linux-kernel - Re: [PATCH] binder: reduce mmap

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Date:   Mon, 18 Feb 2019 19:47:25 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Todd Kjos <tkjos@...gle.com>,
        Martijn Coenen <maco@...roid.com>,
        Arve Hjønnevåg <arve@...roid.com>
Subject: Re: [PATCH] binder: reduce mmap_sem write-side lock

On Mon, Feb 18, 2019 at 09:32:08AM +0100, Greg KH wrote:
> On Mon, Feb 18, 2019 at 05:11:45PM +0900, Minchan Kim wrote:
> > binder has used write-side mmap_sem semaphore to release memory
> > mapped at address space of the process. However, right lock to
> > release pages is down_read, not down_write because page table lock
> > already protects the race for parallel freeing.
> > 
> > Please do not use mmap_sem write-side lock which is well known
> > contented lock.
> > 
> > Cc: Todd Kjos <tkjos@...gle.com>
> > Cc: Martijn Coenen <maco@...roid.com>
> > Cc: Arve Hjønnevåg <arve@...roid.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> >  drivers/android/binder_alloc.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Is this just needed for the recent binder changes that are in
> linux-next, or for older kernels as well?

It has been there for several years but no need to fix older kernels
because down_write of mmap_sem is technically not a bug to stop the
working of binder. Rather than, it's just abuse of the lock so
it's okay to fix only recent kernel.

Thanks.