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b=LSO7R61wL+aLO6KbAwe0jWoNWM1FAzOzCMNhg8aS2Yo8Z6Sh2e6ZhiEgV6gGZp+7g TKvjymHtBxm13ODqKbgk/8bN+82kmzGGzou1qqNE8IFrtUtjc1Ov1XSqjTClGDsYki XidPsUY42Fs64GVSKn80bxAWxBhpjxni5k4O/Ta211VYccQpQpyNpfCvZLZL0f/or1 Sukjag9jvAqv3c941+wWOrAp1H6wCPU01831ivgBotybb7Qoz+CPijLK4zEcLx45KZ IQldhlbIaJrrhBfppaWGoVfa/I35WQo0MMLtRI5zQtXTHHogdNsefN0FtZkdXhvhV7 Ek8K4hCZDCNpQ== Message-ID: <286425bc-e2a2-4e29-abdf-666e798e6cf4@kernel.org> Date: Wed, 22 Apr 2026 09:27:44 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/hmm: Add userfaultfd support to fault handling To: Stanislav Kinsburskii , jgg@ziepe.ca, leon@kernel.org, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <177499585916.169073.8313365481186071940.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net> 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: <177499585916.169073.8313365481186071940.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: A186D40003 X-Stat-Signature: 1c47q4f6i466ynnqfpt5cdeuksm56ws4 X-Rspam-User: X-HE-Tag: 1776842870-359436 X-HE-Meta: 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 quCyX7oC mK4L3QR63yXoELO5lvOTCOHp+IVU4SstoO2syqkgeXNNOp01tEeYjuzK7YVTVLAi/bYK9rrIWXWOCxLvCEH1tATabm4QuI3/GDHfUivWZlg9QAQahaIR/R94uGevDq1g7HC3h7wNR9JBAZREnhGFeDkLTd3EFV+lrCqmZFI3lrZgjwIfe5pZMvqxOQf+zoExS1hV3SsoqmjzP1I380WkXqB2J6fIX15hLeon/Mn0HMlFuE61Vtmbgi4m/FZcTycZKqppJ6u4BCzLIA/kTOegcEralYQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/1/26 00:24, Stanislav Kinsburskii wrote: > Add support for userfaultfd-enabled VMAs to the HMM framework. > > Extract fault handling logic into hmm_handle_mm_fault() to handle both > regular and userfaultfd-backed mappings. The implementation follows > fixup_user_fault() for handling VM_FAULT_RETRY and VM_FAULT_COMPLETED, with > a key difference: instead of retrying or moving forward respectively, > return -EBUSY after reacquiring mmap_read_lock. Since the lock was > released, the VMA could have changed, so defer retry logic to the caller. > > This approach is inefficient for userfaultfd-backed VMAs, as HMM can only > populate one page at a time, but keeps the framework simple by avoiding > complex retry logic within HMM itself. > > Signed-off-by: Stanislav Kinsburskii > --- > mm/hmm.c | 40 ++++++++++++++++++++++++++++++++++++---- > 1 file changed, 36 insertions(+), 4 deletions(-) > > diff --git a/mm/hmm.c b/mm/hmm.c > index f6c4ddff4bd6..d04d68e21473 100644 > --- a/mm/hmm.c > +++ b/mm/hmm.c > @@ -59,6 +59,35 @@ static int hmm_pfns_fill(unsigned long addr, unsigned long end, > return 0; > } > > +static int hmm_handle_mm_fault(struct vm_area_struct *vma, > + unsigned long addr, > + unsigned int fault_flags) > +{ > + int ret; > + > + if (userfaultfd_missing(vma)) { > + struct mm_struct *mm = vma->vm_mm; > + > + fault_flags |= FAULT_FLAG_ALLOW_RETRY | > + FAULT_FLAG_USER; > + > + ret = handle_mm_fault(vma, addr, fault_flags, NULL); > + > + if (ret & (VM_FAULT_COMPLETED | VM_FAULT_RETRY)) { > + mmap_read_lock(mm); > + return -EBUSY; > + } > + > + if (ret & VM_FAULT_ERROR) > + return vm_fault_to_errno(ret, 0); > + } else { > + ret = handle_mm_fault(vma, addr, fault_flags, NULL); > + if (ret & VM_FAULT_ERROR) > + return vm_fault_to_errno(ret, 0); > + } I'm surprised that there is userfaultfd_missing() logic required here at all. What prevents us from always calling handle_mm_fault() in a way + handling return values, such that we will just do the right thing independent of userfaultfd_missing()? -- Cheers, David