From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 DBF4F39C65D for ; Wed, 15 Apr 2026 08:13:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776240794; cv=none; b=Z/oZcjQpMTXF4+FEFzKH7tfK7K/Ken73yci5KrkoAsrGm8lVSPmP7PWmhbyPA310g6l3hRNAIZZFzfneZUbpxkAFhXTW1WjhToaivPA/v0qANdEhml+deGWB/SSJ/KIyXlU98faHlw24y+jDsROfzdxUI5RlnCW8p4UceisLEdM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776240794; c=relaxed/simple; bh=tABW2aXIM2rHOjCqXJFblXdHqk3XbsVf53PkS+Dy154=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mRI+c2l2QHFC3TOKz/e83+ZJ8U5DlfbJ57DuywoYYUqoS5pTlSCfNXNACx8ZWlDy5U7LZgFxrifOFetArQ0cwZVTmhYFANOTCFZrM32J4ACs7QuF9asG73zz2CL3hIdqNPiwt7IyjkbwMemjoFEpf6QaoPwiXu1/TuGetIURVuY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=qfDxXJsp; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="qfDxXJsp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F81BC19424; Wed, 15 Apr 2026 08:13:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776240794; bh=tABW2aXIM2rHOjCqXJFblXdHqk3XbsVf53PkS+Dy154=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=qfDxXJspff7lFWZzIFgIgc/06nA3epYQvHbLxIcOadIfTZ81DX90oolTC50rA4Uqz vnqXJGt5X9uNJ3IP049UTdLvVXLD8UXl39e6VjrrwQbgQedgovW/37zcrvUC2bZJQr e4e/Nchblg5Su9d0xFci5c5kZgKpaUwEcXbySs+kZEEWZjuh5sex1Dv08+rVDjCEyK S54egwSgCY6pvhdpIeWXiexTlwK7SYD8YHyRsY87GrNtGXrreFoH+XA4d0g/TnjeZA tjRK7HIANnMqoRhMW2Mzon3jx4+ykrEonUy/8xu0noRPc5b0/kTpnDGoe+lXHtZrbb WU6VOdJMp4hyg== Message-ID: <6edde1e9-0f42-4dae-b0d1-3f2895f2111b@kernel.org> Date: Wed, 15 Apr 2026 10:13:05 +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 v3 13/13] mm/huge_memory: add and use has_deposited_pgtable() To: Yin Tirui , Lorenzo Stoakes Cc: Andrew Morton , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kiryl Shutsemau , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <41b1ff54-c120-42ae-8b74-54767abf3554@gmail.com> <2f29f66b-46db-4925-b922-4add61b633bf@gmail.com> <53d748d3-4150-4e7b-8c1f-4c58587e9183@kernel.org> <6125defb-3aa4-494c-abed-982be684f839@gmail.com> <6417587a-7e43-4615-9e2c-50a245842f59@kernel.org> 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 4/15/26 05:50, Yin Tirui wrote: > Hi David, > > On 4/15/26 02:15, David Hildenbrand (Arm) wrote: >> On 4/14/26 17:14, Yin Tirui wrote: >>> >>> I did a quick tree-wide grep: >>> $ git grep -l "remap_pfn_range" | xargs grep -l "\.fault\s*=" >>> arch/powerpc/platforms/book3s/vas-api.c >>> drivers/infiniband/hw/hfi1/file_ops.c >>> drivers/uio/uio.c >>> drivers/vfio/pci/vfio_pci_core.c >>> fs/proc/vmcore.c >>> security/selinux/selinuxfs.c >>> >>> It turns out there are two users of this "hybrid" approach in the kernel: >>> 1. fs/proc/vmcore.c: It pre-maps via remap_pfn_range() but registers >>> mmap_vmcore_fault(). >>> 2. arch/powerpc/platforms/book3s/vas-api.c: It pre-maps via >>> remap_pfn_range(), but registers vas_mmap_fault(). >>> >>> >>> How would you suggest we proceed here? >> >> How about we populate PMDs in remap_pfn_range() only if !fault? > > Doing this would at most prevent VMAs with a ->fault() handler from > getting huge mappings, which seems to have little negative impact. > > But wait, dynamic huge mappings are actually created through ->huge_fault(). If my memory serves me right, also fault() can nowadays install PMD mappings. For example, shmem only implements ->fault through shmem_fault() finish_fault() after __do_fault() takes care of that (mapping through a PMD if possible). > > I did a quick grep: > $ git grep -l "remap_pfn_range" | xargs grep -l "\.huge_fault\s*=" > drivers/vfio/pci/vfio_pci_core.c > > This is a false positive. There is no case in the kernel that mixes > remap_pfn_range() and ->huge_fault() on the same VMA. > > What if we use !huge_fault instead, disallowing remap_pfn_range() from > populating PMDs if ->huge_fault() is provided? I think we should just disallow any PMD mappings if we either have ->fault or ->huge_fault. I would assume that ->huge_fault implies >fault, but let's rather be save than sorry. > > Then, when we encounter a huge PMD, we know for sure whether it was > installed through remap_pfn_range() (needs a deposited pgtable) or > ->huge_fault() (no deposit needed, can be refaulted). > >> >> Then, if we have !fault, we know that the PMD is from remap_pfn_range() >> and has a disposed page table. >> >> Would that work? >> > > So for Lorenzo's `has_deposited_pgtable()` helper, we could simply use: > > /* Huge PFN map without a huge_fault handler must deposit */ > if (vma_test(vma, VMA_PFNMAP_BIT)) > return !vma->vm_ops || !vma->vm_ops->huge_fault; As mentioned above, also considering vma->vm_ops->fault; > > > By the way, while auditing this, I noticed that > drivers/gpu/drm/drm_gem_shmem_helper.c calls vmf_insert_pfn_pmd() > directly from its normal ->fault() handler instead of implementing > ->huge_fault(). > If we adopt the `!huge_fault` check above, this DRM driver would be > wrongly classified as needing a deposit. It seems that DRM driver needs > a minor refactoring to properly use ->huge_fault() to keep the MM > semantics clean. No, it's doing something that's allowed. If we call ->fault and there is not PTE table, it may insert a PMD. -- Cheers, David