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 43F6E370D6B for ; Mon, 22 Jun 2026 07:55:48 +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=1782114950; cv=none; b=oeE0rFYz0hrBcBpZN4YD9UyH4qQziV+hCw0BYkLpyZnB00GIpsHmGcCz5FK1QGQUje+eSn7bCa1pA6q+3R6Zm8xK2BnAhjpTg6FmUmchGcZMqYeD08JHMdgAf6rqo9y6Ruupfxx4Hy8mwNbQmYBoCg2siXP7ck6PdftSTS6uuwQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782114950; c=relaxed/simple; bh=iqxjou+mJUyGrelXaq4Vg6dqS2ISV3LPNMnmsr3TDas=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=CZQUmfSwgkQrO2LUghxXRJG6A8VN3YcwgYPBFvmQfOWFufpALwqwAACNRemQAScib8Y4EAPQAHkcEmWNcU8fvTfdnXLEM7vUhLNf6vVw29Pr3xkcFrpNBzVgqks7UDhGAGmcxyYAetk47v5FR7wcsNtLiSS3Bvo2aw2j6y6CZA8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=czajZs6y; 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="czajZs6y" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DE8321F000E9; Mon, 22 Jun 2026 07:55:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782114948; bh=PI6SCYr5oDI+dmRba23U6uw1UDjQEl4kT/Lv5kvNOWU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=czajZs6yEfQODRWp3l7euZ0tzsVCGwORupONsIcPqZXgDJm6CuPyAMkTXa/n9rffc Y+ml8ZcgQOJWVanBoyzTFlbnXBa6VDnXVxMFtV62KFZzkTVjNOnzbOglpk+wDoCbIh j2D1PAdf9RafGBD+6rzndI/LHbvl+URMqkBU7A4xEN8dFWff3nzru5xAKWJSrt/QUK QCFEXPZQx24mcMDPEUdU74U8VNfQ1ZwON5cW+f47Mhuilm+Ap8fxOlhO1VEBhj58BI ZC0JimVqciTQmp1to9HgN2RZjDLbNLMI8XZbjKBhI6wHzXlTPN71eoOvCETLprEu+M GwNWoeH6bnTeg== Message-ID: <6d576c2a-af34-493c-bccb-cf2e035e6506@kernel.org> Date: Mon, 22 Jun 2026 09:55:42 +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 v4 0/2] mm: improve folio refcount scalability To: Zi Yan , ilya.gladyshev@linux.dev, Andrew Morton Cc: ivgorbunov@me.com, Liam.Howlett@oracle.com, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, foxido@foxido.dev, harry.yoo@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, rppt@kernel.org, surenb@google.com, torvalds@linuxfoundation.org, vbabka@suse.cz, willy@infradead.org, yuzhao@google.com, pfalcato@suse.de, kirill@shutemov.name References: <20260608154734.8e4115fde4e2e14a3b6892fb@linux-foundation.org> 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 6/21/26 03:40, Zi Yan wrote: > On Sat Jun 20, 2026 at 2:19 PM EDT, wrote: >>> >>> Thanks. Nice numbers. >>> >>> AI review had some things to say: >>> https://sashiko.dev/#/patchset/df26082871b4c65b2bd38d409026237c08572836@linux.dev >> >> Among some minor issues, it also pointed out a funny ABA race: >> >> ``` >> T1/T2 work with pages of type X. >> T3 works with pages of type Y. >> >> T1: page_dec_and_test() >> T1: -> sub refcount [1 -> 0] >> T1: -> *interrupted* (very bad hypervisor, for example) >> >> T2: optimistic get() [0 -> 1] > > How is this possible? folio_get() and folio_try_get() should prevent > getting a refcount-0 folio; get_page() uses folio_get(); try_get_page() > also checks refcount before ref_inc. With this patch set, it's different: Essentially, there is a short time frame between dropping the refcount to 0 and setting it to PAGEREF_FROZEN_BIT. (1) atomic_sub_and_test() [1 -> 0] (2) atomic_cmpxchg_relaxed(0, PAGEREF_FROZEN_BIT) [0 -> PAGEREF_FROZEN_BIT] Someone in-between (1) and (2) can still "abort" this freeing process by incrementing the refcount from 0 to 1. (2) will detect this and count it as "well, not freed". So with a refcount if 0, the page is "staged for freeing", which can be aborted. > > For your patch, is it because of the separation of refcount-0 and > frozen? The page goes refcount-0 before it is frozen? Exactly that. Refcount 0 is only transitional. > Will it work if > the page is frozen first then gets its refcount to 0? Basically, the > frozen state prevents anyone else messing up with your refcount. > Let me comment the original sequence: ``` T1/T2 work with pages of type X. T3 works with pages of type Y. T1: page_dec_and_test() T1: -> sub refcount [1 -> 0] T1: -> interrupted (very bad hypervisor, for example) -> T1 did (1) but not (2) T2: optimistic get() [0 -> 1] -> T2 intercepted after T1's (1). T2: put page back [1 -> 0] -> T2 started (1) T2: calls dtor for type X, returns into the allocator -> For that to happen it must perform (2) through page_dec_and_test(). -> Page is frozen now. [0 -> PAGEREF_FROZEN_BIT] T3: receives page of type Y, sets refcount to 1 -> Ordinary refcounted allocation. [PAGEREF_FROZEN_BIT -> 1] T3: page_dec_and_test() T3: -> sub refcount [1 -> 0] -> T3 did (1) but not (2) yet. T1 resumes execution T1: -> CAS [0->LOCKED] -> T1 did its (2) now. T1: BUG: calls dtor of type X on page of type Y I am confused about the "dtor of type X vs. type Y". We have the page in our hand, once we won (2) we can lookup the type and do the right thing. I guess the interesting part is where the destructor is called from the outside: static inline void folio_put(struct folio *folio) { if (folio_put_testzero(folio)) __folio_put(folio); } Where we keep operating on it as if it were a folio, although it might now be a non-folio thing. So if T1 is doing a folio_put(), we'd call into page_cache_release() etc with a non-folio thing. But IIUC, that can happen today already when we do folio_try_get(folio)+folio_put(folio) and it wasn't a folio in the first place? I'd assume that will all change once the refcount moves into separately-allocated "struct folios". -- Cheers, David