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 714E7C433EF for ; Thu, 30 Dec 2021 06:00:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236332AbhL3GAK (ORCPT ); Thu, 30 Dec 2021 01:00:10 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40062 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229446AbhL3GAJ (ORCPT ); Thu, 30 Dec 2021 01:00:09 -0500 Received: from mail-yb1-xb2b.google.com (mail-yb1-xb2b.google.com [IPv6:2607:f8b0:4864:20::b2b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8A773C061574 for ; Wed, 29 Dec 2021 22:00:08 -0800 (PST) Received: by mail-yb1-xb2b.google.com with SMTP id i3so54722474ybh.11 for ; Wed, 29 Dec 2021 22:00:08 -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=lYXuL9NU5LzCPQzMYGqg0I+VEYjMhL1XTirwxHq3oIg=; b=qvnEWfVmgZFESuSi9Qa5PzKpTVCpHJTyk601vUFn+q6W3BeHPHFDqGQvF6k7Neg+fo g5Jaf5L+I85IR0SVzqdOXNGwaB8QKZXRTsClCZvasmt27A8I0PTyg0jaYmO84oNC+ZYv xi9ZdF7NhLzpaBv3Mqjdp2CaCph9gXbvVG7uIRVBObnE22DvV8uVdE40O/osioddnH5O t1I/iwEAWfVUlZLsUsNdRpsE6mVBP53p9KosGs8rLkUtbWIe+fmg8pB7fXelc4RKm0rI IbfCuZc3ipvWU7gpfuX1wxaulNm5tpQr2dT6Ae973XsYdhJr437Gf91C+UFEJFssLT8E E5ig== 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=lYXuL9NU5LzCPQzMYGqg0I+VEYjMhL1XTirwxHq3oIg=; b=QAWGm3z3vtSXcaHtYybAIuHW//ogEZvlyrMJY+/AS1AniWXVOobszwt2u6/CjDHHJS sAc1X2smVKqV4PTIIVw3mXDRXww7WJC9C6ESectkWT2G66OIECi37zsBYDVRunECrNvv 0ZNBzed3ErzVvJFBEbJfFVl3bR9YtNGItCNY0MBxsBWmUIEd4R/5EnYyJ6G8jwQd1cGu QbW0SbYSqc7CG4fjpTNqJWAY1yIiIGkwmVn7mRSwklXiBayTZKR/47+ooA4y3ZnlJawu LTCtP+H/CRYGkFmD5i6Upqlq+Qgy3uMr6ZMraM3mJSkE9oN4uIJRC0d7w9qq1qRey0pg jIFA== X-Gm-Message-State: AOAM5321JMGx2Q4rEcjyGkoRMJ40k3HLb6UV366A+z2mB/XBwHY+hHKp tTc8XdgJ3PPUM62EryAwYZ0Wk4E7t1Z6iN4tdO4zLQ== X-Google-Smtp-Source: ABdhPJwtlkvMGG8ueFd0I2R6MMvTgca99twHThfkO6dm7IvNHwKGG7bS3ZDFGfryMgKw5iJYgNHLtsEL0sk8IToPhk8= X-Received: by 2002:a5b:881:: with SMTP id e1mr22171341ybq.602.1640844006794; Wed, 29 Dec 2021 22:00:06 -0800 (PST) MIME-Version: 1.0 References: <20211208212211.2860249-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Wed, 29 Dec 2021 21:59:55 -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 9:23 AM Suren Baghdasaryan wrote: > > 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. After some more digging I think there are two acceptable options: 1. Call unlock_range() under mmap_write_lock and then downgrade it to read lock so that both exit_mmap() and __oom_reap_task_mm() can unmap vmas in parallel like this: if (mm->locked_vm) { mmap_write_lock(mm); unlock_range(mm->mmap, ULONG_MAX); mmap_write_downgrade(mm); } else mmap_read_lock(mm); ... unmap_vmas(&tlb, vma, 0, -1); mmap_read_unlock(mm); mmap_write_lock(mm); free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); ... mm->mmap = NULL; mmap_write_unlock(mm); This way exit_mmap() might block __oom_reap_task_mm() but for a much shorter time during unlock_range() call. 2. Introduce another vm_flag mask similar to VM_LOCKED which is set before munlock_vma_pages_range() clears VM_LOCKED so that __oom_reap_task_mm() can identify vmas being unlocked and skip them. Option 1 seems cleaner to me because it keeps the locking pattern around unlock_range() in exit_mmap() consistent with all other places it is used (in mremap() and munmap()) with mmap_write_lock taken. WDYT? > >