From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id 73C946B0229 for ; Thu, 13 May 2010 09:24:58 -0400 (EDT) Date: Thu, 13 May 2010 14:24:36 +0100 From: Mel Gorman Subject: Re: [PATCH 5/5] extend KSM refcounts to the anon_vma root Message-ID: <20100513132436.GC27949@csn.ul.ie> References: <20100512134111.467fb6c2@annuminas.surriel.com> <20100512210706.GQ24989@csn.ul.ie> <4BEB18FE.1090808@redhat.com> <20100513112603.GB27949@csn.ul.ie> <4BEBFA82.2000301@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <4BEBFA82.2000301@redhat.com> Sender: owner-linux-mm@kvack.org To: Rik van Riel Cc: Andrew Morton , Andrea Arcangeli , Minchan Kim , Linux-MM , KAMEZAWA Hiroyuki , LKML , Linus Torvalds List-ID: On Thu, May 13, 2010 at 09:11:30AM -0400, Rik van Riel wrote: > On 05/13/2010 07:26 AM, Mel Gorman wrote: >> On Wed, May 12, 2010 at 05:09:18PM -0400, Rik van Riel wrote: >>> On 05/12/2010 05:07 PM, Mel Gorman wrote: >>>> On Wed, May 12, 2010 at 01:41:11PM -0400, Rik van Riel wrote: >>>>> Subject: extend KSM refcounts to the anon_vma root >>>>> >>>>> KSM reference counts can cause an anon_vma to exist after the processe >>>>> it belongs to have already exited. Because the anon_vma lock now lives >>>>> in the root anon_vma, we need to ensure that the root anon_vma stays >>>>> around until after all the "child" anon_vmas have been freed. >>>>> >>>>> The obvious way to do this is to have a "child" anon_vma take a >>>>> reference to the root in anon_vma_fork. When the anon_vma is freed >>>>> at munmap or process exit, we drop the refcount in anon_vma_unlink >>>>> and possibly free the root anon_vma. >>>>> >>>>> The KSM anon_vma reference count function also needs to be modified >>>>> to deal with the possibility of freeing 2 levels of anon_vma. The >>>>> easiest way to do this is to break out the KSM magic and make it >>>>> generic. >>>>> >>>>> When compiling without CONFIG_KSM, this code is compiled out. >>>>> >>>>> Signed-off-by: Rik van Riel >>>>> --- >>>>> include/linux/rmap.h | 12 ++++++++++++ >>>>> mm/ksm.c | 17 ++++++----------- >>>>> mm/rmap.c | 45 ++++++++++++++++++++++++++++++++++++++++++++- >>>>> 3 files changed, 62 insertions(+), 12 deletions(-) >>>>> >>>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h >>>>> index 33ffe14..387d40c 100644 >>>>> --- a/include/linux/rmap.h >>>>> +++ b/include/linux/rmap.h >>>>> @@ -126,6 +126,18 @@ int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *); >>>>> void __anon_vma_link(struct vm_area_struct *); >>>>> void anon_vma_free(struct anon_vma *); >>>>> >>>>> +#ifdef CONFIG_KSM >>>>> +static inline void get_anon_vma(struct anon_vma *anon_vma) >>>>> +{ >>>>> + atomic_inc(&anon_vma->ksm_refcount); >>>>> +} >>>>> + >>>>> +void drop_anon_vma(struct anon_vma *); >>>>> +#else >>>>> +#define get_anon_vma(x) do {} while(0) >>>>> +#define drop_anon_vma(x) do {} while(0) >>>>> +#endif >>>>> + >>>>> static inline void anon_vma_merge(struct vm_area_struct *vma, >>>>> struct vm_area_struct *next) >>>>> { >>>>> diff --git a/mm/ksm.c b/mm/ksm.c >>>>> index 7ca0dd7..9f2acc9 100644 >>>>> --- a/mm/ksm.c >>>>> +++ b/mm/ksm.c >>>>> @@ -318,19 +318,14 @@ static void hold_anon_vma(struct rmap_item *rmap_item, >>>>> struct anon_vma *anon_vma) >>>>> { >>>>> rmap_item->anon_vma = anon_vma; >>>>> - atomic_inc(&anon_vma->ksm_refcount); >>>>> + get_anon_vma(anon_vma); >>>>> } >>>> >>>> I'm not quite getting this. Here, we get the local anon_vma so we >>>> increment its reference count and later we drop it but without a >>>> refcount taken on the root anon_vma, why is it guaranteed to stay >>>> around? >>> >>> Because anon_vma_fork takes a reference count on the root anon_vma, >>> the VMA we take a refcount on will either have a refcount on the >>> root, or it is the root. >>> >> >> Sorry, I'm still not getting it. anon_vma_fork keeps the refcount around >> during fork but what about during exit? > > It is kept around all the way from fork until exit. > Ok, now I get it. I was thinking in terms of a reference count being for the duration of a specific operation. In this case, the root anon_vma has an elevated reference count for the lifetime of the anon_vma forest. Thanks. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org