On Fri, Jan 11, 2019 at 06:49:53PM +0300, Yury Kotov wrote: > 10.01.2019, 23:12, "Dr. David Alan Gilbert" <dgilb...@redhat.com>: > > * Yury Kotov (yury-ko...@yandex-team.ru) wrote: > >> Hi, > >> > >> The series adds migration capability which allows to skip 'external' RAM > >> blocks > >> during migration. External block is a RAMBlock which available from the > >> outside > >> of current QEMU process (e.g. file in /dev/shm). It's useful for fast > >> local > >> migration to update QEMU for the running guests. > > > > Hi Yury, > > There have been a few similar patch series around from people wanting > > to do similar things. > > In particular Lai Jiangshan's > > https://lists.nongnu.org/archive/html/qemu-devel/2018-03/msg07511.html > > and Cédric Le Goater wanted to skip regions for a different reason. > > > > We merged some of Cédric's code last year so that we now > > have the qemu_ram_is_migratable() function - and we should be reusing > > that to skip things rather than adding a new check that we have to add > > everywhere. > > > > I didn't see the series, so I'll check it, thanks! > But I saw qemu_ram_is_migratable() function and corresponding patch. > It's very close to my needs, but it works a bit different IIUC: > 1. Not migratable blocks isn't validated (existence and size) during > migration, > 2. "Migratable" state is determined during the block creation time. > Such case isn't valid because of it: > * Source has one migratable and one not migratable RAM blocks, > * Target has the same (idstr) blocks, but both are not migratable. > Thus, target will not expect pages for not migratable blocks. > > > Also, ypu're skipping 'external' things, I think the other suggestion > > was to skip 'shared' things (i.e. anything with share=0); skipping > > share=on cases sounds easier to me. > > I agree that introducing new term is a complication, but 'share' and > 'external' > terms have important differences (I'll describe it below). > > Just to clarify: > * 'share' means that other processes has an access to such memory, > * 'external' means file backed memory.
If you use file backed memory with share=off, writes are not propagated to the file (they are mapped with MAP_PRIVATE). Would you really want to skip file backed memory if it has share=off? > > There is another use case I wanted to support (I had to write about it in > the cover letter, sorry..): > 1. Migrate source VM to file and kill source, > 2. Start target VM and migrate it from file. > In such case source VM may have memory-backend-ram with share=off, it's ok. > > Thus, in the new migration capability I want to migrate memory that meets > three conditions: > 1. The source will not use the memory after migration ends, > 2. The source may exit before target starts (migrate to file), > 3. The target has an access to the memory. > > I think 'external' fits them better than 'share'. > In either case, defining "external" seems tricky. A memory region might be backed by a file on tmpfs or hugetlbfs that was deleted, which makes the file "internal" for practical purposes. QEMU has no way to tell if (3) is really true. -- Eduardo