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 F3A313CC324; Fri, 31 Jul 2026 11:59:20 +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=1785499162; cv=none; b=Rwvlko8TMRA4l2NRKgx/yu+xhFSN6D1CAjO0e/w27uQv3zNsFaOaH9ouoAsmIcA6u6506SHVeDhXDssyXuZ9CXdq4oBfPzOoTmmrlwNt9P3ffEajv1xGoOeOeo+okJx4cA4OTrIl2Tk9+O9FRqGf1i6meBS73L1Qo9l0db9e7dA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785499162; c=relaxed/simple; bh=hEFxzcRWstFUAXCBAAUDu40G2TRn32sP14RQkNifo+8=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=aunrns/nuGvMBBsRuBpOSwMmnpZHakmBgNBZ/20No7uVBs1vBhNLa6DGm0po8WKB7a59QIsyskM+6HkbwoRJYD65wvTB2fwqVfNcnWVoRUt3ZfIAbs4D3MgVLqbgTnREof1ZXpt/X78Odxdr5wAKM6p3qny5HZTYIBZu7Rz8zBo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KvoDkQlu; 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="KvoDkQlu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 75C541F000E9; Fri, 31 Jul 2026 11:59:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785499160; bh=Nm4XNdosDTkLpy1A2APf3Rq2Y20s3PmQ81LS+20iumM=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=KvoDkQluOqIEZqgpMUedMAAOQZiM8Px+cdim13BhGWGcz0luftftif+Cex6mR4bSV gn5P+xU2YooQ3Z2SdV7qPHL2pJ7r6g31jBlgtau48sz4n1qgfWJBrdohaEdZwkECTL Eut9WfJq6LEaJiPWx2qEQ65O0oj9AwH4xpXnDsgw8X8UnhDL05lP4hGQYtH6TRqFpe kk9qzkt+ptAmYy3WQnFunTrOFyvkt8xnhNdDCTsDboPYvLJgdOzltNvsCWbgbWmPKN 96YXjoNb4IYBa1EkNJapNnEmz+0jyskTAUeQnAHQEAaYszPi1OfZmiDEQ4+18Hd18+ ICAFYfm9QPX1A== Message-ID: Date: Fri, 31 Jul 2026 13:59:01 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@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() From: "David Hildenbrand (Arm)" 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> 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 12:22, David Hildenbrand (Arm) wrote: >> >> I didn't really like this either, the drain_state thing is really >> awkward, but in both usages (collect_longterm_unpinnable_folios() and >> guest_memfd), there's an outer loop where if the draining happened on >> the local CPU before it should skip straight to just draining on all the >> other CPUs. > > Just nasty :) > >> >>> I was hoping that we could embed more logic in a helper. The history [1] of the >>> refcount check is rather sad: >>> >>> https://lore.kernel.org/all/c5bac539-fd8a-4db7-c21c-cd3e457eee91@google.com/ >>> >>> ... primarily because of mlock() handling. >>> >> >> In the original code in collect_longterm_unpinnable_folios(), I couldn't >> find anything that handles folio_test_mlock(), so I was lost for a while >> until I realized mlock() doesn't add a refcount to the folio. > > mlocked folios in the mlock cache hold a reference as well. > >> >> Are you kind of proposing an optimization to >> collect_longterm_unpinnable_folios() by adding a check for >> !folio_test_mlock()? I hope we can put that in a separate patch series, >> I'm still hoping to get this series in for 7.3!! > > 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. > >> >>> For guest_memfd(), would mlock() ever apply on a path where you need that check? >>> >> >> For guest_memfd, if a folio were mlocked, unmapping it would fail, and >> so the refcount on the folio would be elevated. The >> kvm_gmem_is_safe_for_conversion() check in the patch after this one >> would fail, correctly. >> >> I really wanted the caller of the lru_add_drain_progressive() function >> to control whether to do the drain (see below), which would solve the >> mlock problem by not assuming the use of folio_expected_ref_count() to >> determine whether to do draining. > > Again, the problem is that on *any* raised reference you would drain. I > was trying to limit the harm. > > [...] > >>> + */ >>> +static void lru_cache_drain_for_folio(const struct folio *folio, >>> + enum lru_cache_drained *drained) >> >> The main thing I wanted in the proposed version >> (lru_add_drain_progressive()), was to let the caller determine whether >> to try or to continue draining. > > Why? > >> >> I wanted the caller to have full control over whether to drain or not, >> so I didn't want to pass folio into the function. I thought the caller >> should first determine whether to drain, then call the function. > > Why? > > That's literally what the existing refcount check tries to do: figure out if > there are LRU caches. > >> >> This version assumes that the caller wants to continue draining based on >> something to do with a folio, and the folio may or may not be on the >> lru_add fbatch at all, which is a little strange to me. > > 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. > >> >> Also, this version enforces a certain definition of "expected" number of >> refcounts on the folio. I guess in this case this definition works with >> guest_memfd, but guest_memfd doesn't support swap so the swapcache check >> in folio_expected_ref_count() isn't necessary. > > ?! > > That's why we have the universal definition of expected references and the > common helper. > > Because pagecache pages commonly don't support the swapcache. > >> Also, if the definition >> folio_expected_ref_count() changes, then guest_memfd is implicitly >> affected. Not sure if the "expected" definition is the same for all >> callers? > > 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). > >> >> I guess if there was a function named >> folio_ref_counts_indicate_presence_only_in_the_filemap() instead of >> folio_expected_ref_count(), then it'd be the perfect function for > > You're not seriously proposing such an abomination I hope? > >> guest_memfd to use. The current definition of folio_expected_ref_count() >> also includes page table mappings, but in this check guest_memfd really >> wants to make sure that there are no page table mappings. > > You can just check early for mappings. > > Remember: this is about LRU draining, *not* about your final > "unexpected references" check. > >> >> This version folds folio_likely_lru_cached() into the check, and I >> adopted the check for guest_memfd because it'd help with huge pages, >> though technically guest_memfd is always 4K now so the check is also >> pointless. > > Who cares if we end up with a common usable helper? We have usless > checks *all over the place* in common helpers. > >> >> I tried a macro version of this where the macro caller can pass in a >> full condition, which would look like this: >> >> lru_add_drain_while(folio_may_be_lru_cached(folio) && >> folio_ref_count(folio) != expected_refcount, >> drain_state); > > Yuk. > >> >> but I thought that just created something people have to jump to, to >> first understand how the macro works, so I left it as an explicit while >> loop instead. >> >>> +{ >>> + if (!folio_likely_lru_cached(folio)) >>> + return false; >>> + >>> + /* Try local draining first, if not already done previously. */ >>> + if (*drained == LRU_CACHE_NOT_DRAINED) { >>> + lru_add_drain(); >>> + *drained = LRU_CACHE_DRAINED; >>> + } >>> + /* Try draining all CPUs next if still not an LRU folio. */ >>> + if (folio_likely_lru_cached(folio) && *drained == LRU_CACHE_DRAINED) { >>> + lru_add_drain_all(); >>> + *drained = LRU_CACHE_DRAINED_ALL; >>> + } >>> +} >>> + >>> /* >>> * Returns the number of collected folios. Return value is always >= 0. >>> */ >>> @@ -2266,9 +2316,9 @@ static unsigned long collect_longterm_unpinnable_folios( >>> struct list_head *movable_folio_list, >>> struct pages_or_folios *pofs) >>> { >>> + enum lru_cache_drained drained = LRU_CACHE_NOT_DRAINED; >> >> I also considered an enum, but it would be another thing to export. I >> was thinking to have drain_state just be opaque to the caller and the >> only thing the caller needs to know is to initialize it to 0. >> >> Perhaps there's a better way to "make it opaque"? > > 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? -- Cheers, David