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 DF22E3DB641; Wed, 14 Oct 2026 18:29:43 +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=1792002585; cv=none; b=AiJuqMBFgzN9mMV28FM1A6hHG4ywOF6Ty2WJrcEhzNsngSqjmSTvOk187pXxWyLkfx8d+vD1IepbDOhLTK0s/tZrgCdNYtYi+RfjX4jPZbu2Bk1iqdJyB9UC9i8zYHuus/AgVG1JnTLzDB7ocekNiatoL8vIWZUXdQZUS5DCw/8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792002585; c=relaxed/simple; bh=IEpek5EVQJlUi70tJL7kLqJX+LTvl3pbxUeeMBzLQ7s=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BjNffRDAgP4cMTqL1or4E1drN5A/tTS1rSeVIi/1Zc8uyMLW84Or5zCKOk8k+KCKlIHfis+YR5tahYVSfkj0hiBE7RNeFi6ThIGWoFUEmASxdtHAQ3N7CQldkyc8asRnlgY4TkMfZvSQvO6buD5Xc02oNfOwvQrKmHZw1V1KMMA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lUh51J//; 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="lUh51J//" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4B4BB1F000C4; Wed, 14 Oct 2026 18:29:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792002583; bh=OTQB0TUAXUD3vxID5BdQah95iIYmmihnkyQRPpRL68Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=lUh51J//ZpG1LvBeZMWXi9C2zDPmftO2Z9kWvh+UGMBT0fEhZz8eLiUf6Fj4NXtSk /zCbkpzo1Edcb+oUVOKw+ZcVasSMAADZ7N6FMj8MIMcK5j2YWXAPdTmgOuaZoOLH3q CCkoio3TrMQuGevPK0CN6yUnk2fMdZw4myNBFpZdZf3H1Eq4eUL6em0pn8vH8Ktt/G MAI9aU4JrPDAwXnJv4JLohBdnPzdUPcpNurDlVmXPCdUR+UJ2TuGRzYungwqomF5oU JlFVawmL65Sb6ivYBQCWZ5F76JGcz7jbmZ0kQuPI+SLNkkA0aiCxkAsQh/dlRPfNUg tlyGqGkPKN3xg== Message-ID: Date: Wed, 14 Oct 2026 20:29:35 +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: [PATCH] mm/userfaultfd: only allocate anon_vma in CoW mappings To: "Lorenzo Stoakes (ARM)" Cc: Muchun Song , Oscar Salvador , Andrew Morton , Mike Rapoport , Peter Xu , Miaohe Lin , Lokesh Gidra , "Liam R. Howlett" , Kyle Zeng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261014-copy_hugetlb_page_range-v1-1-5b68e03fe7fe@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 10/14/26 16:51, Lorenzo Stoakes (ARM) wrote: > On Wed, Oct 14, 2026 at 01:53:11PM +0200, David Hildenbrand (Arm) wrote: >> As uffd_lock_vma() is part of the same lookup machinery and >> also contains a misleading VM_SHARED check, change that one as well >> directly for consistency. > > OK, I think that one isn't _strictly necessary_, but while we're there, > worth doing too, clearly. > > We should also go through and convert the remaining !VM_SHARED checks -> > vma_is_cow_mapping() too if possible, but that's a follow up, obviously! > >> >> A follow-up cleanup will stop taking the hugetlb_vma_lock_read(src_vma) >> lock in copy_hugetlb_page_range() entirely. Further, the usage of > > Hmm really? Is it possible to do that without the hugetlb lock? > Yes, we don't have to sync against PMD sharing if PMD sharing is impossible. This is what I have laying around: commit ad6a5c65bd5fa31d93a2de2d2e5e473b26ad6a3d Author: David Hildenbrand (Arm) Date: Sat Oct 10 13:00:13 2026 +0200 mm/hugetlb: stop syncing against PMD table sharing in copy_hugetlb_page_range() We must never end up in copy_hugetlb_page_range() when PMD sharing applies, yet we still have to perform the hugetlb_vma_lock_read() so hugetlb_walk() is happy. Let's introduce a new vma_supports_pmd_sharing() and use that instead in hugetlb_walk(). Also convert want_pmd_share(), which now looks a lot cleaner, and document why the uffd checks cannot go into __vma_shareable_lock(). We can now relocate our safety checks in copy_hugetlb_page_range() to the beginning of the function. No functional change intended. Signed-off-by: David Hildenbrand (Arm) diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h index 5029c72418631..b2da5d5816944 100644 --- a/include/linux/hugetlb.h +++ b/include/linux/hugetlb.h @@ -1389,6 +1389,20 @@ static inline bool __vma_shareable_lock(struct vm_area_struct *vma) bool __vma_private_lock(struct vm_area_struct *vma); +static inline bool vma_supports_pmd_sharing(struct vm_area_struct *vma) +{ +#ifdef CONFIG_USERFAULTFD + /* + * Keep the UFFD checks out of __vma_shareable_lock(): UFFD flags can + * change at runtime while the hugetlb VMA lock is held. They must not + * cause unlocking or freeing an existing lock to be skipped. + */ + if (uffd_disable_huge_pmd_share(vma)) + return false; +#endif + return __vma_shareable_lock(vma); +} + /* * Safe version of huge_pte_offset() to check the locks. See comments * above huge_pte_offset(). @@ -1406,7 +1420,7 @@ hugetlb_walk(struct vm_area_struct *vma, unsigned long addr, unsigned long sz) * * NOTE: lockdep_is_held() is only defined with CONFIG_LOCKDEP. */ - if (__vma_shareable_lock(vma)) + if (vma_supports_pmd_sharing(vma)) WARN_ON_ONCE(!lockdep_is_held(&vma_lock->rw_sema) && !lockdep_is_held( &vma->vm_file->f_mapping->i_mmap_rwsem)); diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 60f9b209c8db1..606e93e32eefb 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4987,22 +4987,16 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, * Shared mappings only need copying when we could have * information stored in page tables (e.g., userfaultfd PTE * bits or markers), which is incompatible with PMD sharing. - * Still take the hugetlb VMA lock for now to satisfy - * hugetlb_walk()'s locking requirements. */ - hugetlb_vma_lock_read(src_vma); + if (WARN_ON_ONCE(vma_supports_pmd_sharing(src_vma) || + vma_supports_pmd_sharing(dst_vma))) + return -EINVAL; } last_addr_mask = hugetlb_mask_last_page(h); for (addr = src_vma->vm_start; addr < src_vma->vm_end; addr += sz) { spinlock_t *src_ptl, *dst_ptl; - if (WARN_ON_ONCE(want_pmd_share(src_vma, addr) || - want_pmd_share(dst_vma, addr))) { - ret = -EINVAL; - break; - } - src_pte = hugetlb_walk(src_vma, addr, sz); if (!src_pte) { addr |= last_addr_mask; @@ -5147,8 +5141,6 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, if (cow) { raw_write_seqcount_end(&src->write_protect_seq); mmu_notifier_invalidate_range_end(&range); - } else { - hugetlb_vma_unlock_read(src_vma); } return ret; @@ -7005,17 +6997,9 @@ bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr) unsigned long start = addr & PUD_MASK; unsigned long end = start + PUD_SIZE; -#ifdef CONFIG_USERFAULTFD - if (uffd_disable_huge_pmd_share(vma)) - return false; -#endif - /* - * check on proper vm_flags and page table alignment - */ - if (!(vma->vm_flags & VM_MAYSHARE)) - return false; - if (!vma->vm_private_data) /* vma lock required for sharing */ + if (!vma_supports_pmd_sharing(vma)) return false; + if (!range_in_vma(vma, start, end)) return false; return true; Will send that out for the next cycle. > It'd be good in general if we could get rid of that lock as a whole :) but maybe > dreaming. Yeah :) Thanks! -- Cheers, David