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(p200300cbc730430018eb6c63a196d3a2.dip0.t-ipconnect.de. [2003:cb:c730:4300:18eb:6c63:a196:d3a2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432aa70a1d8sm220764755e9.27.2024.11.11.07.19.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 11 Nov 2024 07:19:33 -0800 (PST) Message-ID: Date: Mon, 11 Nov 2024 16:19:31 +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: <6099e202-ef0a-4d21-958c-2c42db43a5bb@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 2YaiV6wrUWg7Rl6J15-w9xgxX7FSFYuJYUAvrPLPN68_1731338375 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 07FDC12001E X-Rspamd-Server: rspam11 X-Stat-Signature: g396kwajjn7c55xqr1aod738tzpo5rts X-HE-Tag: 1731338324-827184 X-HE-Meta: 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 NIIHJqio gJMRiat6nrdZizvM1l7EcKIf+1zJZze/AQq+YZcMvSPl9f+mOAd0ATXAR7SCUr6EfTTIr5uJCCGd/oOXh+rkQPmSXfVpXWGFfJT/MBqEVfK8ctRK3m5k4/tMIj/ZorPOLb/xazH1Hu6Trtl78ea9LnuB6pbkyzZAJNg+U55snH5gaIUp3TajwmoVIHdru9T860pghtmLzHtLq9VP6uEf3PSvQVj8FUQhwWCNTDTnlsIKY6WjWXjOg4NQYSww9AfUx/mp9DmR8lVPU/CMch+LRyAJIl75MA5ThglOO+qFZCedV4la89M7XaYTEAot2OqRN5PRLdvWWEidkg9fApked35l4L0G+0oTiLDjpLuMR9JdWnIY3pypZqEUSQ+SrH4mU8kLe+XjTgP9oNZujTFHOK308DnYt8BcoElTduqlUZRERs2PKzneBQdPEgRmJ0el7xohWSlP4jqO7Xerv0u6YDpURNhCJITKyv3UJGDy+Kd34EkI= 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 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. -- Cheers, David / dhildenb