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 8E86EC433F5 for ; Thu, 16 Dec 2021 17:23:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240190AbhLPRXZ (ORCPT ); Thu, 16 Dec 2021 12:23:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58364 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240040AbhLPRXY (ORCPT ); Thu, 16 Dec 2021 12:23:24 -0500 Received: from mail-yb1-xb32.google.com (mail-yb1-xb32.google.com [IPv6:2607:f8b0:4864:20::b32]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3566AC061574 for ; Thu, 16 Dec 2021 09:23:24 -0800 (PST) Received: by mail-yb1-xb32.google.com with SMTP id d10so66574481ybn.0 for ; Thu, 16 Dec 2021 09:23:24 -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=KOfddnRBlgrRNJUcJFAYmMl+H47vzBbmtc2FFUjJ1No=; b=Yc6wnk82blHZiFBHSFBOZCsbU33ZtmS7OnOP25JEBdLn3pzapUEBPO+ZR+Y5w481i/ l3Mmya+tO61fJof7XmePit0QMOfUw9bYKVhdflL8+bklYF2rpvn1RMBuLjK34LYSximm 6+pmof4aUNAinh1CzukO8E4JXiOvIGWxrcU95SWQh1J2wb0fPhVkUNl1Ncc8+eW2crWJ /5rRohpENH/kVlLx6PGD3yaPGL1SL712hbmhi8FTZoWjUd8SrSqLphfCnRfb0oPmdhZg mL3uvNQRTYEocGJ+kUyNoDRh0G1qL+I9M8d758aBElqO0hsZo0a699PSJLYwxq+CQ7HA HlrA== 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=KOfddnRBlgrRNJUcJFAYmMl+H47vzBbmtc2FFUjJ1No=; b=ErwVUTE7Sbf+5Jj1HVDskbR+v5APtlh9kIeRIz4B/9saSmFKF0dRSiyZmm00oEnTrz gxa0HVYT+T7AjbYop9kEGdgnpl0+U2K+p7/sTE4tJYbwAoRVemkcPOGndNn3pgK726ft hkZI3VM40aMWgNF0KVUMqpR6419XaqnGLs6atBmKExtPiz4TE6hR395/yp1FXcm6nXa+ Jos/+bdvJEQ4aPScNgpGdnYOiVRHnrr8NCVlk4OHcrrBo8zGNW6tjcswfbcM3bN7qn2T K6yZIyxroh4J6l9u5ombg7YnSPUv1HRAc38XYMr8gVvTiiuApLUQg/cHYpgHSqHzoJtY RGXA== X-Gm-Message-State: AOAM531HG0AmrH9yHjQArPiRIMFjgHDqySsaUILtEmk5R2mAQ1vhpfkB Pvhp34BYVdOcGWd4RugIgv1Fss/7ZKZ0HMnmK4W9Zg== X-Google-Smtp-Source: ABdhPJwWv5HgWT/NNBKfZuIQk87EsF7CJpgNK4d7YTR8OVigJcUEq9iqCvp4TdlUeADUbnwvUnXcZWCoCH0NCwZ7+uM= X-Received: by 2002:a25:a448:: with SMTP id f66mr14255921ybi.225.1639675403013; Thu, 16 Dec 2021 09:23:23 -0800 (PST) MIME-Version: 1.0 References: <20211208212211.2860249-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 16 Dec 2021 09:23:12 -0800 Message-ID: Subject: Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap To: Johannes Weiner Cc: Michal Hocko , akpm@linux-foundation.org, rientjes@google.com, willy@infradead.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 16, 2021 at 3:49 AM Johannes Weiner wrote: > > On Wed, Dec 15, 2021 at 06:26:11PM -0800, Suren Baghdasaryan wrote: > > On Thu, Dec 9, 2021 at 9:06 AM Suren Baghdasaryan wrote: > > > > > > On Thu, Dec 9, 2021 at 8:47 AM Michal Hocko wrote: > > > > > > > > On Thu 09-12-21 08:24:04, Suren Baghdasaryan wrote: > > > > > On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko wrote: > > > > > > > > > > > > Do we want this on top? > > > > > > > > > > As we discussed in this thread > > > > > https://lore.kernel.org/all/YY4snVzZZZYhbigV@dhcp22.suse.cz, > > > > > __oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to > > > > > unmap pages in parallel with exit_mmap without blocking each other. > > > > > Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism > > > > > and has a negative impact on performance. So the conclusion of that > > > > > thread I thought was to keep that part. My understanding is that we > > > > > also wanted to remove MMF_OOM_SKIP as a follow-up patch but > > > > > __oom_reap_task_mm would stay. > > > > > > > > OK, then we were talking past each other, I am afraid. I really wanted > > > > to get rid of this oom specific stuff from exit_mmap. It was there out > > > > of necessity. With a proper locking we can finally get rid of the crud. > > > > As I've said previously oom reaping has never been a hot path. > > > > > > > > If we really want to optimize this path then I would much rather see a > > > > generic solution which would allow to move the write lock down after > > > > unmap_vmas. That would require oom reaper to be able to handle mlocked > > > > memory. > > > > > > Ok, let's work on that and when that's done we can get rid of the oom > > > stuff in exit_mmap. I'll look into this over the weekend and will > > > likely be back with questions. > > > > As promised, I have a question: > > Any particular reason why munlock_vma_pages_range clears VM_LOCKED > > before unlocking pages and not after (see: > > https://elixir.bootlin.com/linux/latest/source/mm/mlock.c#L424)? Seems > > to me if VM_LOCKED was reset at the end (with proper ordering) then > > __oom_reap_task_mm would correctly skip VM_LOCKED vmas. > > https://lore.kernel.org/lkml/20180514064824.534798031@linuxfoundation.org/ > > has this explanation: > > > > "Since munlock_vma_pages_range() depends on clearing VM_LOCKED from > > vm_flags before actually doing the munlock to determine if any other > > vmas are locking the same memory, the check for VM_LOCKED in the oom > > reaper is racy." > > > > but "to determine if any other vmas are locking the same memory" > > explanation eludes me... Any insights? > > A page's mlock state is determined by whether any of the vmas that map > it are mlocked. The munlock code does: > > vma->vm_flags &= VM_LOCKED_CLEAR_MASK > TestClearPageMlocked() > isolate_lru_page() > __munlock_isolated_page() > page_mlock() > rmap_walk() # for_each_vma() > page_mlock_one() > (vma->vm_flags & VM_LOCKED) && TestSetPageMlocked() > > If we didn't clear the VM_LOCKED flag first, racing threads could > re-lock pages under us because they see that flag and think our vma > wants those pages mlocked when we're in the process of munlocking. Thanks for the explanation Johannes! So far I didn't find an easy way to let __oom_reap_task_mm() run concurrently with unlock_range(). Will keep exploring. >