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Thu, 20 Feb 2020 11:24:51 +0000 (UTC) Received: from [10.36.118.29] (unknown [10.36.118.29]) by smtp.corp.redhat.com (Postfix) with ESMTP id F343490F79; Thu, 20 Feb 2020 11:24:43 +0000 (UTC) Subject: Re: [PATCH v1 13/13] migrate/ram: Tolerate partially changed mappings in postcopy code To: qemu-devel@nongnu.org References: <20200219161725.115218-1-david@redhat.com> <20200219161725.115218-14-david@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= mQINBFXLn5EBEAC+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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABtCREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT6JAlgEEwEIAEICGwMFCQlmAYAGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheAFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl3pImkCGQEACgkQTd4Q 9wD/g1o+VA//SFvIHUAvul05u6wKv/pIR6aICPdpF9EIgEU448g+7FfDgQwcEny1pbEzAmiw zAXIQ9H0NZh96lcq+yDLtONnXk/bEYWHHUA014A1wqcYNRY8RvY1+eVHb0uu0KYQoXkzvu+s Dncuguk470XPnscL27hs8PgOP6QjG4jt75K2LfZ0eAqTOUCZTJxA8A7E9+XTYuU0hs7QVrWJ jQdFxQbRMrYz7uP8KmTK9/Cnvqehgl4EzyRaZppshruKMeyheBgvgJd5On1wWq4ZUV5PFM4x II3QbD3EJfWbaJMR55jI9dMFa+vK7MFz3rhWOkEx/QR959lfdRSTXdxs8V3zDvChcmRVGN8U Vo93d1YNtWnA9w6oCW1dnDZ4kgQZZSBIjp6iHcA08apzh7DPi08jL7M9UQByeYGr8KuR4i6e RZI6xhlZerUScVzn35ONwOC91VdYiQgjemiVLq1WDDZ3B7DIzUZ4RQTOaIWdtXBWb8zWakt/ ztGhsx0e39Gvt3391O1PgcA7ilhvqrBPemJrlb9xSPPRbaNAW39P8ws/UJnzSJqnHMVxbRZC Am4add/SM+OCP0w3xYss1jy9T+XdZa0lhUvJfLy7tNcjVG/sxkBXOaSC24MFPuwnoC9WvCVQ ZBxouph3kqc4Dt5X1EeXVLeba+466P1fe1rC8MbcwDkoUo65Ag0EVcufkQEQAOfX3n0g0fZz 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+w6DABEBAAGJAiUEGAECAA8FAlXLn5ECGwwFCQlmAYAACgkQTd4Q 9wD/g1qA6w/+M+ggFv+JdVsz5+ZIc6MSyGUozASX+bmIuPeIecc9UsFRatc91LuJCKMkD9Uv GOcWSeFpLrSGRQ1Z7EMzFVU//qVs6uzhsNk0RYMyS0B6oloW3FpyQ+zOVylFWQCzoyyf227y GW8HnXunJSC+4PtlL2AY4yZjAVAPLK2l6mhgClVXTQ/S7cBoTQKP+jvVJOoYkpnFxWE9pn4t H5QIFk7Ip8TKr5k3fXVWk4lnUi9MTF/5L/mWqdyIO1s7cjharQCstfWCzWrVeVctpVoDfJWp 4LwTuQ5yEM2KcPeElLg5fR7WB2zH97oI6/Ko2DlovmfQqXh9xWozQt0iGy5tWzh6I0JrlcxJ ileZWLccC4XKD1037Hy2FLAjzfoWgwBLA6ULu0exOOdIa58H4PsXtkFPrUF980EEibUp0zFz GotRVekFAceUaRvAj7dh76cToeZkfsjAvBVb4COXuhgX6N4pofgNkW2AtgYu1nUsPAo+NftU CxrhjHtLn4QEBpkbErnXQyMjHpIatlYGutVMS91XTQXYydCh5crMPs7hYVsvnmGHIaB9ZMfB njnuI31KBiLUks+paRkHQlFcgS2N3gkRBzH7xSZ+t7Re3jvXdXEzKBbQ+dC3lpJB0wPnyMcX FOTT3aZT7IgePkt5iC/BKBk3hqKteTnJFeVIT7EC+a6YUFg= Organization: Red Hat GmbH Message-ID: <9887ef65-12b1-37d5-cbf3-fda0b7d7fda7@redhat.com> Date: Thu, 20 Feb 2020 12:24:42 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200219161725.115218-14-david@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: y9vuyhgNPMSH-OKsIl6NUg-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Andrea Arcangeli , Eduardo Habkost , Juan Quintela , "Dr . David Alan Gilbert" , Peter Xu , Paolo Bonzini , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 19.02.20 17:17, David Hildenbrand wrote: > When we partially change mappings (e.g., mmap over parts of an existing > mmap) where we have a userfaultfd handler registered, the handler will > implicitly be unregistered from the parts that changed. This is e.g., the > case when doing a qemu_ram_remap(), but is also a preparation for RAM > blocks with resizable allocations and we're shrinking RAM blocks. > > When the mapping is changed and the handler is removed, any waiters are > woken up. Trying to place pages will fail. We can simply ignore erors > due to that when placing pages - as the mapping changed on the migration > destination, also the content is stale. E.g., after shrinking a RAM > block, nobody should be using that memory. After doing a > qemu_ram_remap(), the old memory is expected to have vanished. > > Let's tolerate such errors (but still warn for now) when placing pages. > Also, add a comment why unregistering will continue to work even though > the mapping might have changed. > > Cc: "Dr. David Alan Gilbert" > Cc: Juan Quintela > Cc: Peter Xu > Cc: Andrea Arcangeli > Signed-off-by: David Hildenbrand > --- > migration/postcopy-ram.c | 43 ++++++++++++++++++++++++++++++++++------ > 1 file changed, 37 insertions(+), 6 deletions(-) > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c > index c68caf4e42..df9d27c004 100644 > --- a/migration/postcopy-ram.c > +++ b/migration/postcopy-ram.c > @@ -506,6 +506,13 @@ static int cleanup_range(RAMBlock *rb, void *opaque) > range_struct.start = (uintptr_t)host_addr; > range_struct.len = length; > > + /* > + * In case the mapping was partially changed since we enabled userfault > + * (esp. when whrinking RAM blocks and we have resizable allocations, or > + * via qemu_ram_remap()), the userfaultfd handler was already removed for > + * the mappings that changed. Unregistering will, however, still work and > + * ignore mappings without a registered handler. > + */ > if (ioctl(mis->userfault_fd, UFFDIO_UNREGISTER, &range_struct)) { > error_report("%s: userfault unregister %s", __func__, strerror(errno)); > > @@ -1239,10 +1246,28 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from, > */ > if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, from, pagesize, rb)) { > int e = errno; > - error_report("%s: %s copy host: %p from: %p (size: %zd)", > - __func__, strerror(e), host, from, pagesize); > > - return -e; > + /* > + * When the mapping gets partially changed before we try to place a page > + * (esp. when whrinking RAM blocks and we have resizable allocations, or > + * via qemu_ram_remap()), the userfaultfd handler will be removed and > + * placing pages will fail. In that case, any waiter was already woken > + * up when the mapping was changed. We can safely ignore this, as > + * mappings that change once we're running on the destination imply > + * that memory of these mappings vanishes. Let's still print a warning > + * for now. > + * After talking to Andrea, on mapping changes, no waiter will be woken up automatically. We have to do an UFFDIO_WAKE, which even works when there is no longer a handler registered for that reason. Interesting stuff :) -- Thanks, David / dhildenb