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 A23ACC433F5 for ; Fri, 20 May 2022 15:55:33 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 1467B6B0071; Fri, 20 May 2022 11:55:33 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0F6036B0072; Fri, 20 May 2022 11:55:33 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id EFEFF6B0073; Fri, 20 May 2022 11:55:32 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id E1F036B0071 for ; Fri, 20 May 2022 11:55:32 -0400 (EDT) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id B433A1909 for ; Fri, 20 May 2022 15:55:32 +0000 (UTC) X-FDA: 79486571304.24.B1F38C5 Received: from mail-yb1-f173.google.com (mail-yb1-f173.google.com [209.85.219.173]) by imf01.hostedemail.com (Postfix) with ESMTP id 3EF0B40018 for ; Fri, 20 May 2022 15:55:31 +0000 (UTC) Received: by mail-yb1-f173.google.com with SMTP id f16so14906035ybk.2 for ; Fri, 20 May 2022 08:55:31 -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=qqIlZXHciqSCzGv9K20w6MWTF8E6ifVRlg9P+8G2gKQ=; b=oo+1ocxByaJe982/04okSYCPxdX1W9blLgrX8Fa21vxgElGzE6B/yMLLTUByoo+WPH mBz3f2Ei8URAy3xNduTdzO3blo8zIiPHHdcd5gUVscNDg8M+gjUK8wyHJ+nC1xMC7vD3 pHj+bux8qQeKOobC8z6FBxyc7L1yqqMSwvSKQ2uONAGqa6qMzMh1X8ru9lq06mItoKR7 hCR3d4ZqrzHIfEzYws3kzrcYj0HlTKNkVJXXwcvW4zvbKqxF6DrgANf8ykl3p80CDf6a IhcXnywNslbNPvPRJftRgZDNh/+P7FHemqI9Q4aXoYdjrAMc/0cFzN3mwZuLSxF6bp/O KxFQ== 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=qqIlZXHciqSCzGv9K20w6MWTF8E6ifVRlg9P+8G2gKQ=; b=DNj45L9y1Hu/h4BFF96JzJ8/Kfi1IQ8HNwNHK5AZfnYR5z4+ImqVnBOj33/rQ9Z01G 9Wpi5r5jOa2EczJxqGXYBYDMTmV1L7KdU16Ax1fhH7EBpsSc5C0lSgqcGUju8u35Qg9w ld9kyj9ANsN5m+mLaSWd0rMjzcEftP3thLxhwvk2IsLRdpsK/NN60OgLgSgI5q1lZBIL R4cqWyGughXm8YdLSk7j3xlGLr9tUuQczVwN16eO3kaizRFw7Y5DL1JTFUJ4O+31pBB9 xbPKg+bl3sp95hTCfEg+dtxS4Sr+oWvsiGQQA1lgz9GocQv/Uk1VL1RyXZkUmGWwErUs AxXg== X-Gm-Message-State: AOAM533XRm0hHF6cqEjjoHmZrUUM1LMp+U49I/P77ZMHDmrO5shGoAxH LU6NaM/bierVpq+v4MxRQSK1Lus/j0uLKevMtdSH2w== X-Google-Smtp-Source: ABdhPJzZKZf5fVNn8AONoaSu4ult8oDmS/7X1ySGgimMas0oBwgScpufr6Vecwc4UrWAnzsit/xWKSLeGfwXerC//yE= X-Received: by 2002:a25:1c0b:0:b0:64d:6b11:6a32 with SMTP id c11-20020a251c0b000000b0064d6b116a32mr10536038ybc.441.1653062131138; Fri, 20 May 2022 08:55:31 -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 08:55:20 -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-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3EF0B40018 X-Stat-Signature: knsaxa44czjdzixki1iiottdjfd9gcgf Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=oo+1ocxB; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf01.hostedemail.com: domain of surenb@google.com designates 209.85.219.173 as permitted sender) smtp.mailfrom=surenb@google.com X-Rspam-User: X-HE-Tag: 1653062131-937638 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 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. 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