linux-kernel - Re: [PATCH v3] ANDROID: binder: change down_write to down

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Date:   Thu, 29 Mar 2018 11:42:04 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v3] ANDROID: binder: change down_write to down_read


On Thu, Mar 29, 2018 at 11:37:12AM +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: Todd Kjos <tkjos@...gle.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Arve Hjønnevåg <arve@...roid.com>
> Reviewed-by: Martijn Coenen <maco@...roid.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>

Sent wrong version. Sorry for that. Please ignore this and take it.

>From 480e992d4a650fb98e1397114d75dea7af8e6d0c Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
Date: Wed, 28 Mar 2018 11:32:42 +0900
Subject: [PATCH v3] ANDROID: binder: change down_write to down_read
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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: 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>
---

>From v2:
  * Fix vma->flag setting - Arve

>From v1:
  * remove WARN_ON_ONCE - Greg
  * add reviewed-by - Martijn

Martijn, I took your LGTM of v1 as Reviewed-by. If you don't like it
or want to change it to acked-by, please, tell me.

 drivers/android/binder.c       | 3 ++-
 drivers/android/binder_alloc.c | 6 +++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 764b63a5aade..fe62be7d7113 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -4722,7 +4722,8 @@ 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 = (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..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);
 		vma = alloc->vma;
 	}
 
@@ -288,7 +288,7 @@ 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 +321,7 @@ 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