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 1BF7DC433EF for ; Thu, 9 Dec 2021 08:55:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235293AbhLII7F (ORCPT ); Thu, 9 Dec 2021 03:59:05 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:53798 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233036AbhLII7D (ORCPT ); Thu, 9 Dec 2021 03:59:03 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id D96FA210FE; Thu, 9 Dec 2021 08:55:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1639040129; 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=bNRBNxibij0BqhT+a2ad9uxqy3v5UnEfD1KuJlm212A=; b=LwV8vONzCWy6fixsm6yJhD63ZKwQ4SCRUpTFoZyyY/w2hwfpAlkRltwgUxtdO+6G+RwBs5 Ev8MGMneFW4f1dwuOMGVuCDnIjT++y3Ymt+zKFP6v+qla03TO0MRxKgtereIn1FN2JKKHP 9XhOQ9YUTxlZIlvRO94IdnPyA/cEN/w= 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 628A0A3B81; Thu, 9 Dec 2021 08:55:29 +0000 (UTC) Date: Thu, 9 Dec 2021 09:55:28 +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 v4 1/3] mm: protect free_pgtables with mmap_lock write lock in exit_mmap Message-ID: References: <20211208212211.2860249-1-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211208212211.2860249-1-surenb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 08-12-21 13:22:09, 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 and remove_vma. 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. unlock_range is not safe to be called under read lock. See 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3"). > Note also that because oom-reaper checks VM_LOCKED flag, > unlock_range() should not be allowed to race with it. > Before this patch, remove_vma used to be called with no locks held, > however with fput being executed asynchronously and vm_ops->close > not being allowed to hold mmap_lock (it is called from __split_vma > with mmap_sem held for write), changing that should be fine. > 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 The patch looks good otherwise. Btw. when I was trying to do something similar in the past Hugh has noted that we can get rid of the same lock&&unlock trick in ksm. Maybe you want to have a look at that as well ;) Acked-by: Michal Hocko Thanks! > --- > changes in v4 > - Separated comments describing vm_operations_struct::close locking > requirements into a separate patch, 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); > - mmap_write_lock(mm); > - mmap_write_unlock(mm); > } > > + mmap_write_lock(mm); > if (mm->locked_vm) > unlock_range(mm->mmap, ULONG_MAX); > > arch_exit_mmap(mm); > > vma = mm->mmap; > - if (!vma) /* Can happen if dup_mmap() received an OOM */ > + if (!vma) { > + /* Can happen if dup_mmap() received an OOM */ > + mmap_write_unlock(mm); > return; > + } > > lru_add_drain(); > flush_cache_mm(mm); > @@ -3171,16 +3173,14 @@ void exit_mmap(struct mm_struct *mm) > free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); > tlb_finish_mmu(&tlb); > > - /* > - * Walk the list again, actually closing and freeing it, > - * with preemption enabled, without holding any MM locks. > - */ > + /* Walk the list again, actually closing and freeing it. */ > while (vma) { > if (vma->vm_flags & VM_ACCOUNT) > nr_accounted += vma_pages(vma); > vma = remove_vma(vma); > cond_resched(); > } > + mmap_write_unlock(mm); > vm_unacct_memory(nr_accounted); > } > > -- > 2.34.1.400.ga245620fadb-goog -- Michal Hocko SUSE Labs