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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4F00FC433EF for ; Fri, 20 May 2022 16:17:40 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 76ECE6B0071; Fri, 20 May 2022 12:17:39 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 71E506B0072; Fri, 20 May 2022 12:17:39 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 597DB6B0073; Fri, 20 May 2022 12:17:39 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 49BF06B0071 for ; Fri, 20 May 2022 12:17:39 -0400 (EDT) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 1FA691ADD for ; Fri, 20 May 2022 16:17:39 +0000 (UTC) X-FDA: 79486627038.28.79C6012 Received: from mail-yw1-f176.google.com (mail-yw1-f176.google.com [209.85.128.176]) by imf23.hostedemail.com (Postfix) with ESMTP id D76601400DE for ; Fri, 20 May 2022 16:17:19 +0000 (UTC) Received: by mail-yw1-f176.google.com with SMTP id 00721157ae682-2fed823dd32so91940017b3.12 for ; Fri, 20 May 2022 09:17:38 -0700 (PDT) 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=ONhZlVBI4HpQAKHE0J0FctRsQVKq+x2f4PkT65aBxxg=; b=dmedB56pvgqE7vaxmJFTfngnHOclw61WJgI459OreAQ/EWhACPq1JJTl+tBexbZd3/ clCCW9wARLONeh60N3DHvB3Y1BQSNmngLUMTLBYkUFbVhQ6Zz3TAdZGwd7Tln+nw7vlY EKu+sLeAsRK/llsvB/Yx1bKeOwOhQEFTd1IoLVIkMHpkjORtv2JjeJV7qfRNvyIkyGHN JRCl3PyDvndsxDY2oDtf4qdb8KvcbW1aDVsv71H918s88KQnwhUnG1Jape1XL5qZ/FaG q93yLWhQIvyuE5w/PihuBY6NuvMO0Dj6/jwlAWaDYA9KwYqGc88G1WBjRWJcaX1aRoMs NGEw== 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=ONhZlVBI4HpQAKHE0J0FctRsQVKq+x2f4PkT65aBxxg=; b=fUsU84sUGTO3MVURDY/cdLBmRCpaB/BrEoXlfpIk/B4HkI9XqOMyEtmXPs8+qoP4pw e+6/zr5hhV6WLZBZCPefRRYucmPbQjsDbvfMB4t0WaDYeoxkNc2pU66B8Fq2k7M0W5uO 0zC5jTsrvqwc+Ml7G1tWbMlIe/DcRf0s8R7Wn+8l3CtWe+6dZ3DE2RB44u0Ro+jQHm5p uXUALQ+/i7cTRWBuIM8EieVCDwfp62IBLWxhGMZ+e428gduh6PYAN3+2YA4gaOzx/FYf qz3Znk34BX9OX1B88UXl65g9dYYHO636QtIX2UeHqAROb00tMZa7ZP1z9AMkr6MXpE/G FHAg== X-Gm-Message-State: AOAM5303XAkFbqTbsToH2fu/ifwy4NKxayQvRygdVemkciNIloVeHIWx HRaP1YFI78t8KP7RBmVK/JoEcf4TEE1IDYTK8QsXBw== X-Google-Smtp-Source: ABdhPJxejo+Y7HD7oOOqHmw/rRbhZxiq5xDx278bG4dQMkXbrxvg4BAUzigZhBuAL0J1/93WzXPhH+PrBEck7JSuTNI= X-Received: by 2002:a81:557:0:b0:2ff:a20c:4bfc with SMTP id 84-20020a810557000000b002ffa20c4bfcmr969269ywf.293.1653063457607; Fri, 20 May 2022 09:17:37 -0700 (PDT) MIME-Version: 1.0 References: <20220516075619.1277152-1-surenb@google.com> <20220519202149.3ywynqhbxlzp6uyn@revolver> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 20 May 2022 09:17:26 -0700 Message-ID: Subject: Re: [PATCH v2 1/2] mm: drop oom code from exit_mmap To: Michal Hocko Cc: Liam Howlett , "akpm@linux-foundation.org" , "rientjes@google.com" , "willy@infradead.org" , "hannes@cmpxchg.org" , "guro@fb.com" , "minchan@kernel.org" , "kirill@shutemov.name" , "aarcange@redhat.com" , "brauner@kernel.org" , "hch@infradead.org" , "oleg@redhat.com" , "david@redhat.com" , "jannh@google.com" , "shakeelb@google.com" , "peterx@redhat.com" , "jhubbard@nvidia.com" , "shuah@kernel.org" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "kernel-team@android.com" Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Queue-Id: D76601400DE X-Stat-Signature: omnw9adw1afost813yf593pzrsqjpb7k Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=dmedB56p; spf=pass (imf23.hostedemail.com: domain of surenb@google.com designates 209.85.128.176 as permitted sender) smtp.mailfrom=surenb@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam03 X-HE-Tag: 1653063439-293958 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, May 20, 2022 at 8:55 AM Suren Baghdasaryan wrote: > > On Fri, May 20, 2022 at 12:21 AM Michal Hocko wrote: > > > > On Thu 19-05-22 14:33:03, Suren Baghdasaryan wrote: > > > On Thu, May 19, 2022 at 1:22 PM Liam Howlett wrote: > > [...] > > > > arch_exit_mmap() was called under the write lock before, is it safe to > > > > call it under the read lock? > > > > > > Ah, good catch. I missed at least one call chain which I believe would > > > require arch_exit_mmap() to be called under write lock: > > > > > > arch_exit_mmap > > > ldt_arch_exit_mmap > > > free_ldt_pgtables > > > free_pgd_range > > > > Why would be this a problem? This is LDT mapped into page tables but as > > far as I know oom_reaper cannot really ever see that range because it is > > not really reachable from any VMA. > > Ah, thanks! I didn't realize these page tables are not reachable from > VMAs. The only other call that I'm not sure is ok without mmap write > lock is xen_hvm_exit_mmap: > > arch_exit_mmap > paravirt_arch_exit_mmap > xen_hvm_exit_mmap > > I'll look closer today but if someone can confirm it's safe then my > current patch should be fine as is. My conclusion is that it's safe to call arch_exit_mmap without exclusive mmap lock since the only possible competition is from OOM-killer/process_mrelease which operate on mm->mmap and none of the arch_exit_mmap implementations use mm->mmap. Andrew, sorry for going back and forth. I think the patch is fine as is and can be integrated. Thanks! > Thanks, > Suren. > > > > > > I'll need to check whether arch_exit_mmap() has to be called before > > > unmap_vmas(). If not, we could move it further down when we hold the > > > write lock. > > > Andrew, please remove this patchset from your tree for now until I fix this. > > > > > > > > > > > > > > > > > vma = mm->mmap; > > > > > if (!vma) { > > > > > /* Can happen if dup_mmap() received an OOM */ > > > > > - mmap_write_unlock(mm); > > > > > + mmap_read_unlock(mm); > > > > > return; > > > > > } > > > > > > > > > > @@ -3138,6 +3121,16 @@ void exit_mmap(struct mm_struct *mm) > > > > > /* update_hiwater_rss(mm) here? but nobody should be looking */ > > > > > /* Use -1 here to ensure all VMAs in the mm are unmapped */ > > > > > unmap_vmas(&tlb, vma, 0, -1); > > > > > + mmap_read_unlock(mm); > > > > > + > > > > > + /* > > > > > + * Set MMF_OOM_SKIP to hide this task from the oom killer/reaper > > > > > + * because the memory has been already freed. Do not bother checking > > > > > + * mm_is_oom_victim because setting a bit unconditionally is cheaper. > > > > > + */ > > > > > + set_bit(MMF_OOM_SKIP, &mm->flags); > > > > > + > > > > > + mmap_write_lock(mm); > > > > > > > > Is there a race here? We had a VMA but after the read lock was dropped, > > > > could the oom killer cause the VMA to be invalidated? > > > > Nope, the oom killer itself doesn't do much beyond sending SIGKILL and > > scheduling the victim for the oom_reaper. dup_mmap is holding exclusive > > mmap_lock throughout the whole process. > > > > > > I don't think so > > > > but the comment above about dup_mmap() receiving an OOM makes me > > > > question it. The code before kept the write lock from when the VMA was > > > > found until the end of the mm edits - and it had the check for !vma > > > > within the block itself. We are also hiding it from the oom killer > > > > outside the read lock so it is possible for oom to find it in that > > > > window, right? > > > > The oom killer's victim selection doesn't really depend on the > > mmap_lock. If there is a race and MMF_OOM_SKIP is not set yet then it > > will consider the task and very likely bail out anyway because the > > address space has already been unampped so oom_badness() would consider > > this task boring. > > > > oom_reaper on the other hand would just try to unmap in parallel but > > that is fine regardless of MMF_OOM_SKIP. Seeing the flag would allow to > > bail out early rather than just trying to unmap something that is no > > longer there. The only problem for the oom_reaper is to see page tables > > of the address space disappearing from udner its feet. That is excluded > > by the the exlusive lock and as Suren mentions mm->mmap == NULL check > > if the exit_mmap wins the race. > > -- > > Michal Hocko > > SUSE Labs