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

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Date:   Wed, 9 May 2018 15:40:23 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     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>,
        Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v6] ANDROID: binder: change down_write to down_read

On Tue, May 08, 2018 at 04:08:13PM -0700, Joel Fernandes wrote:
> On Tue, May 08, 2018 at 07:51:01PM +0900, Minchan Kim wrote:
> > On Mon, May 07, 2018 at 10:28:29AM -0700, Joel Fernandes wrote:
> > > On Mon, May 07, 2018 at 11:15:37PM +0900, Minchan Kim wrote:
> > > > binder_update_page_range needs down_write of mmap_sem because
> > > > vm_insert_page need to change vma->vm_flags to VM_MIXEDMAP unless
> > > > it is set. However, when I profile binder working, it seems
> > > > every binder buffers should be mapped in advance by binder_mmap.
> > > > It means we could set VM_MIXEDMAP in binder_mmap time which is
> > > > already hold a mmap_sem as down_write so binder_update_page_range
> > > > doesn't need to hold a mmap_sem as down_write.
> > > > Please use proper API down_read. It would help mmap_sem contention
> > > > problem as well as fixing down_write abuse.
> > > > 
> > > > Ganesh Mahendran tested app launching and binder throughput test
> > > > and he said he couldn't find any problem and I did binder latency
> > > > test per Greg KH request(Thanks Martijn to teach me how I can do)
> > > > I cannot find any problem, too.
> > > > 
> > > > Cc: Ganesh Mahendran <opensource.ganesh@...il.com>
> > > > Cc: Joe Perches <joe@...ches.com>
> > > > Cc: Arve Hjønnevåg <arve@...roid.com>
> > > > Cc: Todd Kjos <tkjos@...gle.com>
> > > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > Reviewed-by: Martijn Coenen <maco@...roid.com>
> > > > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > > > ---
> > > >  drivers/android/binder.c       | 4 +++-
> > > >  drivers/android/binder_alloc.c | 6 +++---
> > > >  2 files changed, 6 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > > > index 4eab5be3d00f..7b8e96f60719 100644
> > > > --- a/drivers/android/binder.c
> > > > +++ b/drivers/android/binder.c
> > > > @@ -4730,7 +4730,9 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
> > > >  		failure_string = "bad vm_flags";
> > > >  		goto err_bad_arg;
> > > >  	}
> > > > -	vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE;
> > > > +	vma->vm_flags |= VM_DONTCOPY | VM_MIXEDMAP;
> > > > +	vma->vm_flags &= ~VM_MAYWRITE;
> > > > +
> > > >  	vma->vm_ops = &binder_vm_ops;
> > > >  	vma->vm_private_data = proc;
> > > >  
> > > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > > > index 5a426c877dfb..4f382d51def1 100644
> > > > --- a/drivers/android/binder_alloc.c
> > > > +++ b/drivers/android/binder_alloc.c
> > > > @@ -219,7 +219,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
> > > >  		mm = alloc->vma_vm_mm;
> > > >  
> > > >  	if (mm) {
> > > > -		down_write(&mm->mmap_sem);
> > > > +		down_read(&mm->mmap_sem);
> > > 
> > > 
> > > Nice. Is there a need to hold the reader-lock at all here? Just curious what
> > > else is it protecting (here or in vm_insert_page).
> > 
> > It should protect vm_area_struct. IOW, when we try insert page into virtual address area,
> > vma shouldn't be changed(ie, unmap/collapse/split).
> 
> When you say unmap, are you talking about pages being unmapped from the VMA
> or something else?

I mean to destroy vm_area_struct itself as well as unmap pages.

> 
> For the collapse/split part, the binder VMA (vm_area_struct) itself isn't
> changed after the initial mmap (AFAIK) so I don't see a need for protection
> there.

There is no way to unmap in runtime? What happens if some buggy applications
do unmap by mistake?
Cannot we access those B's vma from A context?
If B process is exiting, the VMA would be gone while A is accessing.

> 
> Again, I'm not against the patch but thought its good to clarify for myself
> what the usage of the lock is, here.

I know. :)