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 1FD70436BC0 for ; Mon, 5 Oct 2026 09:25:06 +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=1791192308; cv=none; b=bBqHW56xAI80CIvtOu3clb2Khbp3PHgZnA9bHF5R2dkdIwdVQMHGGW5H5I/wQqJUfYrq64qOM6fbzwA375JgOWAdYNAfZtZ7NJK1OwEL7ugglhN84hxjAOlFwodF8r+T20rQN0BPSWQuXCvkrk1RIS1H57P4f6WSuY5mvZWGYqA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791192308; c=relaxed/simple; bh=y1kj9lY97xLVKAI4wcouMS8EPF/7EchJo2f9kuv624o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rN6AUZ8XDNBb6UQRTGruwL38aFhRzSSBAt8o5FKGD53o/rOovozL6JfU2u07SSdtRnvPE/palFWvhV7jxQHSP/Jw9hMFBtJo0/thTKzGrXEpS2z8GiMB8oKV76WIjKjmHIrEkeaXmRCSAlikwa1zPIHebi5IQ0TBYrDXejJRTBc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZuIjpnA8; 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="ZuIjpnA8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2D261F00893; Mon, 5 Oct 2026 09:25:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791192306; bh=byVa2dejwe5R5G4MFSTCg2eRH8VZASl2SG1iY0abgbw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ZuIjpnA8ddooEStEs7wryYcVO1ILKnXO+zFolrn4vCFMg5+znQPR09/KaibiR/dAU REi69GMUP0DrPTrMn+DvbpMk7ZK4Od72xgIPVQButQsZ+RfSQT3ooR4P3q89ViCf/E kzcAHClDGvdGI1Z9YWbZfnSzLHAI+7MO+LOYKdpPjW/4klDa8NgdRqinc7yWKcao06 nDkapS+x5gHgzSgQV1qpJZabM0ttmXMrB6+Si0xhhvInUcrrvlEnHmz/KVWgUp/oIJ 03FYfGFarjbVXAryam41K6H3YWAGhDwQPpsq5LkAsMENFj4mowwULNmnekvXiWrim/ aYKz7Wt1ovTaw== Message-ID: <2b4571c5-8e51-4c91-9d88-11929f200875@kernel.org> Date: Mon, 5 Oct 2026 11:25:01 +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/hugetlb: avoid recursive i_mmap_rwsem in PMD sharing To: Li Zhe , "Jose A. Perez de Azpillaga" , "Lorenzo Stoakes (Arm)" Cc: akpm@linux-foundation.org, muchun.song@linux.dev, osalvador@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260930064308.58159-1-lizhe.67@bytedance.com> <9a7c026e-f5f2-40e0-84aa-919c988cc136@bytedance.com> 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: <9a7c026e-f5f2-40e0-84aa-919c988cc136@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/5/26 11:03, Li Zhe wrote: > On 9/30/26 5:31 PM, Jose A. Perez de Azpillaga wrote: >> On Wed, Sep 30, 2026 at 02:43:08PM +0800, Li Zhe wrote: >>> huge_pmd_share() always takes i_mmap_rwsem for read before looking for a >>> shareable PMD page table. >>> >>> That is unsafe for callers that already hold the same mapping lock for >>> write. In particular, move_hugetlb_page_tables() takes i_mmap_rwsem for >>> write to prevent truncation races, then calls huge_pte_alloc() for the >>> destination address. If the destination PUD is empty and PMD sharing is >>> possible, huge_pte_alloc() can call huge_pmd_share(), which then tries to >>> take the same rwsem for read and can deadlock on itself. >>> >>> PMD sharing is only an optimization. If the read side of i_mmap_rwsem >>> cannot be acquired immediately, fall back to allocating a private PMD >>> table instead of blocking in the sharing path. >>> >>> Signed-off-by: Li Zhe >>> --- >>> mm/hugetlb.c | 14 ++++++++++++-- >>> 1 file changed, 12 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index cea25773a6c95..75631101a6807 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -6995,7 +6995,15 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma, >>> pte_t *spte = NULL; >>> pte_t *pte; >>> >>> - i_mmap_lock_read(mapping); >>> + /* >>> + * Some callers already hold i_mmap_rwsem for write, for example >>> + * move_hugetlb_page_tables(). PMD sharing is only an optimization, so >>> + * fall back to a private PMD table instead of blocking on the same >>> + * rwsem in read mode. >>> + */ >>> + if (!i_mmap_trylock_read(mapping)) >>> + goto alloc; >> try deadlock is not a fallback here. both callers already hold the write >> lock, and that is enough for the walk, so sharing is never attempted on >> these paths. >> >> hugetlb_change_protection() does thr same thing on the uffd-wp path and >> is not in the changelog. >> >> and no Fixes tag either. does this want Cc: stable? > > > Thanks for the review. > > I agree that holding the write lock should be enough for the walk, and > that avoiding PMD sharing on these paths is not the right direction. > > David mentioned that Oscar was planning to send a proper fix for this > issue, so I will wait for that patch instead of moving forward with this > trylock fallback approach. @Lorenzo, if Oscar is too busy, I guess we can paste the overall idea for the fix here as well (publicly)? -- Cheers, David