Messages in this thread | | | Date | Wed, 28 Mar 2018 15:47:08 +0900 | From | Minchan Kim <> | Subject | Re: [PATCH] ANDROID: binder: change down_write to down_read |
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On Wed, Mar 28, 2018 at 07:53:54AM +0200, Greg Kroah-Hartman wrote: > On Wed, Mar 28, 2018 at 11:42:31AM +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 bider_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. > > > > Android suffers from mmap_sem contention so let's reduce mmap_sem > > down_write. > > > > Cc: Martijn Coenen <maco@android.com> > > Cc: Todd Kjos <tkjos@google.com> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > > Signed-off-by: Minchan Kim <minchan@kernel.org> > > --- > > drivers/android/binder.c | 2 +- > > drivers/android/binder_alloc.c | 8 +++++--- > > 2 files changed, 6 insertions(+), 4 deletions(-) > > > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c > > index 764b63a5aade..9a14c6dd60c4 100644 > > --- a/drivers/android/binder.c > > +++ b/drivers/android/binder.c > > @@ -4722,7 +4722,7 @@ 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) & ~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..a184bf12eb15 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); > > vma = alloc->vma; > > } > > > > @@ -229,6 +229,8 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate, > > goto err_no_vma; > > } > > > > + WARN_ON_ONCE(vma && !(vma->vm_flags & VM_MIXEDMAP)); > > What can we do with this warning? What about systems that run in 'panic > on warn' mode? Is this something we should handle directly instead?
Hmm, it seems we don't need above warning there because vm_insert_page will hit BUG_ON in that case. I will remove it at respin. Thanks for the review, Greg.
I will wait other binder maintainer's opinion about below question.
> > it seems > > every binder buffers should be mapped in advance by binder_mmap.
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