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 DE87B3BB685 for ; Thu, 24 Sep 2026 18:52:07 +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=1790275934; cv=none; b=masx1v+iUqFGSgt8iservwWrsgRExh5t7HlaRRZgGh1us7DSXcVmMpxqwa/L8eWi8qEeT0teoOiF0q5e3aMuiqpLaqFKotxLJZSVGC/+lHMA1vu4YrLqkwqy5PTniK6hn+6yp5lBg2NF/zVLH+mWQUkXfHRTgCylryeic+iC0Yg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790275934; c=relaxed/simple; bh=uB7PBE+vNmeeUB+vdMvBt2A8VZS499IMcXGUO94mTWU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=DNWJEdPqIo3lQ4rRodUlJw1U5VLKU5tF7Ea9xKyz+wtOtoykPrNr0+oDilJtDlH3tnDA3Bu3Z4HRDbDqmKYxcwa3IdcYnUhPnsNQLNHDFrMVrpLQHXGAVDIahIlkT8DQXXL862w/idPWsE2XXolaQImdmT3tZCX5tC9GLcoUlqU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hmKwra2T; 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="hmKwra2T" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50E851F00893; Thu, 24 Sep 2026 18:51:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790275924; bh=8fQuNmR+wyKKa/NE7gKqnglhXs6oXfwgf7LqT0NLqJM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hmKwra2TGt4NKXHAA/TkRcGPes5yT9c5nvFOd+aZbIBb7FPyc7k5fI9FVjH9eU4GG sODuhzOfxulRS+W1TQ3EiWAHBIdTotMlSZba1Zftbd3TRwCMVH/BoCBE0RnoxRtNik oXOPyebbT1FC9GA19P4NRoEdFGMIwEzKIX/PEN4ZgpKlbbq7Wn06FFrvEdj9QL+WWx MgKLgK3yz6RTLmkPgA8Xu6Vs+paDS02BGoGLXXkQew1Mxmq75NIxaodkh1HOvLTjr2 AOtX5ao831YTZLHT9N9YMT57zQXlry8BCfVNNahANt8tvX99YtRVHO8EYnTQok5MF+ 50pdsWexQGSSQ== Message-ID: <570cf649-a8bf-4073-be71-207fbb2bef40@kernel.org> Date: Thu, 24 Sep 2026 20:51:50 +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 v6 3/3] mm: implement page refcount locking via dedicated bit To: Ilya Gladyshev , Zi Yan Cc: akpm@linux-foundation.org, andrew+netdev@lunn.ch, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, edumazet@google.com, harry.yoo@oracle.com, hramamurthy@google.com, ivgorbunov@me.com, joshwash@google.com, kirill@shutemov.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, pfalcato@suse.de, rppt@kernel.org, surenb@google.com, torvalds@linuxfoundation.org, vbabka@suse.cz, willy@infradead.org, yuzhao@google.com References: <56e036665dbeb0a8f51d697220d5ed84f210994c@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/23/26 19:34, Ilya Gladyshev wrote: > September 15, 2026 at 5:42 AM, "Zi Yan" wrote: > > >> >> On Sat Sep 12, 2026 at 3:50 PM EDT, Ilya Gladyshev wrote: >> >>> >>> The current page refcount implementation uses a single counter value >>> (zero) as dead. So, to prevent incrementing a dead refcount in >>> folio_try_get(), it fundamentally requires a CAS loop. >>> >>> This CAS loop can act as a serialization point and can become a >>> significant bottleneck during high-frequency file read operations >>> [1][2]. >>> >>> This patch reallocates the refcount value range: >>> >>> (1) refcount < 0 means dead refcount (uninit / frozen) >>> (2) refcount = 0 allowed only as a temporary state (see below) >>> (3) refcount > 0 is a regular reference count >>> >>> In other words, refcount is now split into "dead bit" and a 31-bit >>> counter. >>> >>> Refcount decrement now works as follows: >>> 1. Counter decrement >>> 2. If it is now zero, try to put it deep inside the dead zone >>> (CAS to INT_MIN). Or you can view it as "set up frozen bit and reset >>> counter". >>> 3. This CAS can fail only if someone grabbed a reference in-between -- >>> that's okay, this page is their problem now. >>> >> I am not sure this works. David raised a concern on V4[1], where if a >> folio lands to page_frag_free()'s free_frozen_pages(), it will not be >> properly handled. "this page is their problem" only works if every >> folio/page free functions call a generic folio/page free function that >> checks the page type and does the proper handling. > >>> Refcount decrement now works as follows: >>> 1. Counter decrement >>> 2. If it is now zero, try to put it deep inside the dead zone >>> (CAS to INT_MIN). Or you can view it as "set up frozen bit and reset >>> counter". >>> 3. This CAS can fail only if someone grabbed a reference in-between -- >>> that's okay, this page is their problem now. >> >> I am not sure this works. David raised a concern on V4[1], where if a >> folio lands to page_frag_free()'s free_frozen_pages(), it will not be >> properly handled. "this page is their problem" only works if every >> folio/page free functions call a generic folio/page free function that >> checks the page type and does the proper handling. > > After some thinking, this scenario is indeed possible and problematic. The > complexity of the required setup give me hope that there are some implicit > barriers, however I wasn't able to find them. But I am not an mm guru, > so maybe I missed something :shrug: > > -- Below is a minimal buggy scenario -- > > Thread A: network code that calls page_frag_free() > Thread B: calls folio_try_get(). Maybe it is the DAMON paddr scanner, > maybe this folio was previously used in page cache > (however this noticeably complicates the scenario) > Thread C: user of folio's reincarnation. It also requires destruction via > folio_put() and not via free_frozen_pages() -- for example hugetlb > or memcg charged folio. > > Refcount operations as follows (FR = FROZEN): > > A: DEC 1 -> 0 [ page_ref_dec_and_test() ] > B: INC 0 -> 1 [ folio_try_get() ] => success > B: DEC 1 -> 0 [ page_ref_dec_and_test() ] > B: CAS 0 -> FR [ page_ref_dec_and_test() ] > B: * deallocates via __folio_put() * > > C: * re-allocates page * > C: DEC 1 -> 0 [ page_ref_dec_and_test() ] > A: CAS 0 -> FR [ page_ref_dec_and_test() ] => success > A: * deallocates directly via free_frozen_pages() * => BUG > > This can be fixed with a `s/page_frag_free/folio_put/`-style patch, however > it seems unreasonable... There needs to be a common page freeing function that dipatches stuff. folio_put() is not the best future-proof thing, because not all pages will be folios sooninsh. -- Cheers, David