_FOLIOS
 	def_bool (HUGETLB_PAGE && ARCH_HAS_GIGANTIC_PAGE) || \
 		 (ZONE_DEVICE && HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
 
+config ASYNC_KERNEL_PGTABLE_FREE
+	def_bool n
+
 # TODO: Allow to be enabled without THP
 config ARCH_SUPPORTS_HUGE_PFNMAP
 	def_bool n

mm/pgtable-generic.c

@@ -406,3 +406,40 @@ again:
 	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