[RFC PATCH v2 2/9] mm: pgtable: Add sysctl to enable COW PTE

[Chih-En Lin <shiyn.lin@xxxxxxxxx>]

Add a new sysctl vm.cow_pte to set MMF_COW_PTE_READY flag for enabling
copy-on-write (COW) to the PTE page table during the next time of fork.

Since it has a time gap between using the sysctl to enable the COW PTE
and doing the fork, we use two states to determine the task that wants
to do COW PTE or already doing it.

Signed-off-by: Chih-En Lin <shiyn.lin@xxxxxxxxx>
---
 include/linux/pgtable.h |  6 ++++++
 kernel/fork.c           |  5 +++++
 kernel/sysctl.c         |  8 ++++++++
 mm/Makefile             |  2 +-
 mm/cow_pte.c            | 39 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 59 insertions(+), 1 deletion(-)
 create mode 100644 mm/cow_pte.c

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 014ee8f0fbaab..d03d01aefe989 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -937,6 +937,12 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
 	__ptep_modify_prot_commit(vma, addr, ptep, pte);
 }
 #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
+
+int cow_pte_handler(struct ctl_table *table, int write, void *buffer,
+		    size_t *lenp, loff_t *ppos);
+
+extern int sysctl_cow_pte_pid;
+
 #endif /* CONFIG_MMU */
 
 /*
diff --git a/kernel/fork.c b/kernel/fork.c
index 8a9e92068b150..6981944a7c6ec 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2671,6 +2671,11 @@ pid_t kernel_clone(struct kernel_clone_args *args)
 			trace = 0;
 	}
 
+	if (current->mm && test_bit(MMF_COW_PTE_READY, &current->mm->flags)) {
+		clear_bit(MMF_COW_PTE_READY, &current->mm->flags);
+		set_bit(MMF_COW_PTE, &current->mm->flags);
+	}
+
 	p = copy_process(NULL, trace, NUMA_NO_NODE, args);
 	add_latent_entropy();
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 205d605cacc5b..c4f54412ae3a9 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2360,6 +2360,14 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= mmap_min_addr_handler,
 	},
+	{
+		.procname	= "cow_pte",
+		.data		= &sysctl_cow_pte_pid,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= cow_pte_handler,
+		.extra1		= SYSCTL_ZERO,
+	},
 #endif
 #ifdef CONFIG_NUMA
 	{
diff --git a/mm/Makefile b/mm/Makefile
index 9a564f8364035..7a568d5066ee6 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -40,7 +40,7 @@ mmu-y			:= nommu.o
 mmu-$(CONFIG_MMU)	:= highmem.o memory.o mincore.o \
 			   mlock.o mmap.o mmu_gather.o mprotect.o mremap.o \
 			   msync.o page_vma_mapped.o pagewalk.o \
-			   pgtable-generic.o rmap.o vmalloc.o
+			   pgtable-generic.o rmap.o vmalloc.o cow_pte.o
 
 
 ifdef CONFIG_CROSS_MEMORY_ATTACH
diff --git a/mm/cow_pte.c b/mm/cow_pte.c
new file mode 100644
index 0000000000000..4e50aa4294ce7
--- /dev/null
+++ b/mm/cow_pte.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/sysctl.h>
+#include <linux/pgtable.h>
+#include <linux/sched.h>
+#include <linux/sched/coredump.h>
+#include <linux/pid.h>
+
+/* sysctl will write to this variable */
+int sysctl_cow_pte_pid = -1;
+
+static void set_cow_pte_task(void)
+{
+	struct pid *pid;
+	struct task_struct *task;
+
+	pid = find_get_pid(sysctl_cow_pte_pid);
+	if (!pid) {
+		pr_info("pid %d does not exist\n", sysctl_cow_pte_pid);
+		sysctl_cow_pte_pid = -1;
+		return;
+	}
+	task = get_pid_task(pid, PIDTYPE_PID);
+	if (!test_bit(MMF_COW_PTE, &task->mm->flags))
+		set_bit(MMF_COW_PTE_READY, &task->mm->flags);
+	sysctl_cow_pte_pid = -1;
+}
+
+int cow_pte_handler(struct ctl_table *table, int write, void *buffer,
+		    size_t *lenp, loff_t *ppos)
+{
+	int ret;
+
+	ret = proc_dointvec(table, write, buffer, lenp, ppos);
+
+	if (write && !ret)
+		set_cow_pte_task();
+
+	return ret;
+}
-- 
2.37.3