linux-kernel - Re: [PATCH v1 2/2] binderfs: reserve devices for initial mount

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Date:   Thu, 3 Jan 2019 12:25:24 -0800
From:   Todd Kjos <tkjos@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Todd Kjos <tkjos@...roid.com>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Martijn Coenen <maco@...roid.com>, joel@...lfernandes.org
Subject: Re: [PATCH v1 2/2] binderfs: reserve devices for initial mount

On Sun, Dec 23, 2018 at 6:36 AM Christian Brauner <christian@...uner.io> wrote:
>
> The binderfs instance in the initial ipc namespace will always have a
> reserve of 4 binder devices unless explicitly capped by specifying a lower
> value via the "max" mount option.
> This ensures when binder devices are removed (on accident or on purpose)
> they can always be recreated without risking that all minor numbers have
> already been used up.
>
> Cc: Todd Kjos <tkjos@...gle.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
> v1:
> - patch introduced
> v0:
> - patch not present
> ---
>  drivers/android/binderfs.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
> index 873adda064ac..aa635c7ea727 100644
> --- a/drivers/android/binderfs.c
> +++ b/drivers/android/binderfs.c
> @@ -40,6 +40,8 @@
>  #define INODE_OFFSET 3
>  #define INTSTRLEN 21
>  #define BINDERFS_MAX_MINOR (1U << MINORBITS)
> +/* Ensure that the initial ipc namespace always has a devices available. */
> +#define BINDERFS_MAX_MINOR_CAPPED (BINDERFS_MAX_MINOR - 4)

Why do you ever need more minors per instance than the number of
devices listed in CONFIG_ANDROID_BINDER_DEVICES?

>
>  static struct vfsmount *binderfs_mnt;
>
> @@ -129,11 +131,14 @@ static int binderfs_binder_device_create(struct inode *ref_inode,
>         struct inode *inode = NULL;
>         struct super_block *sb = ref_inode->i_sb;
>         struct binderfs_info *info = sb->s_fs_info;
> +       bool use_reserve = (info->ipc_ns == &init_ipc_ns);
>
>         /* Reserve new minor number for the new device. */
>         mutex_lock(&binderfs_minors_mutex);
>         if (++info->device_count <= info->mount_opts.max)
> -               minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR,
> +               minor = ida_alloc_max(&binderfs_minors,
> +                                     use_reserve ? BINDERFS_MAX_MINOR :
> +                                                   BINDERFS_MAX_MINOR_CAPPED,
>                                       GFP_KERNEL);
>         else
>                 minor = -ENOSPC;
> --
> 2.19.1
>