Re: [RFC PATCH 2/6] mm: clone3: Add CLONE_COW_PGTABLE flag

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Le 19/05/2022 à 20:31, Chih-En Lin a écrit :
> Add CLONE_COW_PGTABLE flag to support clone3() system call to enable the
> Copy-On-Write (COW) mechanism on the page table.

Is that really something we want the user to decide ? Isn't it an 
internal stuff that should be transparent for users ?

As far as I know, there is no way today to decide whether you want COW 
or not for main memory. Why should there be a choice for the COW of page 
tables ?


> 
> Signed-off-by: Chih-En Lin <shiyn.lin@xxxxxxxxx>
> ---
>   include/uapi/linux/sched.h | 1 +
>   kernel/fork.c              | 6 +++++-
>   2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 3bac0a8ceab2..3b92ff589e0f 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -36,6 +36,7 @@
>   /* Flags for the clone3() syscall. */
>   #define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
>   #define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */
> +#define CLONE_COW_PGTABLE 0x400000000ULL /* Copy-On-Write for page table */
> 
>   /*
>    * cloning flags intersect with CSIGNAL so can be used with unshare and clone3
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 35a3beff140b..08cf95201333 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2636,6 +2636,9 @@ pid_t kernel_clone(struct kernel_clone_args *args)
>                          trace = 0;
>          }
> 
> +       if (clone_flags & CLONE_COW_PGTABLE)
> +               set_bit(MMF_COW_PGTABLE, &current->mm->flags);
> +
>          p = copy_process(NULL, trace, NUMA_NO_NODE, args);
>          add_latent_entropy();
> 
> @@ -2860,7 +2863,8 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs)
>   {
>          /* Verify that no unknown flags are passed along. */
>          if (kargs->flags &
> -           ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP))
> +           ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP |
> +                   CLONE_COW_PGTABLE))
>                  return false;
> 
>          /*
> --
> 2.36.1
>