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Date: Mon, 15 Apr 2013 18:27:42 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
X-X-Sender: hugh@eggly.anvils
To: Michel Lespinasse <walken@google.com>
cc: Vivek Goyal <vgoyal@redhat.com>,
        linux kernel mailing list <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: 3.9-rc5: Encountedred INFO: rcu_sched self-detected stall on
 CPU due to 09a9f1d27
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On Mon, 15 Apr 2013, Michel Lespinasse wrote:
> On Mon, Apr 15, 2013 at 2:47 PM, Hugh Dickins <hughd@google.com> wrote:
> > --- 3.9-rc7/mm/mlock.c  2013-04-01 09:08:05.736012852 -0700
> > +++ linux/mm/mlock.c    2013-04-15 14:20:24.454773245 -0700
> > @@ -397,8 +397,7 @@ int __mm_populate(unsigned long start, u
> >         long ret = 0;
> >
> >         VM_BUG_ON(start & ~PAGE_MASK);
> > -       VM_BUG_ON(len != PAGE_ALIGN(len));
> > -       end = start + len;
> > +       end = start + PAGE_ALIGN(len);
> >
> >         for (nstart = start; nstart < end; nstart = nend) {
> >                 /*
> 
> Yes, there was originally an issue with page alignment in mmap as well, which was fixed by making sure mmap now passes the page aligned length to mm_populate().
> 
> sys_brk() passes the length as the difference of two page aligned addresses, so it's fine. But vm_brk() doesn't - it calls do_brk() which page aligns the length, but then vm_brk passes the unaligned length to mm_populate().
> 
> What do you think of the following ? len is already a PAGE_SIZE multiple when called from sys_brk()...

Yes, that's good.

> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 0db0de1c2fbe..6af8b0d1c7db 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2557,10 +2557,6 @@ static unsigned long do_brk(unsigned long addr, unsigned long len)
>  	pgoff_t pgoff = addr >> PAGE_SHIFT;
>  	int error;
>  
> -	len = PAGE_ALIGN(len);
> -	if (!len)
> -		return addr;
> -
>  	flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
>  
>  	error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
> @@ -2643,6 +2639,10 @@ unsigned long vm_brk(unsigned long addr, unsigned long len)
>  	unsigned long ret;
>  	bool populate;
>  
> +	len = PAGE_ALIGN(len);
> +	if (!len)
> +		return addr;
> +
>  	down_write(&mm->mmap_sem);
>  	ret = do_brk(addr, len);
>  	populate = ((mm->def_flags & VM_LOCKED) != 0);
> 
> -- 
> Michel "Walken" Lespinasse
> A program is never fully debugged until the last user dies.