linux-kernel - Re: [PATCH v6] ANDROID: binder: change down_write to down

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Date:   Tue, 15 May 2018 17:56:10 +0900
From:   Minchan Kim <minchan.kernel@...il.com>
To:     Martijn Coenen <maco@...roid.com>
Cc:     Minchan Kim <minchan.kernel@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Joe Perches <joe@...ches.com>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v6] ANDROID: binder: change down_write to down_read

On Tue, May 15, 2018 at 09:46:01AM +0200, Martijn Coenen wrote:

< snip >

> >> About the unmap at runtime part, your commit message was a bit confusing. You
> >> said "every binder buffers should be mapped in advance by binder_mmap." but I
> >> think the new binder shrinker mechanism doesn't make that true anymore.
> >
> > It's good point. I didn't know know that.
> > When I see binder_vm_fault, it emits SIGBUS. That means shrinker cannot zap pages
> > process is using, I think. IOW, every pages for binder are mapped at mmap time
> > and is never mapped in runtime by page fault. Right?
> 
> Right - the address range is allocated once, and an initial amount of
> pages is mapped into it. For every transaction into that process, we
> will see if there's enough pages, and if not allocate so that we have
> enough of them - so this is not done by page fault. The shrinker won't
> touch pages for which a transaction is in progress. Of course a
> process itself could still try to read from an unallocated address,
> but in that case returning SIGBUS and having that process crash seems
> fine.

Thanks for the confirmation.

> 
> I'm also not sure the read lock is needed, but I would need to read a
> whole lot more code to convince myself it's not.

For page zapping, we shouldn't need mmap_sem write lock.
We should replace it with down_read/write, too.

Thanks.