From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 8CBC337F00A for ; Wed, 4 Mar 2026 19:16:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772651793; cv=none; b=M8c0ighlI1Sw9Grg6WjOe7xay3I2nXHRithsLvx9O4ajjn5L//H6MTJo0ABSChEBcgvIo/3XTk+O08VqMfX+/WL2YNH+Ji+JT0oAqTIWBlWaf9uDkSBwKdp3MaVhxZQ/byMyXzGpQc31E2XD+epHnXSlQ2QY7ZzgH9v8wx27fcQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772651793; c=relaxed/simple; bh=7rUPpgokgbaSU3rIz/IH3gMaIW1UQ9YfdPMn576eNtk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=InfhB8ApDg6Hg1P7mcIATmx16R0/eSC2kgz29Mc+gVEMS6QvAUpPScwxAGSzqiGolOP9T4KihvJO369GUuLDiUA5sCRyul2dFROr4PoTlN2rE8lXt+o4+w9Oz1fzIcdr91o8qWnQDaxHQZllxhqaDHJWPomemAH+JuqUr7EtvG0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Hw1uX/ba; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Hw1uX/ba" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A4831C4CEF7; Wed, 4 Mar 2026 19:16:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772651793; bh=7rUPpgokgbaSU3rIz/IH3gMaIW1UQ9YfdPMn576eNtk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Hw1uX/barYICCyEM7B8C3vo1+KRZJfyPlMb8ducnLmhE49Dup1bECeZGlnljBXQ23 89g473DgHrINwx4Hw4KQQq8YohjH0VnXyrYgP+wrS7AR1xjw1m7pw4K2KMpDwgRoRZ R1x/296aUoNdor/3IuFkNtoX4bUWxvUv/oPrp5+Xw1H8+gjBCXI9jxzuY/7V7JBHNQ 3rqYAdY2psj820zRvZczFZ1/MrsYbSAlVDTXdM1JzH527WATvxvszKQ75TYVSNnGeX 9ud4TNRUHKAfa5P1E80r1eJvLcaJvtVL3MnpvExMSuWgS/CYk6oepWSH4JK66mugrG 3B7kAM+GJX/HQ== Message-ID: Date: Wed, 4 Mar 2026 20:16:24 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm: implement page refcount locking via dedicated bit To: Gladyshev Ilya Cc: Andrew Morton , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Harry Yoo , Matthew Wilcox , Yu Zhao , Baolin Wang , Alistair Popple , Gorbunov Ivan , Muchun Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kiryl Shutsemau References: <6bf6eba6e2e6a74e2045a3bd08d58fd91bece7be.1772120327.git.gladyshev.ilya1@h-partners.com> 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: <6bf6eba6e2e6a74e2045a3bd08d58fd91bece7be.1772120327.git.gladyshev.ilya1@h-partners.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit This all made mu brain hurt a little :) > > /** > @@ -176,6 +181,9 @@ static inline int page_ref_sub_and_test(struct page *page, int nr) > { > int ret = atomic_sub_and_test(nr, &page->_refcount); > > + if (ret) > + ret = !atomic_cmpxchg_relaxed(&page->_refcount, 0, PAGEREF_LOCKED_BIT); > + It took me a while to figure out why this can't be just an atomic_or(): even though concurrent page_ref_add_unless_zero() would see a 0 after incrementing it to 1 to back off, there could be yet another concurrent page_ref_add_unless_zero() that would see the transition from 1 to 2 and continue. What is the performance impact on doing the additional atomic_cmpxchg_relaxed() whenever we free a page, in particular, for anonymous memory where we mostly have just a single reference that we drop during munmap() etc? > if (page_ref_tracepoint_active(page_ref_mod_and_test)) > __page_ref_mod_and_test(page, -nr, ret); > return ret; > @@ -204,6 +212,9 @@ static inline int page_ref_dec_and_test(struct page *page) > { > int ret = atomic_dec_and_test(&page->_refcount); > > + if (ret) > + ret = !atomic_cmpxchg_relaxed(&page->_refcount, 0, PAGEREF_LOCKED_BIT); > + > if (page_ref_tracepoint_active(page_ref_mod_and_test)) > __page_ref_mod_and_test(page, -1, ret); > return ret; > @@ -228,14 +239,23 @@ static inline int folio_ref_dec_return(struct folio *folio) > return page_ref_dec_return(&folio->page); > } > > +#define _PAGEREF_LOCKED_LIMIT ((1 << 30) | PAGEREF_LOCKED_BIT) > + > static inline bool page_ref_add_unless_zero(struct page *page, int nr) > { > bool ret = false; > + int val; > > rcu_read_lock(); > /* avoid writing to the vmemmap area being remapped */ > - if (page_count_writable(page)) > - ret = atomic_add_unless(&page->_refcount, nr, 0); > + if (page_count_writable(page)) { > + val = atomic_add_return(nr, &page->_refcount); > + ret = !(val & PAGEREF_LOCKED_BIT); > + > + /* Undo atomic_add() if counter is locked and scary big */ > + while (unlikely((unsigned int)val >= _PAGEREF_LOCKED_LIMIT)) > + val = atomic_cmpxchg_relaxed(&page->_refcount, val, PAGEREF_LOCKED_BIT); I assume we can't do an atomic_dec(), because we might have concurrent unfreezing (or similar things) happening that overwrote whatever was in there. Is it really correct to replace _PAGEREF_LOCKED_LIMIT by PAGEREF_LOCKED_BIT, dropping some unrelated references? I assume the reasoning is that we treat any references with PAGEREF_LOCKED_BIT set as irrelevant and can get overwritten any time. I was wondering is whether page_ref_freeze() could actually leave the references set, and only set the PAGEREF_LOCKED_BIT bit, whereby page_ref_unfreeze() would only clear the PAGEREF_LOCKED_BIT bit. Similarly, the set_page_refcounted() could add a reference and clear the PAGEREF_LOCKED_BIT. That'd be more expensive on the allocation path ... and not sure if that would really help to turn this atomic_cmpxchg_relaxed() into an simpler atomic_dec() my brain could more easily understand :) I think this patch needs a lot more documentation around what the PAGEREF_LOCKED_BIT means, and how this interacts with e.g., the set_page_count() in set_page_refcounted(). In general, I like this! -- Cheers, David