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 6753B3EA94E for ; Wed, 20 May 2026 14:38:52 +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=1779287933; cv=none; b=p1a2Oa5wcWIZk9iIpihE6olssj1M6v4nqrIo1NwVd3fHEA68TKQxyCDtWiMJRTBDv2hXLmHzSbbxmpVu5c4qWJgoLx8qudq3MFdfvw3EzU2h10q2SWLviQdvm/5h5HRQV4HSgP+W/LhPDqWpRu/aPRKQEH3GYunF4tfBe6nIIz8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779287933; c=relaxed/simple; bh=N17wpEv4lx+cnxhYOb/wNvTmLrr603FGXrA1NBm6zPs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Nlrx7pDd2JamDfMloMP2y3p+hA1xiLpnYkTc1HkvTvA60Sa+rMESU+L0vwWkP7NSwnO7ThLs9HQlX0ub7/pUCMmeuZoKjIiVRIWpQ5PEgh4z+XEleQlsx/MFU6gLSXV+Z4XHJCqDcDf2WrhGJMxnatepKTAJhMTvltQfiR3u3+k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PiygwCkV; 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="PiygwCkV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8D58E1F000E9; Wed, 20 May 2026 14:38:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779287932; bh=+7rcEL73sq9QIWTen1JeXHihv7DANv0+QzPRZk/coa8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PiygwCkVUt7wLXGUiPZY86Unli7lub9d3buEBZOr65YRoC1wzH5aT5wxU5wqIS0/N MUHhZlSALuWQo87yE9npFAr27wCLmzgXO9YW1AAcmGEtxvI/cSAG/sRmD3vLRAl4pB CxV33DmR7dwu9CSvSQ6yxcotUq0vrmTMTLg0i/gA7WMqYWeEb6oOR7e9I1AUO/E13r DsqZZNRe9GNynCmMXnCj5aFXi2o6H0uBRsuY2pDyTF8kBlZJwxCEPpNiRiMrEU0qMK fuK2I1wYgDJH1z20JZ7rXkbJfdP+4aME61fgYVka0z3XQrjqCfh+hhtpzzwF70yqrj r+qKSKBGGrisQ== Message-ID: Date: Wed, 20 May 2026 16:38:42 +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 RESEND] userfaultfd: snapshot VMA state across UFFDIO_COPY retry To: Mike Rapoport Cc: Andrew Morton , David Carlier , Heechan Kang , "Liam R. Howlett" , Lorenzo Stoakes , Michael Bommarito , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260519052516.3315196-1-rppt@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 5/20/26 16:12, Mike Rapoport wrote: > On Wed, May 20, 2026 at 03:48:02PM +0200, David Hildenbrand (Arm) wrote: >> On 5/20/26 14:53, Mike Rapoport wrote: >>> >>> Do you mean per-VMA? >>> Per-MM counter would capture unrelated changes, e.g. an masvise() for >>> unrelated range. >> >> I was thinking about a per-MM thing for simplicity. If there were any changes, >> we'd retry (-EAGAIN). >> >> IOW, something like &mm->mm_lock_seq, which we have for per-vma locks already. >> >> Not sure if unrelated changes would really be a problem in practice (another >> thread gabbing the mmap_lock in write mode until we serviced the fault). > > Let me reiterate: > > A thread doing UFFDIO_COPY releases the VMA in mfill_copy_folio_retry(), > re-gets the VMA and checks if the per-MM counter stayed the same. > > If another thread makes any change to VMA while mfill_copy_folio_retry() > waits to re-get the VMA, the counter would be incremented by another > thread. mfill_copy_folio_retry() will see the change after mfill_get_vma() > and will bail out with -EAGAIN. > Yeah. > So if another thread does, e.g. MADV_DONTFORK on completely unrelated VMA, > mfill_copy_folio_retry() will return -EAGAIN and I'm not sure we'll not > break the existing userspace by this. Ah, you mean that -EAGAIN would not be handled by user space already even though documented in the man page ... because it relies on -EAGAIN not happening in specific situations (i.e., copying a single page). That's very likely true and we'd have to retry internally, which makes it more complicated indeed. -- Cheers, David