Re: [PATCH v6] ANDROID: binder: change down_write to down_read

[Minchan Kim]

Hi Joel,

Sorry for the late response. I was off.

On Wed, May 09, 2018 at 04:19:41PM -0700, Joel Fernandes wrote:
> On Wed, May 09, 2018 at 03:40:23PM +0900, Minchan Kim wrote:
> > 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@xxxxxxxxx>
> > > > > > Cc: Joe Perches <joe@xxxxxxxxxxx>
> > > > > > Cc: Arve Hjønnevåg <arve@xxxxxxxxxxx>
> > > > > > Cc: Todd Kjos <tkjos@xxxxxxxxxx>
> > > > > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > > > Reviewed-by: Martijn Coenen <maco@xxxxxxxxxxx>
> > > > > > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > > > > > ---
> > > > > >  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.
> 
> If the VMA is gone, then why is it a good idea to map pages into it anyway?

Hmm, sorry about that. I couldn't understand your point.

> 
> 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?

Thanks.

> 
> The unmap by mistake point is a valid one I guess.
> 
> thanks,
> 
>  - Joel
>