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 6C71745FFAC for ; Tue, 18 Aug 2026 17:53:59 +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=1787075640; cv=none; b=O+Ukqm9Fo30n9DsGbMk31JL3XzpNLYKS0vNP7MV47+Ta5drUYEKcfWmcpfyx2jqt33daW5NHpwbyPVBcJHewrH84CWHQPMHLNxtjGTvqbF7mr0nHV85BOT1kSt6M6CrnuwMdXKx51zRJisJH6iwOqWPPcP05tmWDw5NvjsT9E0c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787075640; c=relaxed/simple; bh=Ok6viy+Cv46/eDSNgRKWLpQVF888330csZct8Y/iSzM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nRakx9NOmwBcahJQeEaDbb2Bo+Xs7fYKroFPtdfFb+MJYOw5a60WX3fBMgObzQzN+eBkkosjRTwULk/VQqkgIkFbtTIHOIlGyft37mbdmQ0qwRs2ePpBGsCC3za/g20KbuAbf0D/ubRr/mkTwZe+ydJ0yFrrr3UiPE/KMuY/J18= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RN4Re/NH; 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="RN4Re/NH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 57DDF1F000E9; Tue, 18 Aug 2026 17:53:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787075639; bh=lzLc+SgSrDwanlsIiH61oiwQSTL3CBqvRLkA4XxeRiE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=RN4Re/NHVv2E/E3x9qkdmHKOxcriA/ngvaff5mFAca3uHQQdnXZ3KFz8ig6E4qG8Z fuGQ2/p3yPM7IKylCzVrIIXqLjtSj3j7ojAReEjpcbPEi5lXOX6F6yjOszTu/77Wp8 nYOK+idzU04CHUr6rYMopFvtTdqqP7LFCDJqc27eSE5nXlX0hE1STMVzrhLAg8j3V6 TN7JKM3sjAmuxXv4kfghW02vPY1ONgq49Zu4CWBF6D9xTLePW6a+jAB/kC0R0YomTb owauZ4QlssR8+Rx8rQAzOlbBC5w2HJnhmb6ydZ5Zt3ztXWWcFjCy7UHNvtZD7BdpVa SVeQkyL720tsw== Message-ID: Date: Tue, 18 Aug 2026 19:53:48 +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 03/12] mm: add PMD swap entry splitting support To: Usama Arif , Andrew Morton , chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org Cc: ying.huang@linux.alibaba.com, Baoquan He , willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, Nico Pache , "Liam R. Howlett" , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, kernel-team@meta.com References: <20260818131202.494754-1-usama.arif@linux.dev> <20260818131202.494754-4-usama.arif@linux.dev> 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: <20260818131202.494754-4-usama.arif@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/18/26 15:09, Usama Arif wrote: > Add a swap branch in __split_huge_pmd_locked() that splits a PMD swap > entry into 512 PTE swap entries. No folio reference is needed because > swap entries point to swap slots rather than pages. Each PTE inherits > the correct sub-slot offset and preserves soft_dirty, uffd_wp, and > exclusive flags. > > The folio_remove_rmap_pmd() gate at the end must inspect old_pmd > rather than *pmd: for a present THP split, *pmd has already been > cleared by pmdp_invalidate(), and that invalidated bit pattern can > decode as a plausible swap entry. > > This branch is reached from the explicit __split_huge_pmd() callers > that hit a non-present PMD: partial-range mprotect / munmap, the > wp_huge_pmd() PMD-COW fallback, and the swap-in / swapoff fallbacks > added in later patches when the cached folio is no longer PMD-sized. > page_vma_mapped_walk() does not iterate PMD swap entries, so > try_to_unmap_one() and try_to_migrate_one() do not reach this branch > and freeze=true cannot occur in this branch today. page and folio > are therefore left uninitialized in the swap branch; a > VM_WARN_ON_ONCE(freeze) catches any future caller that breaks this > invariant before the freeze path dereferences page_to_pfn(page + i) > or put_page(page). > > Signed-off-by: Usama Arif > --- > mm/huge_memory.c | 29 ++++++++++++++++++++++++++++- > 1 file changed, 28 insertions(+), 1 deletion(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 1b6b0aa2baa3b..a473e85d30f51 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3252,6 +3252,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR, > vma, haddr, rmap_flags); > } > + } else if (pmd_is_swap_entry(*pmd)) { > + VM_WARN_ON_ONCE(freeze); > + /* Swap entries have no page for the migration freeze path. */ > + freeze = false; It's odd to VM_WARN_ON_ONCE() and then set freeze=false; I'd just add the comment above the VM_WARN_ON_ONCE() and drop the =false. freeze=true really only applies during page migration, where swap entries don't apply. I think it's time to clean that up ... that is a) Expose a helper called split_pmd_to_migration_entries() that is only used by code that installs migration entries. b) Hide that "freeze" flag from all other file-external functions c) Rename the boolean to "use_migration_entries" Then it's rather clear in this code that this should never happen. > + old_pmd = *pmd; > + soft_dirty = pmd_swp_soft_dirty(old_pmd); > + uffd_wp = pmd_swp_uffd(old_pmd); > + anon_exclusive = pmd_swp_exclusive(old_pmd); > } else { > /* > * Up to this point the pmd is present and huge and userland has > @@ -3388,6 +3396,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > VM_WARN_ON(!pte_none(ptep_get(pte + i))); > set_pte_at(mm, addr, pte + i, entry); > } > + } else if (pmd_is_swap_entry(old_pmd)) { > + softleaf_t sl_entry = softleaf_from_pmd(old_pmd); No existing code uses "sl_entry". Maybe just call it "pmd_swp_entry"/"swp_entry" and below "pte_swp_entry". Apart from that nothing jumped at me :) -- Cheers, David