Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752448AbdF2Iqc (ORCPT ); Thu, 29 Jun 2017 04:46:32 -0400 Received: from mx2.suse.de ([195.135.220.15]:43700 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751861AbdF2IqZ (ORCPT ); Thu, 29 Jun 2017 04:46:25 -0400 Date: Thu, 29 Jun 2017 10:46:21 +0200 From: Michal Hocko To: Andrea Argangeli , Hugh Dickins Cc: Tetsuo Handa , David Rientjes , Oleg Nesterov , Andrew Morton , LKML , linux-mm@kvack.org Subject: Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit_mmap Message-ID: <20170629084621.GE31603@dhcp22.suse.cz> References: <20170626130346.26314-1-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170626130346.26314-1-mhocko@kernel.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5962 Lines: 165 Forgot to CC Hugh. Hugh, Andrew, do you see this could cause any problem wrt. ksm/khugepaged exit path? On Mon 26-06-17 15:03:46, Michal Hocko wrote: > From: Michal Hocko > > David has noticed that the oom killer might kill additional tasks while > the existing victim hasn't terminated yet because the oom_reaper marks > the curent victim MMF_OOM_SKIP too early when mm->mm_users dropped down > to 0. The race is as follows > > oom_reap_task do_exit > exit_mm > __oom_reap_task_mm > mmput > __mmput > mmget_not_zero # fails > exit_mmap # frees memory > set_bit(MMF_OOM_SKIP) > > Currently we are try to reduce a risk of this race by taking oom_lock > and wait for out_of_memory sleep while holding the lock to give the > victim some time to exit. This is quite suboptimal approach because > there is no guarantee the victim (especially a large one) will manage > to unmap its address space and free enough memory to the particular oom > domain which needs a memory (e.g. a specific NUMA node). > > Fix this problem by allowing __oom_reap_task_mm and __mmput path to > race. __oom_reap_task_mm is basically MADV_DONTNEED and that is allowed > to run in parallel with other unmappers (hence the mmap_sem for read). > The only tricky part is we have to exclude page tables tear down and all > operations which modify the address space in the __mmput path. exit_mmap > doesn't expect any other users so it doesn't use any locking. Nothing > really forbids us to use mmap_sem for write, though. In fact we are > already relying on this lock earlier in the __mmput path to synchronize > with ksm and khugepaged. > > Take the exclusive mmap_sem when calling free_pgtables and destroying > vmas to sync with __oom_reap_task_mm which take the lock for read. All > other operations can safely race with the parallel unmap. > > Reported-by: David Rientjes > Fixes: 26db62f179d1 ("oom: keep mm of the killed task available") > Signed-off-by: Michal Hocko > --- > > Hi, > I am sending this as an RFC because I am not yet sure I haven't missed > something subtle here but the appoach should work in principle. I have > run it through some of my OOM stress tests to see if anything blows up > and it all went smoothly. > > The issue has been brought up by David [1]. There were some attempts to > address it in oom proper [2][3] but the first one would cause problems > on their own [4] while the later is just too hairy. > > Thoughts, objections, alternatives? > > [1] http://lkml.kernel.org/r/alpine.DEB.2.10.1706141632100.93071@chino.kir.corp.google.com > [2] http://lkml.kernel.org/r/201706171417.JHG48401.JOQLHMFSVOOFtF@I-love.SAKURA.ne.jp > [3] http://lkml.kernel.org/r/201706220053.v5M0rmOU078764@www262.sakura.ne.jp > [4] http://lkml.kernel.org/r/201706210217.v5L2HAZc081021@www262.sakura.ne.jp > > mm/mmap.c | 7 +++++++ > mm/oom_kill.c | 40 ++-------------------------------------- > 2 files changed, 9 insertions(+), 38 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index 3bd5ecd20d4d..253808e716dc 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -2962,6 +2962,11 @@ void exit_mmap(struct mm_struct *mm) > /* Use -1 here to ensure all VMAs in the mm are unmapped */ > unmap_vmas(&tlb, vma, 0, -1); > > + /* > + * oom reaper might race with exit_mmap so make sure we won't free > + * page tables or unmap VMAs under its feet > + */ > + down_write(&mm->mmap_sem); > free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); > tlb_finish_mmu(&tlb, 0, -1); > > @@ -2974,7 +2979,9 @@ void exit_mmap(struct mm_struct *mm) > nr_accounted += vma_pages(vma); > vma = remove_vma(vma); > } > + mm->mmap = NULL; > vm_unacct_memory(nr_accounted); > + up_write(&mm->mmap_sem); > } > > /* Insert vm structure into process list sorted by address > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index 0e2c925e7826..5dc0ff22d567 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -472,36 +472,8 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) > struct vm_area_struct *vma; > bool ret = true; > > - /* > - * We have to make sure to not race with the victim exit path > - * and cause premature new oom victim selection: > - * __oom_reap_task_mm exit_mm > - * mmget_not_zero > - * mmput > - * atomic_dec_and_test > - * exit_oom_victim > - * [...] > - * out_of_memory > - * select_bad_process > - * # no TIF_MEMDIE task selects new victim > - * unmap_page_range # frees some memory > - */ > - mutex_lock(&oom_lock); > - > - if (!down_read_trylock(&mm->mmap_sem)) { > - ret = false; > - goto unlock_oom; > - } > - > - /* > - * increase mm_users only after we know we will reap something so > - * that the mmput_async is called only when we have reaped something > - * and delayed __mmput doesn't matter that much > - */ > - if (!mmget_not_zero(mm)) { > - up_read(&mm->mmap_sem); > - goto unlock_oom; > - } > + if (!down_read_trylock(&mm->mmap_sem)) > + return false; > > /* > * Tell all users of get_user/copy_from_user etc... that the content > @@ -538,14 +510,6 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) > K(get_mm_counter(mm, MM_SHMEMPAGES))); > up_read(&mm->mmap_sem); > > - /* > - * Drop our reference but make sure the mmput slow path is called from a > - * different context because we shouldn't risk we get stuck there and > - * put the oom_reaper out of the way. > - */ > - mmput_async(mm); > -unlock_oom: > - mutex_unlock(&oom_lock); > return ret; > } > > -- > 2.11.0 > > -- > 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 -- Michal Hocko SUSE Labs