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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 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 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: C3FA740012 X-Stat-Signature: i6pebnx17sisb6zrnsmkexydfax5sy3m X-Rspam-User: X-HE-Tag: 1782114949-773349 X-HE-Meta: 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 heDMI5Ey XBr2apmcbTjGMMZJ2oxUURcI7XMgoMgzH6syLswwzMZeCkRg1qOrEaXK7iPCgiMbhG0vHfq2Qu16RALDIfZ/N3eDjv2rpzW6fXdUf4QzAe2Q0j3Bgs4mqDzXiPI04jlAZzbPa08EF34RjATTjHxGhuYQwa3DW5bVB6KQXSNJGn+PjMIC4B/i2//V5/EQ8OGkNi1oRwNsAEX12tTw5T+qYMFPIUDmI/fudRRIBSestT8QYnHgDmswgd91wxs/Tdk3xv+/vOU8bN9AH7hte48fM1GECcV/TCf9HcWrQxNvgPMR2b8mx21IviWiMfOwfd9ZUPYez Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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