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t=1782116109; bh=I7CiEqdgfo1EMG6k6a44m/PMxSYQ3XbZcJ/q6HRDPVY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=MaV52Ft+ZtcIY4Zuk+MIRru0fDYXiE5R1ETQs63VzVzqC/868iLnr54jcWhhbHiiA quoRqbjsVr//YKLPI/WhBotOpkjMcrdm9yqthzb7vTVTUmIhtwt3WcXFgdc1MLn8gb 8KN3YkDiMgGPDBBvMVzsWN8eigVYSyJ5amadlyaLDudFWOVs7J8L41nAdDi0XgbUEh iQW0/+nsPcwpMVBJuoS6SfaRIFvTyDh+Axq3mU6SRBVnEI3/uGhjDv06aaLy0Ubu9V KzBI+5Vq3hKWaQGR1Lwg1mKpMn0Y7Na2V542ESOsvsjOnM1pD1YBojI/Dz1YIAAgl2 t6sA3VDUcAm5Q== Message-ID: <086b119c-b777-46ae-b087-798ca3e44ecd@kernel.org> Date: Mon, 22 Jun 2026 10:15:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/2] mm: improve folio refcount scalability To: Linus Torvalds , ilya.gladyshev@linux.dev Cc: Andrew Morton , 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, vbabka@suse.cz, willy@infradead.org, yuzhao@google.com, ziy@nvidia.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-Stat-Signature: 8ime39hiye3rcsbwc53zs8pohj1o9syw X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: DB7C92000D X-HE-Tag: 1782116109-474876 X-HE-Meta: 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 yk2jgXJX JpswaRGM1d6QCTZ5tSPTGwhU5fI3dzmRFikWCelv/UUQiD2dlQ3aIQW4N9Ni7+u5FI1uAvAnwwfn7qsEtF/edaRiADsW0oMZOE/RVLf8Ua8IGGDqZOuq5qawYc2vx34xHPRj0+e7Acf/Ga79Pv4ky8t6zKB6TME7sVOKvkdyvxDPdjYKQ5iUxGpu1nsPiddj1fGHXAa2+hvi68tHSTr/37dZngK+f2zKRFQvZkvOfJpiDFrcGB0jKIJqI1/1RSMh989/x Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/21/26 06:46, Linus Torvalds wrote: > On Sat, 20 Jun 2026 at 11:19, wrote: >> >> T2: optimistic get() [0 -> 1] >> T2: put page back [1 -> 0] >> T2: calls dtor for type X, returns into the allocator > > Which optimistic getter does this? > > I didn't go back and look at the series, but isn't the rule that the code does: > > - optimistic get > > - then check that the folio is still valid (*not* using the page > count, but by re-looking it up elsewhere, typically the address space > mapping) > > - put the page if it wasn't valid > > - if it goes to zero, there's no destructor inherent in that I'm also confused about the destructor aspect. Usually we have folio_try_get(folio) ... some checks folio_put(folio); folio_put() is just: if (folio_put_testzero(folio)) __folio_put(folio); If the thing was a folio at the start, but stopped being a folio halfway through folio_put(), I don't see the big difference to the thing not being a folio right from the start. (folio_try_get() would succeed on some random page either way right now) Which destructors are we concerned about? (1) hugetlb destructor If we grabbed a non-hugetlb folio but suddenly have a hugetlb folio after folio_put_testzero(), __folio_put() will just do the right thing and call free_huge_folio(). (2) pagetable_dtor() These destructors are called when we unmap the page table, before actually freeing it. So it wouldn't apply here. Maybe it's about something like void page_frag_free(void *addr) { struct page *page = virt_to_head_page(addr); if (unlikely(put_page_testzero(page))) free_frozen_pages(page, compound_order(page)); } Whereby, if it's a folio, we wouldn't do the right thing as we are not going through __folio_put()? -- Cheers, David