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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 64429CD4F52 for ; Tue, 19 May 2026 08:00:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=RtwfT6r39WqU1vxAeKYG9k5uq6DHBy2YCymS3YntXaw=; b=VSL2OiwtFoDobeCIU0tnqirI8e 9audbbahn7Q+l6METOKlvhasZDi9mK8ToCEtsuGyvuQCfbTAA2XGxiqctMJTvg5RgQaYRYLDHXQ1O pdR+0xxbODLQ/CFNgYXs/oKKZYVrlU0y8CPFBlUhN73OGwtw1T98Jbs2sk6Fvmxjk+zyADS809yul 1NQfLBgIjs860mq91bVb+xc5DxavGJ/yhQ2a/Q4OduoBYVtZkNZE3y+diMw1J14hDzTNAnlz6CXEo 8WmE7EPl8ATeBp86NRPheuaVrRxtvhUcuxj/EKt4S52sT94OoDuPXkpIyVpx7FHaGeEKx1tXP6Ykv FiEf9hRg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wPFNj-00000000ap3-1hZI; Tue, 19 May 2026 08:00:51 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wPFNi-00000000aod-17Qg for linux-arm-kernel@lists.infradead.org; Tue, 19 May 2026 08:00:50 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 781A360208; Tue, 19 May 2026 08:00:49 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 27FD0C2BCB3; Tue, 19 May 2026 08:00:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1779177649; bh=jq+Ic2o6+/T6R+6rKPEYUVCfMRGyHOvp7RzEZ9FUseg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=I7vznwgLR82bFg5okwmiS44hagsTw2fpDtHv0dip23rZYyVv4Z5pV0JENKZenbEPY caLAFtNgxAyhxPdG12x510QK9CG3TrZLhkbICxRov/OQDkC0s4u377E1oMvrrXwpD0 3EKxDFcB9Ep5YgHVziesBC27Eb10HYleQgaos3KsBrEEXh4kLkX4il/gSUs5ieJWF2 xTbsEghuYQZwcY1VgRfOFjtAQnl34fSLoNXrbJE8TSkf8FQx2ojuS/rT2+Y61bn+G/ X5e2Fd3WrZEKYAfmyYW26ySv7pM3wQ/gnB8Wl460a8i+L59que2/pHmk3jQl+hA1Vi v08NGy3tgOARA== Message-ID: <2f02d90d-cdc9-48ef-abe3-99e00f22595f@kernel.org> Date: Tue, 19 May 2026 10:00:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/8] mm: Add ptep_try_install() for lockless empty-slot installs To: Tejun Heo Cc: David Vernet , Andrea Righi , Changwoo Min , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Andrew Morton , Mike Rapoport , Emil Tsalapatis , sched-ext@lists.linux.dev, bpf@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260517211232.1670594-1-tj@kernel.org> <20260517211232.1670594-2-tj@kernel.org> <9ba50fd2-077e-4291-9276-9adb18186873@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 5/18/26 21:53, Tejun Heo wrote: > Hello, > > On Mon, May 18, 2026 at 10:06:29AM +0200, David Hildenbrand (Arm) wrote: >> On 5/17/26 23:12, Tejun Heo wrote: >>> +static inline bool ptep_try_install(pte_t *ptep, pte_t new_pte) >>> +{ >>> + return false; >>> +} >>> +#endif >> >> Ehm, what? >> >> This is a very, very, very bad generic idea/interface. >> >> On which ptes is this supposed to be used? User ptes or kernel ptes? >> >> Surely we don't want this on user ptes. > > Yeah, this is only for the BPF arena PTEs which are already managed in their > own way. I'd be happy to place / gate it however appropriate. So, we only use it within apply_to_page_range() with init_mm, where we don't use page table locks. Usually we seem to grab the &arena->spinlock to protect the kernel page tables. And you're saying that we might get called from a page fault handler where that lock can already be held. Is that really possible? I'd much rather prefer to trylock and retry, unless that can really result in deadlocks. But I have the feeling that such deadlocks should be impossible here. Can you elaborate on the deadlock situation and how that can happen in valid scenarios? The thing is that ptep_try_install() relies on other page table walking and insertion code to do the right thing. Shaky. For example, staring at apply_range_set_cb(), what prevents: (1) apply_range_set_cb() finding pte_none(ptep_get(pte) (2) apply_range_set_scratch_cb() succeeding ptep_try_install() (3) apply_range_set_cb() overwriting the pte with set_pte_at() Between (2) and (3) CPUs could access the scratch PTE. -- Cheers, David