linux-kernel - Re: 3.9-rc5: Encountedred INFO: rcu_sched self-detected stall on CPU due to 09a9f1d27

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Message-ID: <20130416002123.GA29465@google.com>
Date:	Mon, 15 Apr 2013 17:21:23 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: 3.9-rc5: Encountedred INFO: rcu_sched self-detected stall on CPU
 due to 09a9f1d27

On Mon, Apr 15, 2013 at 2:47 PM, Hugh Dickins <hughd@...gle.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()...

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.
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