linux-mips
[Top] [All Lists]

Re: [PATCH 1/1] mmap.c: find_vma: replace if(mm) check with BUG_ON(!mm)

To: Kautuk Consul <consul.kautuk@gmail.com>
Subject: Re: [PATCH 1/1] mmap.c: find_vma: replace if(mm) check with BUG_ON(!mm)
From: Hugh Dickins <hughd@google.com>
Date: Mon, 26 Mar 2012 14:08:00 -0700 (PDT)
Cc: Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=date:from:x-x-sender:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version:content-type; bh=PNY7l+ycCaMh4eq/pnhng52CYH29ubBQRrcDXmprKzw=; b=hcMnZOxh4AqILVASkRDn7l3XJtMDWHMsKuy5DzRPTnnplcfyB2OZkZV1vym55k35xD ckgECh2kHiztsDCI+7iA7Upc55kIF38wYk6CSDb2WD84+2TJZOg2k9aatZrVqposxD4B kQiIwRpckhUHEHFRtHU2nymdxoTzKP36GDy5qIXGFhmnQ0AZ4RT8BDN50+4BKBUFlAEV zHTOvOd8uGWCW6/AaS9Os8m6JXNcOANKARMrhxm3r08rPv/8o56rrFVXbka5AiIxv+D1 WfZ2clpnGl75ZsTBANScY0wA7/hGElte3wfGOVl9M7qNdc5tWGsrt/pc8022q/Obcjbq VTkg==
In-reply-to: <1332777965-2534-1-git-send-email-consul.kautuk@gmail.com>
References: <1332777965-2534-1-git-send-email-consul.kautuk@gmail.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Alpine 2.00 (LSU 1167 2008-08-23)
On Mon, 26 Mar 2012, Kautuk Consul wrote:
> find_vma is called from kernel code where it is absolutely
> sure that the mm_struct arg being passed to it is non-NULL.
> 
> Convert the if check to a BUG_ON.
> This will also serve the purpose of mandating that the execution
> context(user-mode/kernel-mode) be known before find_vma is called.
> 
> Also fixed 2 checkpatch.pl errors in this function for the rb_node
> and vma_tmp local variables.
> 
> I have tested this patch on my x86 PC and there are no BUG_ON crashes
> due to this in the course of normal desktop execution.
> 
> Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>

That seems very reasonable: perhaps there was a reason for checking
find_vma()'s mm way back in the distant past, but I cannot see it now.
But please make two small changes noted below before resubmitting.

Since we ask for mm->mmap_sem to be held before calling find_vma(),
it's hard to reach here with NULL mm.  There are a few strange places
in arch and drivers/media/video which appear to be taking risks by
not holding mmap_sem, but only one of them looks like it _might_ be
endangered by your change.

Ralf, that octeon_flush_cache_sigtramp() in arch/mips/mm/c-octeon.c:
is there ever a danger that it can be called with NULL current->mm?  Is
current->mm set to &init_mm in the initial call from octeon_cache_init()?

> ---
>  mm/mmap.c |   52 ++++++++++++++++++++++++++--------------------------
>  1 files changed, 26 insertions(+), 26 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index a7bf6a3..7589965 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1589,33 +1589,33 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, 
> unsigned long addr)
>  {
>       struct vm_area_struct *vma = NULL;

Please remove the " = NULL": vma is immediately set to mm->mmap_cache.

>  
> -     if (mm) {
> -             /* Check the cache first. */
> -             /* (Cache hit rate is typically around 35%.) */
> -             vma = mm->mmap_cache;
> -             if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
> -                     struct rb_node * rb_node;
> -
> -                     rb_node = mm->mm_rb.rb_node;
> -                     vma = NULL;
> -
> -                     while (rb_node) {
> -                             struct vm_area_struct * vma_tmp;
> -
> -                             vma_tmp = rb_entry(rb_node,
> -                                             struct vm_area_struct, vm_rb);
> -
> -                             if (vma_tmp->vm_end > addr) {
> -                                     vma = vma_tmp;
> -                                     if (vma_tmp->vm_start <= addr)
> -                                             break;
> -                                     rb_node = rb_node->rb_left;
> -                             } else
> -                                     rb_node = rb_node->rb_right;
> -                     }
> -                     if (vma)
> -                             mm->mmap_cache = vma;
> +     BUG_ON(!mm);

And please remove the BUG_ON(!mm): it's a waste of space and time,
it gives very little value over the easily recognizable oops we
shall get from "vma = mm->mmap_cache" with NULL mm.

Thanks,
Hugh

> +
> +     /* Check the cache first. */
> +     /* (Cache hit rate is typically around 35%.) */
> +     vma = mm->mmap_cache;
> +     if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
> +             struct rb_node *rb_node;
> +
> +             rb_node = mm->mm_rb.rb_node;
> +             vma = NULL;
> +
> +             while (rb_node) {
> +                     struct vm_area_struct *vma_tmp;
> +
> +                     vma_tmp = rb_entry(rb_node,
> +                                     struct vm_area_struct, vm_rb);
> +
> +                     if (vma_tmp->vm_end > addr) {
> +                             vma = vma_tmp;
> +                             if (vma_tmp->vm_start <= addr)
> +                                     break;
> +                             rb_node = rb_node->rb_left;
> +                     } else
> +                             rb_node = rb_node->rb_right;
>               }
> +             if (vma)
> +                     mm->mmap_cache = vma;
>       }
>       return vma;
>  }
> -- 
> 1.7.5.4

<Prev in Thread] Current Thread [Next in Thread>