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

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Date:   Thu, 3 Jan 2019 21:34:19 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Todd Kjos <tkjos@...gle.com>
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 Thu, Jan 03, 2019 at 12:25:24PM -0800, Todd Kjos wrote:
> 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?

Are you asking asking why this is 4 and not 3? In that case we should
probably parse CONFIG_ANDROID_BINDER_DEVICES once at init time and then
reserve that many binder devices. Thoughts?

Christian