[PATCH] ANDROID: binder: change down_write to down_read

[PATCH] ANDROID: binder: change down_write to down_read

[Minchan Kim]

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));
+
 	for (page_addr = start; page_addr < end; page_addr += PAGE_SIZE) {
 		int ret;
 		bool on_lru;
@@ -288,7 +290,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 		/* vm_insert_page does not seem to increment the refcount */
 	}
 	if (mm) {
-		up_write(&mm->mmap_sem);
+		up_read(&mm->mmap_sem);
 		mmput(mm);
 	}
 	return 0;
@@ -321,7 +323,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 	}
 err_no_vma:
 	if (mm) {
-		up_write(&mm->mmap_sem);
+		up_read(&mm->mmap_sem);
 		mmput(mm);
 	}
 	return vma ? -ENOMEM : -ESRCH;
-- 
2.17.0.rc1.321.gba9d0f2565-goog

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

[Greg Kroah-Hartman]

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?

thanks,

greg k-h

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

[Minchan Kim]

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.

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

[Martijn Coenen]

On Wed, Mar 28, 2018 at 4:42 AM, Minchan Kim <minchan@kernel.org> 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.

Yeah this is correct - before doing any binder transactions
binder_mmap() must be called, and we do fail transactions if that
hasn't been done yet. LGTM once you're removed the WARN_ON.

Martijn

> 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));
> +
>         for (page_addr = start; page_addr < end; page_addr += PAGE_SIZE) {
>                 int ret;
>                 bool on_lru;
> @@ -288,7 +290,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
>                 /* vm_insert_page does not seem to increment the refcount */
>         }
>         if (mm) {
> -               up_write(&mm->mmap_sem);
> +               up_read(&mm->mmap_sem);
>                 mmput(mm);
>         }
>         return 0;
> @@ -321,7 +323,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
>         }
>  err_no_vma:
>         if (mm) {
> -               up_write(&mm->mmap_sem);
> +               up_read(&mm->mmap_sem);
>                 mmput(mm);
>         }
>         return vma ? -ENOMEM : -ESRCH;
> --
> 2.17.0.rc1.321.gba9d0f2565-goog
>