From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A486FC433EF for ; Mon, 29 Nov 2021 11:25:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234381AbhK2L2t (ORCPT ); Mon, 29 Nov 2021 06:28:49 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:50414 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231848AbhK2L0r (ORCPT ); Mon, 29 Nov 2021 06:26:47 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 8E86F212C9; Mon, 29 Nov 2021 11:23:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1638185008; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=r/Fr5NxP2jCRkLtXeiXZXbWtKkV0jHF3GitFDxVGIWg=; b=BzpRJa2vr9jLIWnSYSIrW8ycKc8vWG2pId4DDQaGtSKAeZSLGq+SKHboviPUK5oJamwTaO nHvEJ704RXZHxIIqGB0Xfd7uEuNtbujL4bVbai2+n80W3c2uuZm6zLBnxpQP8QK2qz432V UezHo2aX7Z/bDeoss0EYlq00t0Z7elk= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id BACC3A3B83; Mon, 29 Nov 2021 11:23:27 +0000 (UTC) Date: Mon, 29 Nov 2021 12:23:23 +0100 From: Michal Hocko To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, rientjes@google.com, willy@infradead.org, hannes@cmpxchg.org, guro@fb.com, riel@surriel.com, minchan@kernel.org, kirill@shutemov.name, aarcange@redhat.com, christian@brauner.io, hch@infradead.org, oleg@redhat.com, david@redhat.com, jannh@google.com, shakeelb@google.com, luto@kernel.org, christian.brauner@ubuntu.com, fweimer@redhat.com, jengelh@inai.de, timmurray@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap Message-ID: References: <20211124235906.14437-1-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211124235906.14437-1-surenb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > oom-reaper and process_mrelease system call should protect against > races with exit_mmap which can destroy page tables while they > walk the VMA tree. oom-reaper protects from that race by setting > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > before taking and releasing mmap_write_lock. process_mrelease has > to elevate mm->mm_users to prevent such race. Both oom-reaper and > process_mrelease hold mmap_read_lock when walking the VMA tree. > The locking rules and mechanisms could be simpler if exit_mmap takes > mmap_write_lock while executing destructive operations such as > free_pgtables. > Change exit_mmap to hold the mmap_write_lock when calling > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > destructive and could run under mmap_read_lock but for simplicity we > take one mmap_write_lock during almost the entire operation. Note > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > should not be allowed to race with it. > In most cases this lock should be uncontended. Previously, Kirill > reported ~4% regression caused by a similar change [1]. We reran the > same test and although the individual results are quite noisy, the > percentiles show lower regression with 1.6% being the worst case [2]. > The change allows oom-reaper and process_mrelease to execute safely > under mmap_read_lock without worries that exit_mmap might destroy page > tables from under them. > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/ > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/ > > Signed-off-by: Suren Baghdasaryan > --- > changes in v2 > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > mm/mmap.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index bfb0ea164a90..f4e09d390a07 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > * __oom_reap_task_mm() will not block. > * > - * This needs to be done before calling munlock_vma_pages_all(), > + * This needs to be done before calling unlock_range(), > * which clears VM_LOCKED, otherwise the oom reaper cannot > * reliably test it. > */ > (void)__oom_reap_task_mm(mm); > > set_bit(MMF_OOM_SKIP, &mm->flags); Why do you keep this in place? Other than that looks OK to me. Maybe we want to add an explicit note that vm_ops::close cannot take mmap_sem in any form. The changelog should also mention that you have considered remove_vma and its previous no MM locking assumption. You can argue that fput is async and close callback shouldn't really need mmap_sem. -- Michal Hocko SUSE Labs