[PATCH v4 01/14] mm: Allow user to control COW PTE via prctl

Chih-En Lin
2023-02-07T03:52:48Z

Add a new prctl, PR_SET_COW_PTE, to allow the user to enable COW PTE.
Since it has a time gap between using the prctl to enable the COW PTE
and doing the fork, we use two states (MMF_COW_PTE_READY and MMF_COW_PTE)
to determine the task that wants to do COW PTE or already doing it.

The MMF_COW_PTE_READY flag marks the task to do COW PTE in the next time
of fork(). During fork(), if MMF_COW_PTE_READY set, fork() will unset the
flag and set the MMF_COW_PTE flag. After that, fork() might shares PTEs
instead of duplicates it.

Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
---
include/linux/sched/coredump.h | 12 +++++++++++-
include/uapi/linux/prctl.h | 6 ++++++
kernel/sys.c | 11 +++++++++++
3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 8270ad7ae14c..570d599ebc85 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -83,7 +83,17 @@ static inline int get_dumpable(struct mm_struct *mm)
#define MMF_HAS_PINNED 27 /* FOLL_PIN has run, never cleared */
#define MMF_DISABLE_THP_MASK (1 << MMF_DISABLE_THP)

+/*
+ * MMF_COW_PTE_READY: Marking the task to do COW PTE in the next time of
+ * fork(). During fork(), if MMF_COW_PTE_READY set, fork() will unset the
+ * flag and set the MMF_COW_PTE flag. After that, fork() might shares PTEs
+ * rather than duplicates it.
+ */
+#define MMF_COW_PTE_READY 29 /* Share PTE tables in next time of fork() */
+#define MMF_COW_PTE 30 /* PTE tables are shared between processes */
+#define MMF_COW_PTE_MASK (1 << MMF_COW_PTE)
+
#define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
- MMF_DISABLE_THP_MASK)
+ MMF_DISABLE_THP_MASK | MMF_COW_PTE_MASK)

#endif /* _LINUX_SCHED_COREDUMP_H */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index a5e06dcbba13..664a3c023019 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -284,4 +284,10 @@ struct prctl_mm_map {
#define PR_SET_VMA 0x53564d41
# define PR_SET_VMA_ANON_NAME 0

+/*
+ * Set the prepare flag, MMF_COW_PTE_READY, to do the share (copy-on-write)
+ * page table in the next time of fork.
+ */
+#define PR_SET_COW_PTE 65
+
#endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 88b31f096fb2..eeab3093026f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2350,6 +2350,14 @@ static int prctl_set_vma(unsigned long opt, unsigned long start,
}
#endif /* CONFIG_ANON_VMA_NAME */

+static int prctl_set_cow_pte(struct mm_struct *mm)
+{
+ if (test_bit(MMF_COW_PTE, &mm->flags))
+ return -EINVAL;
+ set_bit(MMF_COW_PTE_READY, &mm->flags);
+ return 0;
+}
+
SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
unsigned long, arg4, unsigned long, arg5)
{
@@ -2628,6 +2636,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
case PR_SET_VMA:
error = prctl_set_vma(arg2, arg3, arg4, arg5);
break;
+ case PR_SET_COW_PTE:
+ error = prctl_set_cow_pte(me->mm);
+ break;
default:
error = -EINVAL;
break;
--
2.34.1