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[79.242.48.41]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43ec1630f21sm16096415e9.8.2025.04.03.02.18.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 03 Apr 2025 02:18:53 -0700 (PDT) Message-ID: <1577c4be-c6ee-4bc6-bb9c-d0a6d553b156@redhat.com> Date: Thu, 3 Apr 2025 11:18:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings To: Jingbo Xu , Joanne Koong Cc: miklos@szeredi.hu, linux-fsdevel@vger.kernel.org, shakeel.butt@linux.dev, josef@toxicpanda.com, bernd.schubert@fastmail.fm, linux-mm@kvack.org, kernel-team@meta.com, Matthew Wilcox , Zi Yan , Oscar Salvador , Michal Hocko References: <20241122232359.429647-1-joannelkoong@gmail.com> <20241122232359.429647-5-joannelkoong@gmail.com> <1036199a-3145-464b-8bbb-13726be86f46@linux.alibaba.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <1036199a-3145-464b-8bbb-13726be86f46@linux.alibaba.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: VxQH-pn-r1XKW_kgcWe9ld5QDbOnoSfjtluWpc9BLuk_1743671934 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4E94FC0003 X-Stat-Signature: piywxt9tegf1nqj4qia5iwyn4nc6erw1 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1743671938-792519 X-HE-Meta: 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 YTL6Voq/ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 03.04.25 05:31, Jingbo Xu wrote: > > > On 4/3/25 5:34 AM, Joanne Koong wrote: >> On Thu, Dec 19, 2024 at 5:05 AM David Hildenbrand wrote: >>> >>> On 23.11.24 00:23, Joanne Koong wrote: >>>> For migrations called in MIGRATE_SYNC mode, skip migrating the folio if >>>> it is under writeback and has the AS_WRITEBACK_INDETERMINATE flag set on its >>>> mapping. If the AS_WRITEBACK_INDETERMINATE flag is set on the mapping, the >>>> writeback may take an indeterminate amount of time to complete, and >>>> waits may get stuck. >>>> >>>> Signed-off-by: Joanne Koong >>>> Reviewed-by: Shakeel Butt >>>> --- >>>> mm/migrate.c | 5 ++++- >>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/migrate.c b/mm/migrate.c >>>> index df91248755e4..fe73284e5246 100644 >>>> --- a/mm/migrate.c >>>> +++ b/mm/migrate.c >>>> @@ -1260,7 +1260,10 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, >>>> */ >>>> switch (mode) { >>>> case MIGRATE_SYNC: >>>> - break; >>>> + if (!src->mapping || >>>> + !mapping_writeback_indeterminate(src->mapping)) >>>> + break; >>>> + fallthrough; >>>> default: >>>> rc = -EBUSY; >>>> goto out; >>> >>> Ehm, doesn't this mean that any fuse user can essentially completely >>> block CMA allocations, memory compaction, memory hotunplug, memory >>> poisoning... ?! >>> >>> That sounds very bad. >> >> I took a closer look at the migration code and the FUSE code. In the >> migration code in migrate_folio_unmap(), I see that any MIGATE_SYNC >> mode folio lock holds will block migration until that folio is >> unlocked. This is the snippet in migrate_folio_unmap() I'm looking at: >> >> if (!folio_trylock(src)) { >> if (mode == MIGRATE_ASYNC) >> goto out; >> >> if (current->flags & PF_MEMALLOC) >> goto out; >> >> if (mode == MIGRATE_SYNC_LIGHT && !folio_test_uptodate(src)) >> goto out; >> >> folio_lock(src); >> } >> Right, I raised that also in my LSF/MM talk: waiting for readahead currently implies waiting for the folio lock (there is no separate readahead flag like there would be for writeback). The more I look into this and fuse, the more I realize that what fuse does is just completely broken right now. >> If this is all that is needed for a malicious FUSE server to block >> migration, then it makes no difference if AS_WRITEBACK_INDETERMINATE >> mappings are skipped in migration. A malicious server has easier and >> more powerful ways of blocking migration in FUSE than trying to do it >> through writeback. For a malicious fuse server, we in fact wouldn't >> even get far enough to hit writeback - a write triggers >> aops->write_begin() and a malicious server would deliberately hang >> forever while the folio is locked in write_begin(). > > Indeed it seems possible. A malicious FUSE server may already be > capable of blocking the synchronous migration in this way. Yes, I think the conclusion is that we should advise people from not using unprivileged FUSE if they care about any features that rely on page migration or page reclaim. > > >> >> I looked into whether we could eradicate all the places in FUSE where >> we may hold the folio lock for an indeterminate amount of time, >> because if that is possible, then we should not add this writeback way >> for a malicious fuse server to affect migration. But I don't think we >> can, for example taking one case, the folio lock needs to be held as >> we read in the folio from the server when servicing page faults, else >> the page cache would contain stale data if there was a concurrent >> write that happened just before, which would lead to data corruption >> in the filesystem. Imo, we need a more encompassing solution for all >> these cases if we're serious about preventing FUSE from blocking >> migration, which probably looks like a globally enforced default >> timeout of some sort or an mm solution for mitigating the blast radius >> of how much memory can be blocked from migration, but that is outside >> the scope of this patchset and is its own standalone topic. I'm still skeptical about timeouts: we can only get it wrong. I think a proper solution is making these pages movable, which does seem feasible if (a) splice is not involved and (b) we can find a way to not hold the folio lock forever e.g., in the readahead case. Maybe readahead would have to be handled more similar to writeback (e.g., having a separate flag, or using a combination of e.g., writeback+uptodate flag, not sure) In both cases (readahead+writeback), we'd want to call into the FS to migrate a folio that is under readahread/writeback. In case of fuse without splice, a migration might be doable, and as discussed, splice might just be avoided. >> >> I don't see how this patch has any additional negative impact on >> memory migration for the case of malicious servers that the server >> can't already (and more easily) do. In fact, this patchset if anything >> helps memory given that malicious servers now can't also trigger page >> allocations for temp pages that would never get freed. >> > > If that's true, maybe we could drop this patch out of this patchset? So > that both before and after this patchset, synchronous migration could be > blocked by a malicious FUSE server, while the usability of continuous > memory (CMA) won't be affected. I had exactly the same thought: if we can block forever on the folio lock, there is no need for AS_WRITEBACK_INDETERMINATE. It's already all completely broken. -- Cheers, David / dhildenb