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 4F4D7C433F5 for ; Thu, 9 Dec 2021 19:03:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230127AbhLITHB (ORCPT ); Thu, 9 Dec 2021 14:07:01 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44534 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230148AbhLITG5 (ORCPT ); Thu, 9 Dec 2021 14:06:57 -0500 Received: from mail-yb1-xb2e.google.com (mail-yb1-xb2e.google.com [IPv6:2607:f8b0:4864:20::b2e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E8F10C0617A1 for ; Thu, 9 Dec 2021 11:03:23 -0800 (PST) Received: by mail-yb1-xb2e.google.com with SMTP id e136so15987423ybc.4 for ; Thu, 09 Dec 2021 11:03:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=mFgPKxEL0IYHP9XylpFOWxnMIuX8VMB5Vdn/l5Gnhv4=; b=d49r+a/yxhwOKt2CtOzzbORMPT/pqNpe+rgLAEHE+SZtZY+J0Q/dASqGQEoOO63AKl IZGCxDaRUpLkaPs65s0MelNCEZMOzGWu1jxmrZ5NMk5dQFW7Au7fieqDSWvLVewsPghi JXyseLJkwccRHATEPFZhMQW8coGfhXWVFI5DXDa2n/mkNMcBfzlrWlcmzrrNPZi9GbN5 EXjFZQ23bMTrQD0h2s3J5ByCxNqWXe7wGhQr+exDwJ6XnHRB0+m+hrj0kNvx7hVgwMJd 0IeO9FYOCWP8SCL3UyqUcFOiT6+XAFctnrpvoAonrahMEdKNAy5CdS57TCljfW/BBsbw +BTg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=mFgPKxEL0IYHP9XylpFOWxnMIuX8VMB5Vdn/l5Gnhv4=; b=c57HePOVPPr/dTnFBmYOlaLte5tsyJC4YJ2Di5nll6+hnggFauL3jd+IFwwAHJvXtV w28GUX3QpfJ7uezO1HZLZD8HwgW6SWV1wK/ajW/FJ9zutCKPZ0c4AdADt8eWbEg4+awh S0b7uNkEnBKULHfRMNDTIG0s2T6VpVzXs3PPP5FtR7/xGk/fNPqJHwO3v3I2rrUvwiXi 6VwGRl95pNSjVCrTBxe/sZjtyFePRSpNwJY468z6FAXs5CG7EgiFmu0/Z/nRBG3DyL24 BCnbwVULxbDnetzlDAniTsUVQR99mbwHCy5qflj83FUI0PQ3p+Lvfpk7VdHXGQP/mi1u CnqA== X-Gm-Message-State: AOAM530skfGAYmO8sZFZBU3H5Faw2Aj5vpdst7v8EZdAPNwjDVPJ3dbb 7c5GqrusPPylkman3gbjpQ8s3YmjMsYIJXoT3PkFzQ== X-Google-Smtp-Source: ABdhPJyLoK6MoV/dr1IqHWQLckho4ke9rkGF44TsXb9R/qALOruPqSksrOVV/ixlpa0mwThmIPfWCovcEqSJsBovAWE= X-Received: by 2002:a25:610d:: with SMTP id v13mr7892030ybb.1.1639076602764; Thu, 09 Dec 2021 11:03:22 -0800 (PST) MIME-Version: 1.0 References: <20211208212211.2860249-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 9 Dec 2021 11:03:11 -0800 Message-ID: Subject: Re: [PATCH v4 1/3] mm: protect free_pgtables with mmap_lock write lock in exit_mmap To: Michal Hocko 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 9, 2021 at 12:55 AM Michal Hocko wrote: > > 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"). Ok, I'll remove the sentence above. Is my understanding correct that it is unsafe only because oom-reaper can't deal with VM_LOCKED, otherwise it would be fine? > > > 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 > ;) I'll take a look after we cleanup this path completely (oom pieces included). > > Acked-by: Michal Hocko Thanks! > > 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