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 7C3C955C1A7 for ; Tue, 22 Sep 2026 14:48:49 +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=1790088530; cv=none; b=KfafzCyxynBaTXiy4Vnq///Ru1IyMpQme5Zf5sVH01FPquv1sZSvbUTK8YsgDaWcKbnQ2ZY3g2hRKk9T/TUOtkfdsDboz4ZcMWnUtlE9sKhaiK2Ly/zF19G595+xY3XTO8/CRLVzYWe8JZdgRMe7FnjY1RnC0jChWLzfPTM3QYI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790088530; c=relaxed/simple; bh=cUW7Lep4DhVG+1N/uF5trcKujEDTkiBzzMD6eXtyhfk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ri4wfQOpLa/Nd7TDo72jrcIpTnMd81s9J8sJ7jV3FgXnezsrORQ3oRhS7itIP5OiHAwJHhS8bX9gmhwuerl2ge7PEUR8fjaw3cDs19K3IUku4qJI+NqpFdJAhFpXHiUdN5Gf6tv1dj5SGcqfT4qseVn4BwXGMLTg1VlhKozPP+s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KlCNBB/p; 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="KlCNBB/p" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DFEA11F000FF; Tue, 22 Sep 2026 14:48:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790088529; bh=2nC+4Iujr6aKs/X6DmrJUSeJqe7aXLMr7UtX3jEw9Y8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=KlCNBB/pRmBcWRkN5rgO1CAIPNCqGtKjNZQDWlNRIpflyWihuVVSoB1WuY85jHBg2 i254OvM/DNaWmjsj294C5s6McXHOL9dxgMgQE69ZbesKz3zHghquFjmsm10I3dy/+N 7vKwjWbUCFw3R9s9cjoIs4y0qbACZUKITuIXJJAoLimblf7Ss16+tLRkP+GHhix5EW YjwKbEhfOIPrS8ipScASQxJ6qcNXHz7ocvy2CIVE7E3I93XSMtoSJ/unpOtRx2nMs3 KT41buPpA6i8lEwxzora/x71CKTIGHAUT8Le8f5TbuxfcNs9441IqxMFKA5JWsETXo pxd8tftqpLGVA== Message-ID: Date: Tue, 22 Sep 2026 16:48:35 +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 08/29] mm: recognize PMD swap entries in the softleaf layer 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, qi.zheng@linux.dev, luizcap@redhat.com, kernel-team@meta.com References: <20260914122950.3283997-1-usama.arif@linux.dev> <20260914122950.3283997-9-usama.arif@linux.dev> <47ff450a-c2df-4520-8589-63dbb1746f80@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: <47ff450a-c2df-4520-8589-63dbb1746f80@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/22/26 15:08, Usama Arif wrote: > > > On 18/09/2026 22:56, David Hildenbrand (Arm) wrote: >> On 9/14/26 14:27, Usama Arif wrote: >>> Reclaim splits a PMD-mapped anonymous THP into PTE-level swap entries >>> before unmapping it, so an ordinary swap entry has never had to appear in a >>> PMD. Later patches install one there instead, and the softleaf layer is >> >> "Prepare for ..." >> >>> where every consumer decodes non-present PMDs. >>> >>> Accept swap entries as valid PMD softleaves and add pmd_is_swap_entry(). >>> A swap entry carries no PFN, so make pmd_softleaf_to_folio() warn and >>> return NULL rather than interpret a swap offset as a page frame number. >>> >>> Unlike migration and device-private entries, a PMD swap entry can also >>> carry the swap-exclusive marker, which softleaf_from_pmd() has to strip >>> before decoding. Strip all three overlays unconditionally while we are >>> here: each clear is a plain bit clear, so testing first only buys a branch. >>> >>> Signed-off-by: Usama Arif >>> --- >> >> >> [...] >> >>> +/** >>> + * pmd_is_swap_entry() - Does this PMD entry encode an actual swap entry? >>> + * @pmd: PMD entry. >>> + * >>> + * Returns: true if the PMD encodes a swap entry, otherwise false. >>> + */ >>> +static inline bool pmd_is_swap_entry(pmd_t pmd) >>> +{ >>> + return softleaf_is_swap(softleaf_from_pmd(pmd)); >>> } >> >> We don't have a pte_is_swap_entry(), I guess because we always want to handle >> different softleaf types through softleaf_from_pte() first. >> >>> >>> /** >>> * pmd_is_valid_softleaf() - Is this PMD entry a valid softleaf entry? >>> * @pmd: PMD entry. >>> * >>> - * PMD leaf entries are valid only if they are device private or migration >>> - * entries. This function asserts that a PMD leaf entry is valid in this >>> - * respect. >>> + * PMD leaf entries are valid only if they are device private, migration, >>> + * or swap entries. This function asserts that a PMD leaf entry is valid >>> + * in this respect. >>> * >>> * Returns: true if the PMD entry is a valid leaf entry, otherwise false. >>> */ >>> @@ -660,10 +670,12 @@ static inline bool pmd_is_valid_softleaf(pmd_t pmd) >>> * pmd_softleaf_to_folio() - Convert the PMD softleaf entry to a folio. >>> * @pmd: PMD entry. >>> * >>> - * The PMD entry is expected to be a valid PMD softleaf entry. >>> + * The PMD entry is expected to be a valid PMD softleaf entry that references a >>> + * PFN, that is a migration or device private entry. A PMD swap entry is a valid >>> + * softleaf entry but encodes swap slots rather than a PFN, so it has no folio. >>> * >>> - * Returns: the folio the softleaf entry references if this is a valid softleaf >>> - * entry, otherwise NULL. >>> + * Returns: the folio the softleaf entry references, or NULL if the entry is not >>> + * a valid PMD softleaf entry or does not reference a PFN. >>> */ >>> static inline struct folio *pmd_softleaf_to_folio(pmd_t pmd) >>> { >>> @@ -673,6 +685,10 @@ static inline struct folio *pmd_softleaf_to_folio(pmd_t pmd) >>> VM_WARN_ON_ONCE(true); >>> return NULL; >>> } >>> + if (!softleaf_has_pfn(entry)) { >>> + VM_WARN_ON_ONCE(true); >>> + return NULL; >> softleaf_to_folio() does a >> >> VM_WARN_ON_ONCE(!softleaf_has_pfn(entry)); >> >> Just curious: why is that not sufficient? Which could would we expect to call >> into this fuction with nonesense? (same applies to the existing runtime check) >> > > Because softleaf_to_folio() warns after the damage: > > static inline struct folio *softleaf_to_folio(softleaf_t entry) > { > struct folio *folio = pfn_folio(softleaf_to_pfn(entry)); > > VM_WARN_ON_ONCE(!softleaf_has_pfn(entry)); > > pfn_folio() is page_folio(pfn_to_page(pfn)), and page_folio() is > _compound_head(), i.e. READ_ONCE(page->compound_info). So a swap > offset interpreted as a PFN is already dereferenced by the time the > warning runs. Heh, and in softleaf_to_pfn() we have another VM_WARN_ON_ONCE(!softleaf_has_pfn(entry)); So the one in softleaf_to_page() is rather useless already? :) -- Cheers, David