From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 479912C859; Wed, 4 Feb 2026 21:45:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770241545; cv=none; b=CbQSDEm27eYPaKTVUKdNFaaAFS0jvSSG0G2uwPXxcGlgiMI6DQFb5fTVjOSfr7HsOMye5TngtCPL1TokEqTWQS++9tSmbySHFuXDTOJ+VNeXUyGjetmMep9RM4QgnE8A0td2HjNsCzscSLjNMdy2Wnzbasc2V6RLYxQMjmP+62E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770241545; c=relaxed/simple; bh=T6VunRLRhUokHR96PR2K9QEe7u1JQv9PsMwPfT1O/zM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=plOT+MnEOfMfo2J7yR7wtfiCQQSWBNCgKeF0LFzlueUdVpMyTzTi9mIjvmgqV0y5iT/Cp0lfygbta4atnjk0r88z6fVy/EzLPmZmbVTpSMbLbImuiIVQvUhTGF8YZJlF10v4z//Dk8rAysRAbFFu5soycgv5VnpZzw/UFnmdnyQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gwupwBD4; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gwupwBD4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1002DC4CEF7; Wed, 4 Feb 2026 21:45:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770241544; bh=T6VunRLRhUokHR96PR2K9QEe7u1JQv9PsMwPfT1O/zM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=gwupwBD4a46yby1AEJaJ55CTKEz9vPRUj2PD5w+THn7OXVOTgjRpIx3piaawh3eHa LTy2RBIBE+FN0hU2pbAuEgv/oVYUGcgO2SUbYzgcGruNxRpeu3wZZ6pL7NlU9D7CGE x51t/VcOO3MgmiRtd+TC9M0l0EuZvuLnHaDSyP+w2wvIFxks0CQdpiJd7o0TW5sH6Q nbIU9n3y15FOyu7bn8QqM3YthsMMIS0AVNuDr2wRFwMFt+bNYVf2pZ+vsbW+4ZbBnX nARbP1zYZUKS/ynHMzq/i/k7NSz5/1N3yHH3+QUEY4EpZkZV9SqEtVR+CfWRwDpYJF Ft95csc7W9fTA== Message-ID: <16e5a36e-fff0-4a54-9c5c-a8e411659108@kernel.org> Date: Wed, 4 Feb 2026 22:45:40 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] KVM: guest_memfd: Disable VMA merging with VM_DONTEXPAND To: Sean Christopherson , Ackerley Tng Cc: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, syzkaller-bugs@googlegroups.com, michael.roth@amd.com, vannapurve@google.com, kartikey406@gmail.com References: <697d115a.050a0220.1d61ec.0004.GAE@google.com> <20260204170144.2904483-1-ackerleytng@google.com> From: "David Hildenbrand (arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/4/26 22:37, Sean Christopherson wrote: > On Wed, Feb 04, 2026, Ackerley Tng wrote: >> Ackerley Tng writes: >> >>> #syz test: git://git.kernel.org/pub/scm/virt/kvm/kvm.git next >>> >>> guest_memfd VMAs don't need to be merged, > > Why not? There are benefits to merging VMAs that have nothing to do with folios. > E.g. map 1GiB of guest_memfd with 512*512 4KiB VMAs, and then it becomes quite > desirable to merge all of those VMAs into one. > > Creating _hugepages_ doesn't add value, but that's not the same things as merging > VMAs. > >>> especially now, since guest_memfd only supports PAGE_SIZE folios. >>> >>> Set VM_DONTEXPAND on guest_memfd VMAs. >> >> Local tests and syzbot agree that this fixes the issue identified. :) >> >> I would like to look into madvise(MADV_COLLAPSE) and uprobes triggering >> mapping/folio collapsing before submitting a full patch series. >> >> David, Michael, Vishal, what do you think of the choice of setting >> VM_DONTEXPAND to disable khugepaged? > > I'm not one of the above, but for me it feels very much like treating a symptom > and not fixing the underlying cause. And you are spot-on :) > > It seems like what KVM should do is not block one path that triggers hugepage > processing, but instead flat out disallow creating hugepages. Unfortunately, > AFAICT, there's no existing way to prevent madvise() from clearing VM_NOHUGEPAGE, > so we can't simply force that flag. > > I'd prefer not to special case guest_memfd, a la devdax, but I also want to address > this head-on, not by removing a tangentially related trigger. VM_NOHUGEPAGE also smells like the wrong thing. This is a file limitation. !thp_vma_allowable_order() must take care of that somehow down in __thp_vma_allowable_orders(), by checking the file). Likely the file_thp_enabled() check is the culprit with CONFIG_READ_ONLY_THP_FOR_FS? Maybe we need a flag to say "even not CONFIG_READ_ONLY_THP_FOR_FS". I wonder how we handle that for secretmem. Too late for me, going to bed :) -- Cheers, David