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 B1AD7C4829A for ; Tue, 13 Feb 2024 15:30:30 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4TZ4ys30qpz3dfM for ; Wed, 14 Feb 2024 02:30:29 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=ryan.roberts@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4TZ4yP4Dm1z3c8Y for ; Wed, 14 Feb 2024 02:30:02 +1100 (AEDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 19912DA7; Tue, 13 Feb 2024 07:30:11 -0800 (PST) Received: from [10.1.36.184] (XHFQ2J9959.cambridge.arm.com [10.1.36.184]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 37F103F7B4; Tue, 13 Feb 2024 07:29:26 -0800 (PST) Message-ID: Date: Tue, 13 Feb 2024 15:29:24 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings Content-Language: en-GB To: David Hildenbrand , Mark Rutland References: <20240202080756.1453939-1-ryan.roberts@arm.com> <20240202080756.1453939-20-ryan.roberts@arm.com> <502a3ea7-fd86-4314-8292-c7999eda92eb@arm.com> <427ba87a-7dd0-4f3e-861f-fe6946b7cd97@redhat.com> <55a1e0ef-14b3-4311-b2aa-a6add76fa2ed@redhat.com> From: Ryan Roberts In-Reply-To: <55a1e0ef-14b3-4311-b2aa-a6add76fa2ed@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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: Kefeng Wang , x86@kernel.org, Catalin Marinas , 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 , "Aneesh Kumar K.V" , Ingo Molnar , Zi Yan , "Naveen N. Rao" , John Hubbard , Nicholas Piggin , 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 12/02/2024 16:24, David Hildenbrand wrote: > On 12.02.24 16:34, Ryan Roberts wrote: >> On 12/02/2024 15:26, David Hildenbrand wrote: >>> On 12.02.24 15:45, Ryan Roberts wrote: >>>> On 12/02/2024 13:54, David Hildenbrand wrote: >>>>>>> If so, I wonder if we could instead do that comparison modulo the >>>>>>> access/dirty >>>>>>> bits, >>>>>> >>>>>> I think that would work - but will need to think a bit more on it. >>>>>> >>>>>>> and leave ptep_get_lockless() only reading a single entry? >>>>>> >>>>>> I think we will need to do something a bit less fragile. ptep_get() does >>>>>> collect >>>>>> the access/dirty bits so its confusing if ptep_get_lockless() doesn't >>>>>> IMHO. So >>>>>> we will likely want to rename the function and make its documentation >>>>>> explicit >>>>>> that it does not return those bits. >>>>>> >>>>>> ptep_get_lockless_noyoungdirty()? yuk... Any ideas? >>>>>> >>>>>> Of course if I could convince you the current implementation is safe, I >>>>>> might be >>>>>> able to sidestep this optimization until a later date? >>>>> >>>>> As discussed (and pointed out abive), there might be quite some callsites >>>>> where >>>>> we don't really care about uptodate accessed/dirty bits -- where ptep_get() is >>>>> used nowadays. >>>>> >>>>> One way to approach that I had in mind was having an explicit interface: >>>>> >>>>> ptep_get() >>>>> ptep_get_uptodate() >>>>> ptep_get_lockless() >>>>> ptep_get_lockless_uptodate() >>>> >>>> Yes, I like the direction of this. I guess we anticipate that call sites >>>> requiring the "_uptodate" variant will be the minority so it makes sense to use >>>> the current names for the "_not_uptodate" variants? But to do a slow migration, >>>> it might be better/safer to have the weaker variant use the new name - that >>>> would allow us to downgrade one at a time? >>> >>> Yes, I was primarily struggling with names. Likely it makes sense to either have >>> two completely new function names, or use the new name only for the "faster but >>> less precise" variant. >>> >>>> >>>>> >>>>> Especially the last one might not be needed. >>>> I've done a scan through the code and agree with Mark's original conclusions. >>>> Additionally, huge_pte_alloc() (which isn't used for arm64) doesn't rely on >>>> access/dirty info. So I think I could migrate everything to the weaker variant >>>> fairly easily. >>>> >>>>> >>>>> Futher, "uptodate" might not be the best choice because of PageUptodate() and >>>>> friends. But it's better than "youngdirty"/"noyoungdirty" IMHO. >>>> >>>> Certainly agree with "noyoungdirty" being a horrible name. How about "_sync" / >>>> "_nosync"? >>> >>> I could live with >>> >>> ptep_get_sync() >>> ptep_get_nosync() >>> >>> with proper documentation :) >> >> but could you live with: >> >> ptep_get() >> ptep_get_nosync() >> ptep_get_lockless_nosync() >> >> ? >> >> So leave the "slower, more precise" version with the existing name. > > Sure. > I'm just implementing this (as a separate RFC), and had an alternative idea for naming/semantics: ptep_get() ptep_get_norecency() ptep_get_lockless() ptep_get_lockless_norecency() The "_norecency" versions explicitly clear the access/dirty bits. This is useful for the "compare to original pte to check we are not racing" pattern: pte = ptep_get_lockless_norecency(ptep) ... if (!pte_same(pte, ptep_get_norecency(ptep))) // RACE! ... With the "_nosync" semantic, the access/dirty bits may or may not be set, so the user has to explicitly clear them to do the comparison. (although I considered a pte_same_nosync() that would clear the bits for you - but that name is pretty naff). Although the _norecency semantic requires always explicitly clearing the bits, so may be infinitesimally slower, it gives a very clear expectation that the access/dirty bits are always clear and I think that's conveyed well in the name too. Thoughts?