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Fri, 14 Feb 2020 18:27:08 +0000 (UTC) Received: from [10.36.118.137] (unknown [10.36.118.137]) by smtp.corp.redhat.com (Postfix) with ESMTP id 94571790D1; Fri, 14 Feb 2020 18:27:00 +0000 (UTC) Subject: Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating To: Peter Xu References: <20200213172016.196609-1-david@redhat.com> <20200214102514.GB3283@work-vm> <30a66b1d-184e-a684-d0d2-c3921366b478@redhat.com> <20200214104230.GC3283@work-vm> <20200214110226.GD3283@work-vm> <9a15fd0e-77d1-b3a0-4824-665f85f79c71@redhat.com> <20200214172933.GC1163818@xz-x1> <9597c9a9-05d2-d99d-e153-8ffc112910bc@redhat.com> <20200214181146.GE1163818@xz-x1> 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: Date: Fri, 14 Feb 2020 19:26:59 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200214181146.GE1163818@xz-x1> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: BG523zJ7NQ68HLa_fFrlFw-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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: Eduardo Habkost , Juan Quintela , "Michael S. Tsirkin" , Richard Henderson , qemu-devel@nongnu.org, Shameerali Kolothum Thodi , "Dr. David Alan Gilbert" , Shannon Zhao , Paolo Bonzini , Igor Mammedov , =?UTF-8?Q?Alex_Benn=c3=a9e?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 14.02.20 19:11, Peter Xu wrote: > On Fri, Feb 14, 2020 at 06:32:35PM +0100, David Hildenbrand wrote: >> On 14.02.20 18:29, Peter Xu wrote: >>> On Fri, Feb 14, 2020 at 01:02:46PM +0100, David Hildenbrand wrote: >>>> From c0049ac2e95d6756037db918852c507fb88297d9 Mon Sep 17 00:00:00 2001 >>>> From: David Hildenbrand >>>> Date: Fri, 14 Feb 2020 13:01:03 +0100 >>>> Subject: [PATCH v1] tmp >>>> >>>> Signed-off-by: David Hildenbrand >>>> --- >>>> migration/migration.c | 9 +++++++-- >>>> migration/migration.h | 1 + >>>> migration/ram.c | 42 ++++++++++++++++++++++++++++++++++++++++++ >>>> 3 files changed, 50 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/migration/migration.c b/migration/migration.c >>>> index 3a21a4686c..0e7efe2920 100644 >>>> --- a/migration/migration.c >>>> +++ b/migration/migration.c >>>> @@ -175,13 +175,18 @@ void migration_object_init(void) >>>> } >>>> } >>>> =20 >>>> +void migration_cancel(void) >>>> +{ >>>> + migrate_fd_cancel(current_migration); >>>> +} >>>> + >>>> void migration_shutdown(void) >>>> { >>>> /* >>>> * Cancel the current migration - that will (eventually) >>>> * stop the migration using this structure >>>> */ >>>> - migrate_fd_cancel(current_migration); >>>> + migration_cancel(); >>>> object_unref(OBJECT(current_migration)); >>>> } >>>> =20 >>>> @@ -2019,7 +2024,7 @@ void qmp_migrate(const char *uri, bool has_blk, = bool blk, >>>> =20 >>>> void qmp_migrate_cancel(Error **errp) >>>> { >>>> - migrate_fd_cancel(migrate_get_current()); >>>> + migration_cancel(); >>>> } >>>> =20 >>>> void qmp_migrate_continue(MigrationStatus state, Error **errp) >>>> diff --git a/migration/migration.h b/migration/migration.h >>>> index 8473ddfc88..79fd74afa5 100644 >>>> --- a/migration/migration.h >>>> +++ b/migration/migration.h >>>> @@ -343,5 +343,6 @@ int foreach_not_ignored_block(RAMBlockIterFunc fun= c, void *opaque); >>>> void migration_make_urgent_request(void); >>>> void migration_consume_urgent_request(void); >>>> bool migration_rate_limit(void); >>>> +void migration_cancel(void); >>>> =20 >>>> #endif >>>> diff --git a/migration/ram.c b/migration/ram.c >>>> index ed23ed1c7c..f86f32b453 100644 >>>> --- a/migration/ram.c >>>> +++ b/migration/ram.c >>>> @@ -52,6 +52,7 @@ >>>> #include "migration/colo.h" >>>> #include "block.h" >>>> #include "sysemu/sysemu.h" >>>> +#include "sysemu/runstate.h" >>>> #include "savevm.h" >>>> #include "qemu/iov.h" >>>> #include "multifd.h" >>>> @@ -3710,8 +3711,49 @@ static SaveVMHandlers savevm_ram_handlers =3D { >>>> .resume_prepare =3D ram_resume_prepare, >>>> }; >>>> =20 >>>> +static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host= , >>>> + size_t old_size, size_t new_siz= e) >>>> +{ >>>> + /* >>>> + * We don't care about resizes triggered on incoming migration (w= hen >>>> + * syncing ram blocks), or of course, when no migration is going = on. >>>> + */ >>>> + if (migration_is_idle() || !runstate_is_running()) { >>>> + return; >>>> + } >>> >>> I feel like migration_is_idle() check is enough. Firstly, I feel like >>> we allow migration even with VM stopped. At the meantime, if VM is >>> not running, I see no reason that this resizing will happen after all? = :) >> >> Migration code resizes ram blocks when synchronizing the ram state. See >> qemu_ram_resize() in ram_load_precopy() >> >> That happens on incoming migration while the vm is stopped. >=20 > Ah so your comment is for that which I misread. I'm surprised even > the incoming migration will set MigrationState and migration_is_idle() I was already surprised by that :) And states are not well document (IOW, not documented) > will return false for it, since we've got MigrationIncomingState after > all. But yeh that seems to be correct. >=20 > Then it seems fine. It's just a bit unclear even with the comment. > Another alternative is we only register this resize() hook when > migration starts, and unregister it when stopped. Then we can even > drop the migration_is_idle(). Yeah, I prefer the current code because its simpler (and I don't have to worry about races when registering/unregistering notifiers, because locking in combination with migration is still a big puzzle to me) I'll try to make the comment clearer, thanks! >=20 >> >>> >>>> + >>>> + if (!postcopy_is_running()) { >>>> + Error *err =3D NULL; >>>> + >>>> + /* >>>> + * Precopy code cannot deal with the size of ram blocks chang= ing at >>>> + * random points in time. We're still running on the source, = abort >>>> + * the migration and continue running here. Make sure to wait= until >>>> + * migration was canceled. >>>> + */ >>>> + error_setg(&err, "RAM resized during precopy."); >>>> + migrate_set_error(migrate_get_current(), err); >>>> + error_free(err); >>>> + migration_cancel(); >>>> + } else { >>>> + /* >>>> + * Postcopy code cannot deal with the size of ram blocks chan= ging at >>>> + * random points in time. We're running on the target. Fail h= ard. >>>> + * >>>> + * TODO: How to handle this in a better way? >>>> + */ >>>> + error_report("RAM resized during postcopy."); >>>> + exit(-1); >>> >>> Now I'm rethinking the postcopy case.... >>> >>> ram_dirty_bitmap_reload() should only happen during the postcopy >>> recovery, and when that happens the VM should be stopped on both >>> sides. Which means, ram resizing should not trigger there... >> >> But that guest got the chance to run for a bit and eventually reboot >> AFAIK. Also, there are other data races possible when used_length >> suddenly changes, this is just the most obvious one where things will; >> get screwed up. >=20 > Right, the major one could be in ram_load_postcopy() where we call > host_from_ram_block_offset(). However if FW extension is the major > use case then it seems to still work (still better than crashing, > isn't it? :). "Let's close our eyes and hope it will never happen" ? :) No, I don't like that. This screams for a better solution long term, and until then a proper fencing IMHO. We're making here wild guesses about data races and why they might not be that bad in certain cases (did I mention load/store tearing? used_length is not an atomic value ...). --=20 Thanks, David / dhildenb