Re: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

[Andy Lutomirski]

On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> The I/O bitmap is duplicated on fork. That's wasting memory and slows down
> fork. There is no point to do so. As long as the bitmap is not modified it
> can be shared between threads and processes.
>
> Add a refcount and just share it on fork. If a task modifies the bitmap
> then it has to do the duplication if and only if it is shared.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
> V2: New patch
> ---
>  arch/x86/include/asm/iobitmap.h |    5 +++++
>  arch/x86/kernel/ioport.c        |   38 ++++++++++++++++++++++++++++++++------
>  arch/x86/kernel/process.c       |   39 ++++++---------------------------------
>  3 files changed, 43 insertions(+), 39 deletions(-)
>
> --- a/arch/x86/include/asm/iobitmap.h
> +++ b/arch/x86/include/asm/iobitmap.h
> @@ -2,10 +2,12 @@
>  #ifndef _ASM_X86_IOBITMAP_H
>  #define _ASM_X86_IOBITMAP_H
>
> +#include <linux/refcount.h>
>  #include <asm/processor.h>
>
>  struct io_bitmap {
>         u64                     sequence;
> +       refcount_t              refcnt;
>         unsigned int            io_bitmap_max;
>         union {
>                 unsigned long   bits[IO_BITMAP_LONGS];
> @@ -13,6 +15,9 @@ struct io_bitmap {
>         };
>  };
>
> +struct task_struct;
> +
> +void io_bitmap_share(struct task_struct *tsk);
>  void io_bitmap_exit(void);
>
>  void tss_update_io_bitmap(void);
> --- a/arch/x86/kernel/ioport.c
> +++ b/arch/x86/kernel/ioport.c
> @@ -16,6 +16,17 @@
>
>  static atomic64_t io_bitmap_sequence;
>
> +void io_bitmap_share(struct task_struct *tsk)
> + {
> +       /*
> +        * Take a refcount on current's bitmap. It can be used by
> +        * both tasks as long as none of them changes the bitmap.
> +        */
> +       refcount_inc(&current->thread.io_bitmap->refcnt);
> +       tsk->thread.io_bitmap = current->thread.io_bitmap;
> +       set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
> +}
> +
>  void io_bitmap_exit(void)
>  {
>         struct io_bitmap *iobm = current->thread.io_bitmap;
> @@ -25,7 +36,8 @@ void io_bitmap_exit(void)
>         clear_thread_flag(TIF_IO_BITMAP);
>         tss_update_io_bitmap();
>         preempt_enable();
> -       kfree(iobm);
> +       if (iobm && refcount_dec_and_test(&iobm->refcnt))
> +               kfree(iobm);
>  }
>
>  /*
> @@ -59,8 +71,26 @@ long ksys_ioperm(unsigned long from, uns
>                         return -ENOMEM;
>
>                 memset(iobm->bits, 0xff, sizeof(iobm->bits));
> +               refcount_set(&iobm->refcnt, 1);
> +       }
> +
> +       /*
> +        * If the bitmap is not shared, then nothing can take a refcount as
> +        * current can obviously not fork at the same time. If it's shared
> +        * duplicate it and drop the refcount on the original one.
> +        */
> +       if (refcount_read(&iobm->refcnt) > 1) {
> +               iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
> +               if (!iobm)
> +                       return -ENOMEM;
> +               io_bitmap_exit();

And change the refcount to 1?

>         }
>

Otherwise:

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

(I'm about to send, and I see PeterZ beat me to the punch.  You can
still have the ack, though.)