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 D2318331ED4 for ; Tue, 18 Aug 2026 07:30:47 +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=1787038248; cv=none; b=i1O7ywiPojc8bFpMYESycQPy78xkR7b25GfTiNFVriHhpKlnDp3k197judyBhzDRFiE2lE7X2VPhAMw6LWTVDwANXa6QQup+cfS6Rn7/zASMCIM7GMsnFjxUkZKry+Olg3zuLtStdL51DXl/OiCDWX5vHYrzMfBluUW9cE4bjSI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787038248; c=relaxed/simple; bh=BwurLIq3HCpOJJ5GvgkL2TrtnkBfZhW9ZjeB6/tlSCs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bR+C6Oxh2ky2Wti0vWXmMgxljVPX+PrDp1uMVDxhtCOeRukFMr7dYXBfRR5XP3a59eMjhDSsFNgT7MkTFwGWdUqolnSx2hJNAO4XXFPjKRDsffeUQ7zvLk0eIY8Pwd45emp/wyHtDXOJYBOQ6MOzMAowMqfmY9w5EWDxMcFivDo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YCApkB98; 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="YCApkB98" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 432DD1F00A3A; Tue, 18 Aug 2026 07:30:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787038247; bh=F3+9yofyMB4DtAL+J7TlokgqleL68Gal+la2A8GH2EA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=YCApkB98bjF2ENMhASp4r3DdYbm3aea8c+dCZWnQdvq0xs/ZmvhNepadolKrRhcZ0 NIUtKoAfeTZs1fe+1qoortKcNpBKzu8gXjnNCnS7LXgkcg8fxYWPgs32AVHEFrzi+1 Zz0Q6kuoYVmvrETQQa4KDh78vSxYOTu77KCUI6HRbaatuNztRwtRUGCsbKc8iBryNm Ih1jTPpgwnjc0tWpuQwcwdI+6k4HnoZj5vXqb7zXP6SmIhG936m2w65Eulg6WVjBOY eHSipTtxkECUllX0VWdVf7w2NXMxwsQESm7LCqgc3hn+Qo8t9yhljWzw4D3LhSmfXb 9XRAsN8cPq/NA== Message-ID: Date: Tue, 18 Aug 2026 09:30:40 +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 v4] mm: Use a folio in the softleaf_is_device_private path To: Hongfu Li Cc: akpm@linux-foundation.org, liam@infradead.org, lihongfu@kylinos.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org References: <20260818021326.5910-1-hongfu.li@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: <20260818021326.5910-1-hongfu.li@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/18/26 04:13, Hongfu Li wrote: >>> So after migrate_to_ram() the folio might have been split or otherwise somehow >>> the page doesn't belong to the same locked, refcount-incremented folio it did >>> before? >> >> I suspect a split. >> >>> >>> That's kinda a footgun... but this documents it at least. >>> >>> I think a comment explaining how this can happen would be helpful though as this >>> doesn't seem intuitive. >> >> I think, conceptually, calling into something that consumes a page (vmf->page) >> always needs care when operating on folios. >> >> Passing the vmf to some callback might be the odd thing here, because the >> vmf->page contract is not really clear. > > I'm very sorry for introducing this regression. > Thank you for identifying and testing this issue. > > I will double-check this code path and apply your fix to run further > tests locally. > > Would it make sense to add a comment here like the below, to make this > subtle behavior clearer for future readers? > > diff --git a/mm/memory.c b/mm/memory.c > index 4134ac607ee0..2b6d6c863ecb 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4937,6 +4937,12 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > pte_unmap_unlock(vmf->pte, vmf->ptl); > pgmap = page_pgmap(vmf->page); > ret = pgmap->ops->migrate_to_ram(vmf); > + /* > + * migrate_to_ram() can split a large folio, updating > + * vmf->page to a different folio. Re-fetch folio for > + * correct unlock/put. > + */ "migrate_to_ram() might have split the folio." Should be sufficient I guess. -- Cheers, David