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 60299C433EF for ; Tue, 7 Dec 2021 10:10:22 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id D88236B00A9; Tue, 7 Dec 2021 05:10:11 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id D378F6B00AA; Tue, 7 Dec 2021 05:10:11 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BFEAE6B00AB; Tue, 7 Dec 2021 05:10:11 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0055.hostedemail.com [216.40.44.55]) by kanga.kvack.org (Postfix) with ESMTP id AD3106B00A9 for ; Tue, 7 Dec 2021 05:10:11 -0500 (EST) Received: from smtpin13.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 6CA461BAF9FBB for ; Tue, 7 Dec 2021 10:10:01 +0000 (UTC) X-FDA: 78890577402.13.2A9BCD5 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by imf08.hostedemail.com (Postfix) with ESMTP id E60803000098 for ; Tue, 7 Dec 2021 10:10:00 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id A974D1FD2F; Tue, 7 Dec 2021 10:09:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1638871799; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Lfsjs8k6aVAvDBSMSsv9epTpHubEEZQUmQTxDrYjVW8=; b=sCopTl9C7JY48GuMafPEE5asyJ/ioU/Kv8o+Ek/Mimy0Qgph6G4OJ56S7Fcpmoswgubgh+ IbG6dg/wuKVHdvZR522xnp+1I+CoriW/ldCqlKhy3N7hKQO7yzdN2Ib9Qg07jv8h7+BNx2 z8LpMefUeVVwwOMSdFvjYsZvRXSKu40= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 709E8A3B87; Tue, 7 Dec 2021 10:09:59 +0000 (UTC) Date: Tue, 7 Dec 2021 11:09:59 +0100 From: Michal Hocko To: Suren Baghdasaryan 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 Subject: Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap Message-ID: References: <20211124235906.14437-1-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: E60803000098 X-Stat-Signature: i88smbixsgarhtkkrcroa78rfedi37mi Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=sCopTl9C; spf=pass (imf08.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.29 as permitted sender) smtp.mailfrom=mhocko@suse.com; dmarc=pass (policy=quarantine) header.from=suse.com X-HE-Tag: 1638871800-552925 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 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. > > 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. -- Michal Hocko SUSE Labs