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(p200300cbc70cde0012008636b63b0f43.dip0.t-ipconnect.de. [2003:cb:c70c:de00:1200:8636:b63b:f43]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3825493ee59sm4842152f8f.103.2024.11.21.04.15.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Nov 2024 04:15:22 -0800 (PST) Message-ID: Date: Thu, 21 Nov 2024 13:15:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [ISSUE] split_folio() and dirty IOMAP folios From: David Hildenbrand To: Matthew Wilcox Cc: linux-fsdevel@vger.kernel.org, "linux-mm@kvack.org" , kvm@vger.kernel.org, Zi Yan , Christian Brauner , "Darrick J. Wong" , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Thomas Huth References: <4febc035-a4ff-4afe-a9a0-d127826852a9@redhat.com> <6099e202-ef0a-4d21-958c-2c42db43a5bb@redhat.com> 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: ozp5SmL5J-GEVT_-WLcWS4GbGrNPxrOYqw9dImrEAZk_1732191323 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 57EDEC0005 X-Stat-Signature: 5agr6gz3eo9rdth5734i31ms6cwj5cnf X-Rspam-User: X-HE-Tag: 1732191263-255203 X-HE-Meta: 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 BOpkUKO3 0ra7tXxq+J/yqs8qgT/8zeEly5cT+H6c8vlaO5HZLs31bvQ0Z7Iklqd3DMaz6de0i19JDjpNapHa6U2pOu0ZJZPdUXnyXQNRQIhBSzbaRdY1PBHvj6l/gGT6uVzOn7Bk7dul1ZLA2rlnZ/03xvOjFfS2RAasgtVAzV8KQ/Me2FcrlvFCASX3UOLEtXrToDpMCKfCgEO/vOaa4yp/gx7EqhItg4STMNtfUYrR1iopDYCxzGMlNMqJpOMI0fEIUEcAgecgHJIvUU+8uqexT9W0WGynZhFNxCp8sLImIpxxFLTO7A+BrtG8hKqBCrVVYkq87J7X+mfcCYk4EgbDBf3hgL5u3e6qayNGeqCsd+b7h5N0SDaye+iQLtS4nZTJD3A+U1p/KnJFUHN6SsbJ4GnJT5bfT01TE7JG74cGP5DBihhLciIIvnkRjn35sqm+3rSemoTuXoYUmsiU2fQp1dGhDyCecyMZdy5BBxmvPAqT1aYAW0TM= 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 11.11.24 16:19, David Hildenbrand wrote: > On 08.11.24 10:11, David Hildenbrand wrote: >> On 07.11.24 21:20, Matthew Wilcox wrote: >>> On Thu, Nov 07, 2024 at 05:34:40PM +0100, David Hildenbrand wrote: >>>> On 07.11.24 17:09, Matthew Wilcox wrote: >>>>> On Thu, Nov 07, 2024 at 04:07:08PM +0100, David Hildenbrand wrote: >>>>>> I'm debugging an interesting problem: split_folio() will fail on dirty >>>>>> folios on XFS, and I am not sure who will trigger the writeback in a timely >>>>>> manner so code relying on the split to work at some point (in sane setups >>>>>> where page pinning is not applicable) can make progress. >>>>> >>>>> You could call something like filemap_write_and_wait_range()? >>>> >>>> Thanks, have to look into some details of that. >>>> >>>> Looks like the folio_clear_dirty_for_io() is buried in >>>> folio_prepare_writeback(), so that part is taken care of. >>>> >>>> Guess I have to fo from folio to "mapping,lstart,lend" such that >>>> __filemap_fdatawrite_range() would look up the folio again. Sounds doable. >>>> >>>> (I assume I have to drop the folio lock+reference before calling that) >>> >>> I was thinking you'd do it higher in the callchain than >>> gmap_make_secure(). Presumably userspace says "I want to make this >>> 256MB range secure" and we can start by writing back that entire >>> 256MB chunk of address space. >>> >>> That doesn't prevent anybody from dirtying it in-between, of course, >>> so you can still get -EBUSY and have to loop round again. >> >> I'm afraid that won't really work. >> >> On the one hand, we might be allocating these pages (+disk blocks) >> during the unpack operation -- where we essentially trigger page faults >> first using gmap_fault() -- so the pages might not even exist before the >> gmap_make_secure() during unpack. One work around would be to >> preallocate+writeback from user space, but it doesn't sound quite right. >> >> But the bigger problem I see is that the initial "unpack" operation is >> not the only case where we trigger this conversion to "secure" state. >> Once the VM is running, we can see calls on arbitrary guest memory even >> during page faults, when gmap_make_secure() is called via >> gmap_convert_to_secure(). >> >> >> I'm still not sure why we see essentially no progress being made, even >> though we temporarily drop the PTL, mmap lock, folio lock, folio ref ... >> maybe related to us triggering a write fault that somehow ends up >> setting the folio dirty :/ Or because writeback is simply too slow / >> backs off. >> >> I'll play with handling -EBUSY from split_folio() differently: if the >> folio is under writeback, wait on that. If the folio is dirty, trigger >> writeback. And I'll look into whether we really need a writable PTE, I >> suspect not, because we are not actually "modifying" page content. > > The following hack makes it fly: > > case -E2BIG: > folio_lock(folio); > rc = split_folio(folio); > + if (rc == -EBUSY) { > + if (folio_test_dirty(folio) && !folio_test_anon(folio) && > + folio->mapping) { > + struct address_space *mapping = folio->mapping; > + loff_t lstart = folio_pos(folio); > + loff_t lend = lstart + folio_size(folio); > + > + folio_unlock(folio); > + /* Mapping can go away ... */ > + filemap_write_and_wait_range(mapping, lstart, lend); > + } else { > + folio_unlock(folio); > + } > + folio_wait_writeback(folio); > + folio_lock(folio); > + split_folio(folio); > + folio_unlock(folio); > + folio_put(folio); > + return -EAGAIN; > + } > folio_unlock(folio); > folio_put(folio); > > > I think the reason why we don't make any progress on s390x is that the writeback will > mark the folio clean and turn the folio read-only in the page tables as well. So when we > lookup the folio again in the page table, we see that the PTE is not writable and > trigger a write fault ... > > ... the write fault will mark the folio dirty again, so the split will never succeed. > > In above diff, we really must try the split_folio() a second time after waiting, otherwise we > run into the same endless loop. > > > I'm still not 100% sure if we need a writable PTE; after all we are not modifying page content. > But that's just a side effect of not being able to wait for the split_folio() to make progress > in the writeback case so we can retry the split again. After discussing this with Darrick and Willy yesterday, I think the reason we need a writable PTE is because we *might* modify page content: "Requests the Ultravisor to make a page accessible to a guest. If it's brought in the first time, it will be cleared. If it has been exported before, it will be decrypted and integrity checked." So we'll be effectively modifying the page content we will read when the (now secure) page is in the unprotected/exported state. That makes things more complicated, unfortunately :) -- Cheers, David / dhildenb