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

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Message-ID: <CAHRSSEz9pJY1gs5VuWs-R4dmWgME4ocQkVGcNvpX1TVUWpdE0w@mail.gmail.com>
Date:   Thu, 3 Jan 2019 13:47:13 -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 Thu, Jan 3, 2019 at 12:34 PM Christian Brauner <christian@...uner.io> wrote:
>
> 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?

I'm asking why CONFIG_ANDROID_BINDER_DEVICES isn't the source of truth
for the number of devices (it normally specifies 3 devices, but could
be different). I can't think of a reason why you'd want
CONFIG_MAX_MINOR_CAPPED to be different than the number of devices
indicated by CONFIG_ANDROID_BINDER_DEVICES and having it specified in
two places seems error prone.

>
> Christian