linux-kernel - Re: [PATCH v4] ANDROID: binder: change down_write to down

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Message-ID: <1522306123.2210.28.camel@perches.com>
Date:   Wed, 28 Mar 2018 23:48:43 -0700
From:   Joe Perches <joe@...ches.com>
To:     Minchan Kim <minchan@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v4] ANDROID: binder: change down_write to down_read

On Thu, 2018-03-29 at 15:40 +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

typo trivia: binder_mmap

> already hold a mmap_sem as down_write so binder_update_page_range
> doesn't need to hold a mmap_sem as down_write.

and style trivia:

> diff --git 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;

Perhaps this is clearer as

	vma->vm_flags |= VM_DONTCOPY | VM_MIXEDMAP;
	vma->vm_flags &= ~VM_MAYWRITE;