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 C27C9568FAC for ; Wed, 9 Sep 2026 14:08:03 +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=1788962885; cv=none; b=hPvW74MSTOkSgNsLh6L6PdTjqwn5UDybaY1eA7Gx0D85ZkkuKNqzTe6YrgjoZbz2aItz3ohVs/IVgiy5C59trOLrGutnLIqXi4FE7jVuATELZUCnIWToJ+GyVtLV0lWKfSuvzj/aFPNm4PonrT6k+M02ZElREBj6XMsmn1NsKp8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788962885; c=relaxed/simple; bh=RWamXQB5lXNQZK4wY26JhvFyuxLqlBrnxJnzqcIbJto=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=eP3zoSgkhM0DvBEMmVtFzAfJrSC0BZW9VFe797I6PDXeXox0jaM1b2ZsLtWX+KeHUMho+NvoEacTeo1rAylA3Egen3UepIDidvJTGzPh2TT0k4xAyTZ919DMvPwfriV3P71a/WE7crdkR9iAgIX5ZQdQo4hqQvdI9M9JWtGbzs8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fHiQnsD7; 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="fHiQnsD7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9011A1F00A3A; Wed, 9 Sep 2026 14:07:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788962883; bh=VDyWmM40aiKxKbQ6FoW/MOveUW0JhOGKh0dq4VK12n0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fHiQnsD74keMxuuA2tajHNZcaFeCB4NkxmK8z2CQ7BeRU2CpuTeQCmXhPg7N3FRV5 EjuYtAjpIbsL3Ro+Z9NrrC+e8MazPnwsUteb74EApWrDV08qjGM/hUQfkcOxtWJSf6 7rF6NYfn0N4Dp/Mc3H6QAUF2jZoYrbp+DZWKNTazmi/WhMkbIrjZzEzBWE82fXLyt0 clmrkMabbXuBy/vXbGLcU6ib701LtotePPJghNZXVPYgi80eJmKVef9GFYXnfhxZJU kebP2Abi03MXzEmX+3WAYSf4jDdwPQewH5jBt/41TQWaEBl5kyEMblFBrEng1mEk7o ezXBT/E2eDUtQ== Message-ID: <0569af12-8e32-457d-9d1f-9b879d6485d0@kernel.org> Date: Wed, 9 Sep 2026 16:07:53 +0200 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 v3 01/14] mm/zsmalloc: replace PG_private with pointer comparison To: Zi Yan , "Matthew Wilcox (Oracle)" , Andrew Morton , Muchun Song , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Gregory Price , Ying Huang , Alistair Popple , Johannes Weiner , Qi Zheng , Shakeel Butt , Kairui Song Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Minchan Kim , Sergey Senozhatsky References: <20260907-remove-pg_private-v3-0-6ae22f9d9272@nvidia.com> <20260907-remove-pg_private-v3-1-6ae22f9d9272@nvidia.com> <60fe39c2-d92f-4bd0-be33-04d702f8f753@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 9/8/26 17:21, Zi Yan wrote: > On Tue Sep 8, 2026 at 11:07 AM EDT, David Hildenbrand (Arm) wrote: >> On 9/8/26 04:56, Zi Yan wrote: >>> zsmalloc uses PG_private to indicate first zpdesc in a zspage chain. It is >>> equivalent to check zpdesc == zspage->first_zpdesc. Replace >>> is_first_zpdesc() with zpdesc == zspage->first_zpdesc in obj_allocated(). >>> >>> For get_first_zpdesc(), first_zpdesc is from zspage->first_zpdesc, so >>> replace is_first_zpdesc() with first_zpdesc->zspage == zspage, the second >>> requirement of a zspage chain, where all zpdescs point to the same zspage. >>> >>> is_first_zpdesc(), is only used in VM_BUG_ON_PAGE(), so performance impact >>> should be negligible. While at it, change VM_BUG_ON() to >>> VM_WARN_ON_ONCE_PAGE(). >>> >>> It prepares for a future commit that remove PG_private. >>> >>> No functional change intended. >>> >>> Assisted-by: Claude:claude-opus-4-8 >>> Assisted-by: Codex:gpt-5 >> >> Assied-by: LLM :) >> >>> To: Minchan Kim >>> To: Sergey Senozhatsky >>> To: Andrew Morton >>> Cc: linux-mm@kvack.org >>> Cc: linux-kernel@vger.kernel.org >>> Acked-by: Johannes Weiner >>> Signed-off-by: Zi Yan >>> --- >> >> Nice cleanup, one comment below. >> >> Acked-by: David Hildenbrand (Arm) > > Thanks. >> >>> mm/zpdesc.h | 2 +- >>> mm/zsmalloc.c | 24 ++++++------------------ >>> 2 files changed, 7 insertions(+), 19 deletions(-) >>> >>> diff --git a/mm/zpdesc.h b/mm/zpdesc.h >>> index b8258dc78548d..4fd81c2e80769 100644 >>> --- a/mm/zpdesc.h >>> +++ b/mm/zpdesc.h >>> @@ -26,8 +26,8 @@ >>> * with memcg_data. >>> * >>> * Page flags used: >>> - * * PG_private identifies the first component page. >>> * * PG_locked is used by page migration code. >>> + * The first component page has zpdesc->zspage->first_zpdesc == zpdesc >>> */ >>> struct zpdesc { >>> unsigned long flags; >>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c >>> index 825022a7a328f..24526185fd31f 100644 >>> --- a/mm/zsmalloc.c >>> +++ b/mm/zsmalloc.c >>> @@ -290,11 +290,6 @@ struct zs_pool { >>> atomic_t compaction_in_progress; >>> }; >>> >>> -static inline void zpdesc_set_first(struct zpdesc *zpdesc) >>> -{ >>> - SetPagePrivate(zpdesc_page(zpdesc)); >>> -} >>> - >>> static inline void zpdesc_inc_zone_page_state(struct zpdesc *zpdesc) >>> { >>> inc_zone_page_state(zpdesc_page(zpdesc), NR_ZSPAGES); >>> @@ -476,11 +471,6 @@ static void record_obj(unsigned long handle, unsigned long obj) >>> WRITE_ONCE(*(unsigned long *)handle, obj); >>> } >>> >>> -static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc) >>> -{ >>> - return PagePrivate(zpdesc_page(zpdesc)); >>> -} >> >> I would keep this helper, reworking it and moving all comments regarding *how* >> first detection works in there >> >> -static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc) >> +static inline bool __maybe_unused is_first_zpdesc(const struct zpdesc *zpdesc) >> { >> - return PagePrivate(zpdesc_page(zpdesc)); >> + /* Only the first zpdesc has its zspage->first_zpdesc set to itself. */ >> + return zpdesc == zpdesc->zspage->first_zpdesc; >> } > > That is the version I had in RFC[1]. But how it works is not only Ah, great minds think alike. > 1) "Only the first zpdesc has its zspage->first_zpdesc set to itself.", but also > 2) all zpdescs point to the same zspage. > > obj_allocated() checks for 1, since zspage comes from zspage and > the requirment 2 is tautology. > > get_first_zpdesc() checks for 2, since first_zpdesc comes from zspage's > first_zpdesc, another tautology. > > I also had a version to spell out both[2], but Johannes did not lik it, yeah, agreed. > because it[2] is a "super predicates", making the required checks unclear. Oh well :) Acked-by: David Hildenbrand (Arm) -- Cheers, David