From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 6E7423B05AB; Fri, 31 Jul 2026 14:09:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785506976; cv=none; b=Vxlc7g8d8cIxxfpmZPK/xYes0T7ya49emh0LPWsFOvE8CF53du6Ufyubl8DCAzacUKDQ3SPdrbqvVIWiTT8CepZUtVjH0duEBhs9qbPXUMbdDVQvWZNzpW6T9TC75Z2/RucdIftKuLej6ucl70BaNjhmZDeV9NyhnNn961BCwFk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785506976; c=relaxed/simple; bh=UnQ9KLFzpH85iNns8RJwyWUv6nsgrfZ1j6W+9EkGkks=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IxlcgbpI20MBHFd+C9BL/8jsdHH0D/GibREvujN+yp5XQ6zq1E065/m9e/DwW9RcRoAaEYyCLJQ5B+hMe7S6esOG7Sitl/euxXQGmxsm3+eyzHVeuG93GZwP/JSog1LK/279BjOQA6ITNI6j+w9VbLDMXkhkzaUAnj+y29oMNcI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Sma4NySL; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Sma4NySL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 540F41F000E9; Fri, 31 Jul 2026 14:09:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785506975; bh=ru6sr9Ag52v+hywgrI7YJPD9LSqOe1cCd8UnQArdNRU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Sma4NySLrk2LpeqI8MxMincsPL/fzm3kYiTO8kNADhS+us0NITNzTIGxCmkU+N4sQ YrbO9rn6nonWncSBSuz8cZR3dPHFpnyzkvmco3JxwQW2dl09xKaLBWwPEO/xqwTsp7 cMmYUDQVEW7nIrE3SpG3hLyTnQsDi2FiGXtTMWq9CmNJ+h3d4jNRE9aLZ+zSecSh5r 59wJgcyU5VeQbgnXgpoIbDpDlz23/l3BY+n4nDS/AoDeUFtcRayGoy6Zk5xSgxszco YtenKl9VnwSJTtmREWhip2wdMkHfAoitI2bjdG69H3tTF1LOHjU11I+Ned9jZPvoth fEIHze/FxQPsw== Message-ID: <60552bee-3326-4917-b00f-5fb7ba3b8adb@kernel.org> Date: Fri, 31 Jul 2026 16:09:14 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 14/41] mm: swap: Introduce lru_add_drain_progressive() To: Ackerley Tng , aik@amd.com, andrew.jones@linux.dev, binbin.wu@linux.intel.com, brauner@kernel.org, chao.p.peng@linux.intel.com, jmattson@google.com, jthoughton@google.com, michael.roth@amd.com, oupton@kernel.org, pankaj.gupta@amd.com, qperret@google.com, rick.p.edgecombe@intel.com, rientjes@google.com, shivankg@amd.com, steven.price@arm.com, tabba@google.com, willy@infradead.org, wyihan@google.com, yan.y.zhao@intel.com, forkloop@google.com, pratyush@kernel.org, suzuki.poulose@arm.com, aneesh.kumar@kernel.org, liam@infradead.org, Paolo Bonzini , Sean Christopherson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Baoquan He , Jason Gunthorpe , John Hubbard , Peter Xu , Vlastimil Babka , "hughd@google.com" Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev References: <20260728-gmem-inplace-conversion-v9-0-35f9aec2aed2@google.com> <20260728-gmem-inplace-conversion-v9-14-35f9aec2aed2@google.com> <4460e252-b6de-4df3-bea8-0368a9d0f4ff@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/31/26 15:03, Ackerley Tng wrote: > "David Hildenbrand (Arm)" writes: > >> On 7/31/26 12:22, David Hildenbrand (Arm) wrote: >>> >>> Just nasty :) >>> >>> >>> mlocked folios in the mlock cache hold a reference as well. >>> >>> >>> I was trying to avoid messing with the refcount for ordinary LRU cache pages. >>> mlock() should be a corner case for guest_memfd. >>> >>> Relying on the refcount just means that one unconditionally performs a lot of LRU >>> cache draining even though it doesn't make any sense. >>> >>> >>> Again, the problem is that on *any* raised reference you would drain. I >>> was trying to limit the harm. >>> >>> [...] >>> >>> >>> Why? >>> >>> >>> Why? >>> >>> That's literally what the existing refcount check tries to do: figure out if >>> there are LRU caches. >>> >>> >>> I really don't understand what you are trying to say. >>> >>> Draining only makes sense if something is on the LRU cache. And there are >>> better ways of checking that than relying only on even less precise refcounts. >>> >>> If someone wants to do an early refcount check to abort the overall >>> operation, that's fine. >>> >>> >>> ?! >>> >>> That's why we have the universal definition of expected references and the >>> common helper. >>> >>> Because pagecache pages commonly don't support the swapcache. >>> >>> >>> It must, because that is used all over the place. The only thing it >>> cannot deal with is references held by the caller (which could be supplied >>> through and "additional references" parameter like we do elsewhere). >>> >>> >>> You're not seriously proposing such an abomination I hope? >>> >>> >>> You can just check early for mappings. >>> >>> Remember: this is about LRU draining, *not* about your final >>> "unexpected references" check. >>> >>> >>> Who cares if we end up with a common usable helper? We have usless >>> checks *all over the place* in common helpers. >>> >>> >>> Yuk. >>> >>> >>> Putting an enum into a header is a problem in which universe? :) >>> >>> Ackerley, please stop making up stuff. Having generic helper is not a problem. Doing >>> checks in common helpers is not a problem. Putting enums in headers is not a problem. >>> >>> Your version is just bad. >>> >>> I can later try something that keeps the questionable refcount checks in place, >>> maybe that could do as a temporary solution until Hugh possibly finds a way to >>> remove the need for draining entirely. >>> >> >> And looking at it, the current folio_ref_count() in GUP is entirely wrong >> and always makes us drain local+all. >> >> Let me fix that first, which will also modify this code. >> >> I'd say, to unblock gmem we could have something minimal that I would clean >> up later, but I think the problem is that lru_add_drain() / lru_add_drain_all() >> is not exported, right? >> > > My bad, I totally mixed up refcounting to drain with refcounting for the > safe conversion and didn't have a complete understanding of mlock(). > > The problem for gmem was that if there was an elevated refcount on some > folio in the range, it might be because the folio was in the lru_add > fbatch. Right. > > If it were in the lru_add fbatch, it would be fine to just drain the > lru_add fbatch and proceed with the conversion. > > lru_add_drain() isn't sufficient since the conversion might be handled > on one CPU when the folio is on another CPU's fbatch, so I had to do > lru_add_drain_all(), but lru_add_drain_all() causes IPIs which are > expensive. Yes. You should probably do an early folio_maybe_dma_pinned() || folio_mapped() check and just return -- don't drain. > > In an earlier revision I exported lru_add_drain_all(). My understanding > of the discussion at guest_memfd biweekly was that Sean didn't want us > to export lru_add_drain_all() now, and then end up exporting some other > function and unexporting lru_add_drain_all(). To avoid exporting and > unexporting, we then said we should refactor now. > > Happy to go with your proposal too. Yeah, I have to look into this more closely. I'm now convinced that we really have to use the refcount for now, but hopefully we can limit it on folio_expected_refcount() internally + additional references from the caller. I'm quite busy today, but let me flesh something out that actually works. -- Cheers, David