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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (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 1BC6CC48BC3 for ; Mon, 19 Feb 2024 15:19:16 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4TdmR63Myrz3dTp for ; Tue, 20 Feb 2024 02:19:14 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=kernel.org (client-ip=145.40.73.55; helo=sin.source.kernel.org; envelope-from=cmarinas@kernel.org; receiver=lists.ozlabs.org) Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4TdmQb18b1z2ykt for ; Tue, 20 Feb 2024 02:18:47 +1100 (AEDT) Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sin.source.kernel.org (Postfix) with ESMTP id 51CD9CE127E; Mon, 19 Feb 2024 15:18:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 179F3C433F1; Mon, 19 Feb 2024 15:18:35 +0000 (UTC) Date: Mon, 19 Feb 2024 15:18:33 +0000 From: Catalin Marinas To: Ryan Roberts Subject: Re: [PATCH v6 12/18] arm64/mm: Wire up PTE_CONT for user mappings Message-ID: References: <20240215103205.2607016-1-ryan.roberts@arm.com> <20240215103205.2607016-13-ryan.roberts@arm.com> <892caa6a-e4fe-4009-aa33-0570526961c5@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <892caa6a-e4fe-4009-aa33-0570526961c5@arm.com> X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Kefeng Wang , x86@kernel.org, David Hildenbrand , Yang Shi , Dave Hansen , linux-mm@kvack.org, Andrey Ryabinin , "H. Peter Anvin" , Will Deacon , Ard Biesheuvel , Marc Zyngier , Alistair Popple , Barry Song <21cnbao@gmail.com>, Matthew Wilcox , Ingo Molnar , Zi Yan , John Hubbard , Borislav Petkov , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, James Morse , Andrew Morton , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Fri, Feb 16, 2024 at 12:53:43PM +0000, Ryan Roberts wrote: > On 16/02/2024 12:25, Catalin Marinas wrote: > > On Thu, Feb 15, 2024 at 10:31:59AM +0000, Ryan Roberts wrote: > >> +pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) > >> +{ > >> + /* > >> + * Gather access/dirty bits, which may be populated in any of the ptes > >> + * of the contig range. We may not be holding the PTL, so any contiguous > >> + * range may be unfolded/modified/refolded under our feet. Therefore we > >> + * ensure we read a _consistent_ contpte range by checking that all ptes > >> + * in the range are valid and have CONT_PTE set, that all pfns are > >> + * contiguous and that all pgprots are the same (ignoring access/dirty). > >> + * If we find a pte that is not consistent, then we must be racing with > >> + * an update so start again. If the target pte does not have CONT_PTE > >> + * set then that is considered consistent on its own because it is not > >> + * part of a contpte range. > >> +*/ [...] > > After writing the comments above, I think I figured out that the whole > > point of this loop is to check that the ptes in the contig range are > > still consistent and the only variation allowed is the dirty/young > > state to be passed to the orig_pte returned. The original pte may have > > been updated by the time this loop finishes but I don't think it > > matters, it wouldn't be any different than reading a single pte and > > returning it while it is being updated. > > Correct. The pte can be updated at any time, before after or during the reads. > That was always the case. But now we have to cope with a whole contpte block > being repainted while we are reading it. So we are just checking to make sure > that all the ptes that we read from the contpte block are consistent with > eachother and therefore we can trust that the access/dirty bits we gathered are > consistent. I've been thinking a bit more about this - do any of the callers of ptep_get_lockless() check the dirty/access bits? The only one that seems to care is ptdump but in that case I'd rather see the raw bits for debugging rather than propagating the dirty/access bits to the rest in the contig range. So with some clearer documentation on the requirements, I think we don't need an arm64-specific ptep_get_lockless() (unless I missed something). -- Catalin