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 7740751596A for ; Tue, 22 Sep 2026 11:47:04 +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=1790077625; cv=none; b=e2FceGT/t8I/gnKaevcrPWzDwI0HltpM8AQ3diOs22QobJrmt299NX1qcrCrDkGcZUMdPE6wRCwTqKIc8vWXcpz+KyEBnGJj/9i+wyYc0skMkv2osJviM5E7UJ4Myh9pDbY5Zjx6fuEbLGBetPc3Un+4q93FXJHMD0wXkXARamQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790077625; c=relaxed/simple; bh=MH1V6cSXSxrjq8Z1OY0uVfiqxHoJKoXvtlO0RzQyNE4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jkLTdRYhSTLmfoLDi8P0HNwFWEKt48Dy8EN+oK2sDw0m/Y8JMRO06aIAiYbSeRktlsVroKak/HvlewQczS2185SiAKmcXzTQvh5P05wUTDaI+CPvVAYMVsCyh/4Yy0GKLzko1TL+ce3rRqb95o9m2CkhdMBATNAWtmVjndEId0k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=iLoYgScJ; 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="iLoYgScJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E48B01F000FF; Tue, 22 Sep 2026 11:46:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790077624; bh=c+DKAj4xzdIdJKSxQBQcBD03qNvXqcqF0lY3BnsNpe0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=iLoYgScJP8kyzXcT27ks9DHRBs9LUcWMlsVKfWujGuDqvoNr6FSuHvHB9/SoFCLpZ O32AE0nCsHqqF7z8754fWqJHurtvL4n/fjjS2I7gphbJZ6D6zV+XjQ5hxGt1/Qwk2W mjNmJ9CGfKoy1zhGkRuTBiuYMOm3uvBKZqDlVsTZXCGc38ApHeh9D9ojsJOxpU5+Uv VVuP4G9IkioxURcBOpKQlZaU7hlpEqzzrebwItu+acqyPCIlvyTgiinzlO0QjB4+Up 983xFLBA0M4YrSlIuCJvyVCHZx36nUuziYpWWfITljmkXGhPjSMLvScuDrNrwDQky5 58uEhwfCy+S3A== Message-ID: <4101d661-7b16-401c-aaef-b87e81fdaee3@kernel.org> Date: Tue, 22 Sep 2026 13:46:51 +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: [RESEND v7 11/29] mm: split PMD swap entries into PTE swap entries To: Kiryl Shutsemau , Usama Arif Cc: Andrew Morton , chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, 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, 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, qi.zheng@linux.dev, luizcap@redhat.com, kernel-team@meta.com References: <20260914122950.3283997-1-usama.arif@linux.dev> <20260914122950.3283997-12-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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/16/26 17:08, Kiryl Shutsemau wrote: > On Mon, Sep 14, 2026 at 05:28:01AM -0700, Usama Arif wrote: >> Once a PMD can hold a swap entry, everything that splits a PMD - mprotect() >> or munmap() over part of the range, MADV_FREE, a pagewalk with no PMD >> handler - has to be able to split that entry too, or the callers that rely >> on split_huge_pmd() to hand them a PTE table would find the PMD unchanged. >> >> No reference counting is needed: a swap entry pins no folio, and swap_map >> is already one per slot, so the PTEs simply take over what the PMD held. >> >> The migration-only entry point cannot reach the new branch, because >> page_vma_mapped_walk() never hands back a swap PMD for the folio being >> migrated. Warn if that ever changes, and force the regular split anyway, >> since the branch leaves folio and page uninitialised. >> >> Test the pre-split old_pmd rather than re-reading *pmd in the trailing >> folio_remove_rmap_pmd() gate, so every entry-type test in the function >> interrogates the same snapshot. That part is cosmetic: pmdp_invalidate() >> leaves the PMD present as far as software is concerned. >> >> Signed-off-by: Usama Arif >> --- >> mm/huge_memory.c | 36 +++++++++++++++++++++++++++++++++++- >> 1 file changed, 35 insertions(+), 1 deletion(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 873887aed0bc2..0e347a545588c 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3304,6 +3304,21 @@ 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)) { >> + /* >> + * A PMD swap entry has no page, so it cannot be turned into >> + * PTE migration entries. page_vma_mapped_walk() never hands >> + * one back for the folio being migrated, so this should not >> + * happen; warn, but also force the regular split so that a >> + * broken invariant cannot make the code below dereference the >> + * uninitialised folio and page. >> + */ > > The comment can be shorter. > >> + VM_WARN_ON_ONCE(use_migration_entries); >> + use_migration_entries = false; >> + 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); > > The logic looks right to me, but __split_huge_pmd_locked() is getting > awkward. It is close to 300 lines with two if-else chains that have to > be kept in sync. > > Can we have a preparatory patch that moves the PTE-install loops into > per-type helpers? > > split_pmd_into_migration_ptes(), split_pmd_into_device_private_ptes(), > split_pmd_into_present_ptes(). There were recently patches about related cleanups: https://lore.kernel.org/r/cover.1787941780.git.yintirui@gmail.com I'm fine with cleaning this up later (I hope we can land this series in 7.4). -- Cheers, David