From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id 9F71D6B0292 for ; Mon, 24 Jul 2017 03:23:42 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id k71so23630853wrc.15 for ; Mon, 24 Jul 2017 00:23:42 -0700 (PDT) Received: from mail-wr0-f194.google.com (mail-wr0-f194.google.com. [209.85.128.194]) by mx.google.com with ESMTPS id 69si915059wme.212.2017.07.24.00.23.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 24 Jul 2017 00:23:41 -0700 (PDT) Received: by mail-wr0-f194.google.com with SMTP id y43so16718325wrd.0 for ; Mon, 24 Jul 2017 00:23:40 -0700 (PDT) From: Michal Hocko Subject: [PATCH] mm, oom: allow oom reaper to race with exit_mmap Date: Mon, 24 Jul 2017 09:23:32 +0200 Message-Id: <20170724072332.31903-1-mhocko@kernel.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: David Rientjes , Tetsuo Handa , Oleg Nesterov , Hugh Dickins , Andrea Arcangeli , linux-mm@kvack.org, LKML , Michal Hocko From: Michal Hocko David has noticed that the oom killer might kill additional tasks while the exiting oom 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) The victim is still visible to the OOM killer until it is unhashed. Currently we 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 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. Changes - bail on null mm->mmap early as per David Rientjes Reported-by: David Rientjes Fixes: 26db62f179d1 ("oom: keep mm of the killed task available") Signed-off-by: Michal Hocko --- Hi, I've sent this as an RFC [1] previously and it seems that the original issue has been resolved [2], although an explicit tested-by would be appreciated of course. Hugh has pointed out [3] that using mmap_sem in exit_mmap will allow to drop the tricky down_write(mmap_sem); up_write(mmap_sem); in both paths. I hope I will get to that in a forseeable future. I am not yet sure this is important enough to merge to stable trees, I would rather wait for a report to show up. [1] http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@kernel.org [2] http://lkml.kernel.org/r/alpine.DEB.2.10.1707111336250.60183@chino.kir.corp.google.com [3] http://lkml.kernel.org/r/alpine.LSU.2.11.1707191716030.2055@eggly.anvils mm/mmap.c | 7 +++++++ mm/oom_kill.c | 45 +++++++-------------------------------------- 2 files changed, 14 insertions(+), 38 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 24e9261bdcc0..0eeb658caa30 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2993,6 +2993,11 @@ 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); @@ -3005,7 +3010,9 @@ 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 9e8b4f030c1c..a6dabe3691c1 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -470,40 +470,15 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) { struct mmu_gather tlb; 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; trace_skip_task_reaping(tsk->pid); - goto unlock_oom; + return false; } - /* - * 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); - trace_skip_task_reaping(tsk->pid); - goto unlock_oom; - } + /* There is nothing to reap so bail out without signs in the log */ + if (!mm->mmap) + goto unlock; trace_start_task_reaping(tsk->pid); @@ -540,17 +515,11 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) K(get_mm_counter(mm, MM_ANONPAGES)), K(get_mm_counter(mm, MM_FILEPAGES)), 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); trace_finish_task_reaping(tsk->pid); -unlock_oom: - mutex_unlock(&oom_lock); +unlock: + up_read(&mm->mmap_sem); + return ret; } -- 2.13.2 -- 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