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 D8A56C433F5 for ; Tue, 7 Dec 2021 16:50:37 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 477256B00AA; Tue, 7 Dec 2021 11:50:27 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 3FD936B00AB; Tue, 7 Dec 2021 11:50:27 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2509F6B00AC; Tue, 7 Dec 2021 11:50:27 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0040.hostedemail.com [216.40.44.40]) by kanga.kvack.org (Postfix) with ESMTP id 0F57F6B00AA for ; Tue, 7 Dec 2021 11:50:27 -0500 (EST) Received: from smtpin07.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id C6667894C2 for ; Tue, 7 Dec 2021 16:50:16 +0000 (UTC) X-FDA: 78891586032.07.0555FFD Received: from mail-yb1-f171.google.com (mail-yb1-f171.google.com [209.85.219.171]) by imf27.hostedemail.com (Postfix) with ESMTP id 61071700009E for ; Tue, 7 Dec 2021 16:50:16 +0000 (UTC) Received: by mail-yb1-f171.google.com with SMTP id v203so42508730ybe.6 for ; Tue, 07 Dec 2021 08:50:16 -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=Coee/ZPuNSVySMXoX42VaPVzCeLKQsHWecglETbTUjs=; b=FwdeANTcQCdcMtKb07Jo5JVa2I7xGWSfrOieceySp/BgWpC645eRgjwctWMOmTjl2+ lRNOpacBvqvdBOzzug95tj1BTnC3yVmaPilJyZ13goMcI9xx4wANJbDfLUbTJWcr99zU mvqCi1ybyqLuoFFI4u78/1rlnbwKAMf7lB81K4+NkJcioCWPJVNebdOdFYEb5UDPefbw 1ztIak4J2YBT7ZCOaHF/G01xiNz2g/mzbT3cM9UrGw9l5KZK9u2a/33eyj0sVmT3EJyl 65v7Ju28jqwCxrV765ZNv/vmIXjvRamunMkYebDOF943qCzJUSeVRyWwMxptvv+TgMGD Y8Jg== 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=Coee/ZPuNSVySMXoX42VaPVzCeLKQsHWecglETbTUjs=; b=DFAj/kr20zrnJyTp0MgU78GdNAULjxuBznHBvB/8n4gxbCJN/Z/JV7ifGhRGH1EC05 MBeys8HLKZ10kEJKLnBa/QtXLh4etzEhRHoVPeeoIljLk46C4yNmtN1xCnmhbk9QOlaM pp4Nv+RDEFRhf2IexIgebHgwOyG6iwAiZ/z6cpBUNUOi7Dmg1A7W5t0+Y/75ITW0HyPu ldYzif31AX0HGxZzK2c+Flh7oldr1ouKxjHT2LZ06+OzJeF8rh1uyKZdfP9b5sF+tP5i TIxOQC80N3SYYSCGabG9e3ByzEUTn6feBIB0dnnmuIkZZdb6Bcq7BIXj78WbscXs6mTe am0g== X-Gm-Message-State: AOAM532vMe2ppYP2Sd+mmrz68GTDTWqF7+coqrePB4v9zkAijxZmhOFm lfMKSWKkF0YOL2IGd0v3S+/ylup/6hfezqrqhS32+g== X-Google-Smtp-Source: ABdhPJy8WYKqDTiM71KToLHA3KaZzhA9PEA2rdqcL15s+hZPvYSLPvyCNAqwdaum6x6MU+qhdUsoOgnKFI47oYJYLbQ= X-Received: by 2002:a25:6ec5:: with SMTP id j188mr52452546ybc.602.1638895815343; Tue, 07 Dec 2021 08:50:15 -0800 (PST) MIME-Version: 1.0 References: <20211124235906.14437-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Tue, 7 Dec 2021 08:50:04 -0800 Message-ID: Subject: Re: [PATCH v2 1/2] 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" X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 61071700009E X-Stat-Signature: fu4j8r6zctefw64dtihfgaypgir7wwbf Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=FwdeANTc; spf=pass (imf27.hostedemail.com: domain of surenb@google.com designates 209.85.219.171 as permitted sender) smtp.mailfrom=surenb@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1638895816-438629 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 Tue, Dec 7, 2021 at 2:10 AM Michal Hocko wrote: > > On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote: > > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko wrote: > > > > > > On Wed 24-11-21 15:59:05, 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. 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. Note > > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > > > > should not be allowed to race with it. > > > > 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 > > > > --- > > > > changes in v2 > > > > - Moved mmap_write_unlock to cover remove_vma loop as well, 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); > > > > > > Why do you keep this in place? > > > > Sorry for the delay, I was out last week. > > I missed your comment about removing MMF_OOM_SKIP at > > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz > > I'll look into removing it in a separate patch, which I think would be cleaner. > > The point of this code was to sync up the oom_repaer and exit_mmap. Now > that your patch uses proper locking for that to happen then MMF_OOM_SKIP > is not really necessary. IIRC all you need to guarantee is that the vma > tree is empty when exit_mmap does all its work - i.e set mm->mmap to > NULL. You can do that after remove_vma loop but it would be equally safe > at any time after vma = mm->mmap as the loop relies on the vma chain. > Doing that after would be slightly nicer if you ask me. Will do. But if you don't mind I'll post the removal of MMF_OOM_SKIP as a separate patch. This patchset has already been extensively tested and it will be easier for me to test MMF_OOM_SKIP removal separately. > > > > Other than that looks OK to me. Maybe we want to add an explicit note > > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > > should also mention that you have considered remove_vma and its previous > > > no MM locking assumption. You can argue that fput is async and close > > > callback shouldn't really need mmap_sem. > > > > Should I post another version of this patch with the patch description > > clarifying these points and additional comments as you suggested? > > Yes please. Will re-post. So, to clarify, we want: - Patch description to include explanation that remove_vma is now being called under MM lock but this should not be a problem because fput and vm_ops->close do not and should not take mmap_sem. - Add a comment for vm_ops->close that the callback should not take mmap_sem, with explanation that __split_vma and exit_mmap use this callback with the mmap_sem write lock taken. Is that correct? > > -- > Michal Hocko > SUSE Labs