From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793896845; b=n+Pjm8VpfV/AK+BCndId6Mor62/kE81AP+bAHAYA/GhFBgb6efoAttX2v4MHRxG+bVp1 U7e5M4p1L9ArZ7eYkFpdIxRMPi2W/juaZhp1X7mKq61h/vbn2wKqshZbYB8sY7m9MwmXt xh/iaWo/0DZUqpFBAgHo7kJkgJDMBpUujmhUoBhajtM1ZGYYvBs0y3oLbsGhKpin6R5EI X9NTzCCQFEW0D/qN+duUk1dWtmJS4N8dO09yjvLlj6cKvJy3/0W4nm7AaCJji0Z1gJHWx noze5JQijfemGnj9cn5Nhf0pWsVDg6MPiO9hR0aBbP9na+iTMVO1NyH46rvy1mUp1Hw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793896845; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=jMfT6NnqJNQ4fwmo+zlAfszpntP7F4+aKhwQgsQvMfU=; b=D4+PNwhAvYWV9Ti4ueoKxTISS5XMvs+NfgPhE2+xRSucDMgoQ9K+g6tWAPeFljJP0wvp F9CHP6yn2ItPFDX3czWRGsSo+31hQvOAeri30xvgqiC6RmtjK7eqScmFMuqQkKwZ0ZhLS ysU3xoEHfV555gAYx30CLms9LIVVlzkp9s3M5SvhIciRKPljqjgX6ooC8xX1UAEObJ2Sd qOc2Qx3l76xpSIlUk2IG/MyfEYk+z8FtSiyVUOhb3rrg7Xj2Emv0CyLjubpCk5iDv4oFl 7daFFJGB+8rwDK0DcInF+RcvwEoutrIHrIdFdUMSI7xXxd4DFx/RGiuHyryK+NBKoog== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=eRTmDFF2; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 25809606A33B; Thu, 5 Nov 2026 16:40:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=eRTmDFF2 Received: by smtp.kernel.org (Postfix) with ESMTPSA id BE99E1F000C1; Thu, 5 Nov 2026 16:40:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793896844; bh=jMfT6NnqJNQ4fwmo+zlAfszpntP7F4+aKhwQgsQvMfU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=eRTmDFF21AokwEeLbpwgXPGufwdoan/3wTSHEhhcxjntte+tq/knqfW+SQtBcfKIX oDKY1eSAyS2RmFB+kVayVtrMrjXegac0cFwPwntDKHf+VWHV5AH97rP9Fk5PP7e6+z aGLlouWS7XmXh9xJWrJuL/CeImXsADQQnhRGK5eKGks5/XstygPJqB19ZJyF+C7Pur Ne9m0o5gMuet9ljDF9tNmp9It4Jhh8zosFfhaiOHcDuFpWREsdnmJZ0Pu6LKkINTyI KPkeU/fyQLg//pjEkhF3IsdOQ7czbi1Ck5RocPrgMLHf2QkxiJq7XuUUZJu3w6ZyTS kKZA4N/B/9eHA== Message-ID: Date: Thu, 5 Nov 2026 17:40:39 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/khugepaged: free unused page table when setting a huge PMD fails To: Kiryl Shutsemau Cc: Andrew Morton , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Qi Zheng , Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261105-khugepaged_set_pmd-v1-1-cb79ab117199@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 11/5/26 14:04, Kiryl Shutsemau wrote: > On Thu, Nov 05, 2026 at 01:02:08PM +0000, Kiryl Shutsemau wrote: >> On Thu, Nov 05, 2026 at 01:16:05PM +0100, David Hildenbrand (Arm) wrote: >>> do_set_pmd() can allocate vmf.prealloc_pte on architectures that need a >>> deposited page table, such as PowerPC with the hash MMU. If the PMD is >>> no longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK >>> without consuming that table. >>> >>> Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent >>> userfaultfd operation can install a PTE table before taking the folio >>> lock. If that races with set_huge_pmd(), the unused preallocated table >>> is lost when its local vm_fault goes out of scope. >>> >>> Free vmf.prealloc_pte on failure, as do_fault() already does. Successful >>> PMD installation consumes the table and clears the pointer. >>> >>> Having to do this cleanup outside of do_set_pmd() is rather nasty, but >>> other callers depend on it already having been allocated. This needs a >>> cleanup at some point. >> >> I hate this being outside of do_set_pmd(). >> >> What about making do_set_pmd() freeing ->prealloc_pte if it was >> allocated inside? Nothing changes for caller. > > Or if we want to keep in set_huge_pmd() then set_huge_pmd() has to > allocate it too. At least, it makes some sense. It's all rather nasty. I thought the callers depend on it, but if we just free in the function it will behave the same way independent of arch_needs_pgtable_deposit(), so that makes sense. >From 4e8d0a928f0b21e0583dac9234fb1821f3c973b2 Mon Sep 17 00:00:00 2001 From: "David Hildenbrand (Arm)" Date: Wed, 4 Nov 2026 20:56:37 +0100 Subject: [PATCH] mm: free unused page table allocated by do_set_pmd() do_set_pmd() can allocate vmf->prealloc_pte on architectures that need a deposited page table, such as PowerPC with the hash MMU. If the PMD is no longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK without consuming that table. Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent userfaultfd operation can install a PTE table before taking the folio lock. If that races with set_huge_pmd(), the unused preallocated table is lost when its local vm_fault goes out of scope. Track whether do_set_pmd() allocated the table and free it after dropping the PMD lock if it was not consumed. Preserve any preallocated table supplied by the caller for PTE fallback. Successful PMD installation consumes the table and clears the pointer. Returning without a preallocated table matches the existing behavior on architectures that do not need a deposited table. All callers already handle that case. Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()") Cc: stable@vger.kernel.org Signed-off-by: David Hildenbrand (Arm) --- mm/memory.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/mm/memory.c b/mm/memory.c index becf53a2a0944..421064aef3cf0 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5687,6 +5687,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa struct vm_area_struct *vma = vmf->vma; bool write = vmf->flags & FAULT_FLAG_WRITE; unsigned long haddr = vmf->address & HPAGE_PMD_MASK; + bool pgtable_allocated = false; pmd_t entry; vm_fault_t ret = VM_FAULT_FALLBACK; @@ -5725,6 +5726,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa vmf->prealloc_pte = pte_alloc_one(vma->vm_mm); if (!vmf->prealloc_pte) return VM_FAULT_OOM; + pgtable_allocated = true; } vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd); @@ -5755,6 +5757,10 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa count_vm_event(THP_FILE_MAPPED); out: spin_unlock(vmf->ptl); + if (pgtable_allocated && vmf->prealloc_pte) { + pte_free(vma->vm_mm, vmf->prealloc_pte); + vmf->prealloc_pte = NULL; + } return ret; } #else -- 2.43.0 Apparently we might be allocating in do_set_pmd() from a RCU critical section ... maybe I'll find the time to look into that. -- Cheers, David