Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750916Ab2JREHO (ORCPT ); Thu, 18 Oct 2012 00:07:14 -0400 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:41061 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750703Ab2JREHM (ORCPT ); Thu, 18 Oct 2012 00:07:12 -0400 X-SecurityPolicyCheck: OK by SHieldMailChecker v1.8.4 Message-ID: <507F803A.8000900@jp.fujitsu.com> Date: Thu, 18 Oct 2012 13:06:18 +0900 From: Kamezawa Hiroyuki User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:16.0) Gecko/20121010 Thunderbird/16.0.1 MIME-Version: 1.0 To: David Rientjes CC: Linus Torvalds , Andrew Morton , Dave Jones , KOSAKI Motohiro , bhutchings@solarflare.com, Konstantin Khlebnikov , Naoya Horiguchi , Hugh Dickins , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside spinlock when reading numa_maps References: <20121017040515.GA13505@redhat.com> <20121017181413.GA16805@redhat.com> <20121017193229.GC16805@redhat.com> <20121017194501.GA24400@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4238 Lines: 128 (2012/10/18 6:31), David Rientjes wrote: > As a result of commit 32f8516a8c73 ("mm, mempolicy: fix printing stack > contents in numa_maps"), the mutex protecting a shared policy can be > inadvertently taken while holding task_lock(task). > > Recently, commit b22d127a39dd ("mempolicy: fix a race in > shared_policy_replace()") switched the spinlock within a shared policy to > a mutex so sp_alloc() could block. Thus, a refcount must be grabbed on > all mempolicies returned by get_vma_policy() so it isn't freed while being > passed to mpol_to_str() when reading /proc/pid/numa_maps. > > This patch only takes task_lock() while dereferencing task->mempolicy in > get_vma_policy() if it's non-NULL in the lockess check to increment its > refcount. This ensures it will remain in memory until dropped by > __mpol_put() after mpol_to_str() is called. > > Refcounts of shared policies are grabbed by the ->get_policy() function of > the vma, all others will be grabbed directly in get_vma_policy(). Now > that this is done, all callers now unconditionally drop the refcount. > please add original problem description.... from your 1st patch. > When reading /proc/pid/numa_maps, it's possible to return the contents of > the stack where the mempolicy string should be printed if the policy gets > freed from beneath us. > > This happens because mpol_to_str() may return an error the > stack-allocated buffer is then printed without ever being stored. ..... Hmm, I've read the whole thread again...and, I'm sorry if I misunderstand something. I think Kosaki mentioned the commit 52cd3b0740. It avoids refcounting in get_vma_policy() because it's called every time alloc_pages_vma() is called, at every page fault. So, it seems he doesn't agree this fix because of performance concern on big NUMA, Can't we have another way to fix ? like this ? too ugly ? Again, I'm sorry if I misunderstand the points. == From bfe7e2ab1c1375b134ec12efce6517149318f75d Mon Sep 17 00:00:00 2001 From: KAMEZAWA Hiroyuki Date: Thu, 18 Oct 2012 13:17:25 +0900 Subject: [PATCH] hold task->mempolicy while numa_maps scans. /proc//numa_maps scans vma and show mempolicy under mmap_sem. It sometimes accesses task->mempolicy which can be freed without mmap_sem and numa_maps can show some garbage while scanning. This patch tries to take reference count of task->mempolicy at reading numa_maps before calling get_vma_policy(). By this, task->mempolicy will not be freed until numa_maps reaches its end. Signed-off-by: KAMEZAWA Hiroyuki --- fs/proc/task_mmu.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 14df880..d92e868 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -94,6 +94,11 @@ static void vma_stop(struct proc_maps_private *priv, struct vm_area_struct *vma) { if (vma && vma != priv->tail_vma) { struct mm_struct *mm = vma->vm_mm; +#ifdef CONFIG_NUMA + task_lock(priv->task); + __mpol_put(priv->task->mempolicy); + task_unlock(priv->task); +#endif up_read(&mm->mmap_sem); mmput(mm); } @@ -130,6 +135,16 @@ static void *m_start(struct seq_file *m, loff_t *pos) return mm; down_read(&mm->mmap_sem); + /* + * task->mempolicy can be freed even if mmap_sem is down (see kernel/exit.c) + * We grab refcount for stable access. + * repleacement of task->mmpolicy is guarded by mmap_sem. + */ +#ifdef CONFIG_NUMA + task_lock(priv->task); + mpol_get(priv->task->mempolicy); + task_unlock(priv->task); +#endif tail_vma = get_gate_vma(priv->task->mm); priv->tail_vma = tail_vma; @@ -161,6 +176,11 @@ out: /* End of vmas has been reached */ m->version = (tail_vma != NULL)? 0: -1UL; +#ifdef CONFIG_NUMA + task_lock(priv->task); + __mpol_put(priv->task->mempolicy); + task_unlock(priv->task); +#endif up_read(&mm->mmap_sem); mmput(mm); return tail_vma; -- 1.7.10.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/